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Planning for the 2020 Season: Resistance Management Guidelines for Fungicides Used for Downy Mildew of Grape

By Andy Muza, Penn State Extension – Erie County

Downy mildew has been a major problem for many grape growers in Pennsylvania (outside of Erie County) over the last few years. This is no surprise, considering the amount of rainfall that has occurred throughout areas of PA during the past few seasons. As a result, downy mildew inoculum levels may be high in many vineyards at the start of the 2020 growing season.

Therefore, in preparation for planning your downy mildew management program (and reducing the risk of fungicide resistance) for the 2020 season, the following information from the 2019 New York and Pennsylvania Pest Management Guidelines for Grapes is provided (1). 

Fungicide Resistance Risks 

Fungicides that have similar chemical structures and share common modes of action are classified together into Groups. Resistance risk for each fungicide group is considered as either: LOWMEDIUMHIGH or Resistance not known (2). “The likelihood and speed of resistance development largely depends upon whether the fungicide affects a single metabolic site (single-site) within the fungus or multiple sites (multi-site). High-risk products have a single site of action or those for which disease resistance populations have been discovered. Medium-risk products are associated with fungicides where resistance is seen with the mutation of more than one target site or resistance formation is less frequent than that of high risk. Low-risk fungicides are characterized by a very rare or undocumented occurrence of resistance after many years of use” (3).

FRAC Codes

The Fungicide Resistance Action Committee (FRAC) developed a Code, consisting of letters and/or numbers, to distinguish different fungicide groups based on their mode of action (2). Each fungicide product includes a FRAC Code on the fungicide label.

Eleven different modes of action groups [FRAC Codes – M 01, M 03, M 04, 4, 11, 21, 22, 33, 40, 43, 45] are included in Tables 3.2.1 and 3.2.2 on pages 46 – 48 of the 2019 New York and Pennsylvania Pest Management Guidelines for Grapes  that are rated excellent (++++) to good (+++) for management of downy mildew (1).
NOTE: PRESIDIO (fluopicolide – FRAC Code 43) is not included in the information below. The manufacturer has pulled the grape use from the PRESIDIO label, and any new product will not be legal for use on grapes. However, grape growers will be able to legally use up old stock of PRESIDIO with the grape use pattern on the label. 

Multi-Site Fungicides

  • FRAC Code M 01 – The common name of the active ingredient in this group is copper.
  • FRAC Code M 03 – The common names of active ingredients in this group include mancozeb and ziram.
  • FRAC Code M 04 – The common name of an active ingredient in this group includes captan.

Resistance Risk: LOW. No signs of resistance developing.

Fungicide Products containing active ingredients listed within FRAC Codes: M 01, M 03, M 04

  • COPPER products (several formulations) – copper
  • DITHANE, MANZATE and PENNCOZEB products – mancozeb
  • ZIRAM – ziram
  • CAPTAN products – captan

Resistance Management Guideline: Multi-site fungicides are important tools in a resistance management program (4). Copper, mancozeb and captan products provide good control of downy mildew while ziram provides moderate control. These fungicides can also be used, as either a tank mix partner or as a co-formulation product, with fungicides that are designated as HIGH or MEDIUM risk for downy mildew resistance. Use of multi-site fungicides, as either a co-formulation product or as a tank mix partner, can: improve disease control; reduce the risk of resistance development; or provide a measure of control if resistance is already present in a vineyard (5).

Be sure to read the label of the products used for information such as: maximum allowable rates/A/season, compatibility and phytotoxicity precautions, preharvest and reentry intervals. 

FRAC Code 4 – The common name of an active ingredient in this group includes mefenoxam.

Resistance Risk: HIGH

Fungicide Products containing an active ingredient listed within FRAC Code 4

  • RIDOMIL GOLD/COPPER – a co-formulation product containing mefanoxam + copper hydroxide.
  • RIDOMIL GOLD MZ WG – a co-formulation product containing mefanoxam + mancozeb.

RESISTANCE WARNINGRidomil Gold is an outstanding fungicide against downy mildew, but the causal organism (Plasmopara viticola) can develop resistance to it very quickly when the product is used intensively. This fungicide became ineffective in the humid viticultural regions of Europe soon after its introduction many years ago.

Resistance Management Guideline: To reduce the risk of developing resistance:

  • DO NOT make more than two applications of RIDOMIL GOLD per season (MZ and copper formulations combined). The conservative (safer) strategy is only one application per season. 
  • DO NOT make two consecutive applications of RIDOMIL GOLD (MZ and/or copper formulations) per season.
  • ROTATE RIDOMIL GOLD products with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 
  • DO NOT attempt “rescue” treatments with RIDOMIL GOLD if an epidemic is in progress.

FRAC Code 11 – Common names of active ingredients in this group include: azoxystrobin, kresoxim-methyl, mandestrobin, pyraclostrobin, trifloxystrobin and fenamidone.

Resistance Risk: HIGH

Fungicide Products containing active ingredients listed within FRAC Code 11

  • ABOUND, AZAKA 2.08 SC – azoxystrobin
  • QUADRIS TOP – a co-formulation product containing azoxystrobin + difenoconazole
  • TOPGUARD EQ – a co-formulation product containing azoxystrobin + flutriafol
  • DEXTER MAX – a co-formulation product containing azoxystrobin + mancozeb
  • SOVRAN 50WG – kresoxim-methyl
  • INTUITY 4SC- mandestrobin
  • PRISTINE 38WG – a co-formulation product containing pyraclostrobin + boscalid 
  • FLINT, FLINT EXTRA – trifloxystrobin
  • LUNA SENSATION – a co-formulation product containing trifloxystrobin + fluopyram
  • REASON – fenamidone. Although not a strobilurin, fenamidone has the same biochemical mode of activity. Cross resistance has been shown between all FRAC Code 11 fungicides.

RESISTANCE WARNINGDowny mildew resistance to the strobilurin (FRAC Code 11) fungicides has occurred in multiple vineyards throughout New York, various mid-Atlantic regions, and probably Pennsylvania. It is now risky to rely on FRAC Code 11 fungicides for control of either downy mildew or powdery mildew. When such resistance occurs, none of the FRAC Code 11 fungicides will provide commercial control of downy mildew, and they must be combined with an effective rate of an unrelated fungicide to avoid potential crop loss. 

Resistance Management Guideline: 

  • DO NOT make more than two applications per season of all FRAC Code 11 products (combined).
  • DO NOT make two consecutive applications of a FRAC Code 11 product. 
  • ROTATE FRAC Code 11 products with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 

FRAC Code 21 – The common name of an active ingredient in this group includes cyazofamid.

Resistance Risk: Unknown but assumed to be MEDIUM to HIGH. 

Fungicide Product containing an active ingredient listed within FRAC Code 21

  • RANMAN – cyazofamid

Resistance Management Guideline: 

  • DO NOT make more than two applications per season of RANMAN.
  • DO NOT make two consecutive applications of RANMAN. 
  • ROTATE RANMAN with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 
  • Tank Mixing RANMAN with a phosphorus acid product (e.g., ProPhyt, Phostrol) has provided excellent control of downy mildew.

FRAC Code 22 – The common name of an active ingredient in this group includes zoxamide.

Resistance Risk: MEDIUM

Fungicide Product containing an active ingredient listed within FRAC Code 22.

  • GAVEL 75DF – a co-formulation product containing zoxamide + mancozeb. The resistance risk for zoxamideis MEDIUM and the resistance risk for mancozeb is LOW.

This product when applied at the labeled rate of 2.0 – 2.5 lbs/A provides the same amount of mancozeb as 1.8 – 2.2 lbs of standard 75DF formulations of other mancozeb products (e.g., Dithane, Penncozeb). For control of diseases other than downy mildew, GAVEL 75DF should be applied with sufficient quantities of another mancozeb product to provide a dosage equivalent to 3 – 4 lbs/A of the 75DF formulations of a solo mancozeb product.

Resistance Management Guideline: 

  • DO NOT make more than three applications per season of GAVEL 75DF.
  • DO NOT make more than two consecutive applications of GAVEL 75DF. 
  • ROTATE GAVEL 75DF with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 

FRAC Code 33 – The common name of an active ingredient in this group includes phosphorous acid (various formulations). A number of products containing phosphorous acid (also called “phosphite” or “phosphonate”) are sold as nutritional supplements and “plant conditioners,” but only a few are registered for disease control on grapes; two that have proven efficacy in NY spray trials are ProPhyt and Phostrol, although others (e.g., Rampart, Reveille, Fosphite) have been effective in commercial use.

Resistance Risk: MEDIUM

Fungicide Products containing active ingredients listed within FRAC Code 33

  • PHOSTROL, PROPHYT, RAMPART, etc. – phosphorous acid

RESISTANCE WARNINGDowny Mildew resistance to phosphorous acid products has occurred when they have been used intensively on other crops, and a reduction in performance has been noted in several New York vineyards over the past few years, although resistance has not been proven conclusively. 

Resistance Management Guideline:

  • DO NOT make more than three applications per season of phosphorous acid products.
  • DO NOT make more than two consecutive applications of a phosphorous acid product. 
  • ROTATE phosphorous acid products with an unrelated fungicide (different Frac Code) having efficacy against downy mildew.

FRAC Code 40 – Common names of active ingredients in this group include dimethomorph and mandipropamid.

Resistance Risk: MEDIUM

Fungicide Products containing active ingredients listed within FRAC Code 40.

  • ZAMPRO – a co-formulation product containing dimethomorph + ametoctradin
  • REVUS 2SC – mandipropamid
  • REVUS TOP 4SC – a co-formulation product containing mandipropamid + difenoconazole

RESISTANCE WARNINGDowny mildew resistance to FRAC Code 40 fungicides has been found in three vineyards in Virginia and one in North Carolina in 2018. 

Resistance Management Guideline: 

  • DO NOT make more than three applications per season of Frac 40 products (ZAMPRO, REVUS/REVUS TOP) combined.
  • DO NOT make more than two consecutive applications of a Frac 40 product. 
  • ROTATE Frac 40 products with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 

FRAC Code 45 – The common name of an active ingredient in this group includes ametoctradin.

Resistance Risk: assumed to be MEDIUM to HIGH. 

Fungicide Product containing an active ingredient listed within FRAC Code 45:  

Zampro – a co-formulation product containing ametoctradin + dimethomorph 

Resistance Management Guideline: Same as listed for ZAMPRO under FRAC 40 above.

References:

  1. 2019 New York and Pennsylvania Pest Management Guidelines for Grapes. Weigle, T. H., and A. J. Muza. Cornell and Penn State Extension. 168 pp. https://cropandpestguides.cce.cornell.edu/Guidelines/2019/Grapes/index1.aspx
  2. Frac Code List 2019: Fungal control agents sorted by cross resistance pattern and mode of action (including FRAC Code numbering).https://www.frac.info/docs/default-source/publications/frac-code-list/frac-code-list-2019.pdf
  3. Raised Resistance Risks.https://pesticidestewardship.org/resistance/fungicide-resistance/raised-resistance-risks/
  4. Importance of multisite fungicides in managing pathogen resistance. https://www.frac.info/docs/default-source/publications/frac-recommendations-for-multisites/frac-statement-on-multisite-fungicides-2018.pdf?sfvrsn=19544b9a_2
  5. FRAC recommendations for fungicide mixtures designed to delay resistance evolution. https://www.frac.info/docs/default-source/publications/frac-recommendations-for-fungicide-mixtures/frac-recommendations-for-fungicide-mixtures—january-2010.pdf?sfvrsn=7e9d419a_4

A Complex Late Season Bunch Rot

By Andy Muza, Penn State Extension – Erie County

Harvest season in Pennsylvania is upon us or soon will be (depending on your varieties and where your vineyards are located), so late season bunch rots become a major concern for wine grape growers. A complex late season rot not controlled by fungicide applications is Sour Rot.  

Question: What can you get when you combine: tight clustered varieties; yeast; acetic acid bacteria; berry injury; and fruit flies?
Answer– Sour Rot.

Over the last few years extensive research, by Wendy McFadden-Smith and her colleagues at OMAFRA in Ontario and Megan Hall, Wayne Wilcox and Greg Loeb at Cornell, has greatly increased our knowledge of the Sour Rot syndrome. The following information is a brief summary of what the research revealed.

How do you know if the rot in your clusters is sour rot?
Sour rot has been defined by Megan Hall and Wayne Wilcox as, “a specific syndrome, characterized by the oxidation of the berry skin and the smell of acetic acid (vinegar) emanating from diseased berries.”

Therefore, field diagnosis is by both sight and smell. In white varieties, berry skins turn brown and in red varieties, berries have a reddish – purple discoloration (Figure 1). Infected berries degrade and have a vinegarlike odor. This syndrome is usually associated with large populations of fruit flies.

Development of sour rot
A wide variety of yeasts and bacteria naturally occur on and in grapes in the vineyard. Yeasts, whether in the vineyard or in the wine cellar, do what they do best. That is, they convert sugars in grape juice to alcohol (i.e., ethanol). Likewise, acetic acid bacteria (e.g., Acetobacter sppGluconobacter spp), whether in the vineyard or in the wine cellar, do what they do best. These bacteria convert ethanol into acetic acid (i.e., vinegar) in the presence of oxygen. Injured berries provide the gateway for bacteria, oxygen and insects (most commonly fruit flies) to enter berries.

The presence of fruit flies has been discovered to be a key component in the sour rot syndrome (Figure 2). Experiments showed that without fruit flies the symptoms of sour rot did not develop. Fruit flies spp. (e.g., common fruit fly, Drosophila melanogaster; and spotted wing drosophila, Drosophila suzukii) are attracted to injured berries via the smell of acetic acid and ethanol. As fruit flies feed and deposit eggs they spread yeast and bacteria from their bodies or gut contents throughout the clusters. However, the complete role that fruit flies contribute in sour rot development is not yet fully understood. Megan Hall, now at the University of Missouri, is continuing research to determine the complete picture of the fruit fly connection in sour rot development.

Management 

Cultural practices– Cultural practices play a critical role in the management of grape diseases and sour rot is no exception. Canopy management techniques, such as shoot thinning/positioning and leaf removal around clusters, provide better air flow and sun exposure thus reducing a more favorable microclimate for disease development.                 In addition, this opens up the canopy to better spray penetration.
Hall and Wilcox also showed that a vertical shoot position training system significantly reduced sour rot compared to a high wire trellis system. This should be taken into consideration if you are planning on planting a new vineyard with tight clustered, thin skinned varieties.

Berry Injury– The management of berry injury can be broken into 2 categories:
1)  What we cannot control, and 2)  What we can control. 

  1. What we cannot control – the weather.
    The most widespread cause of late season injury to berries in our region is due to rainfall events which cause berries to split or pull away from their pedicels.  Tight clustered, thin skinned varieties (such as Pinot Noir, Riesling, Vignoles, etc.) are the most susceptible to this injury and to sour rot and botrytis development.
    Unfortunately, tropical storms can and sometimes do occur around harvest, spreading excessive rainfall, resulting in berry splitting. The best we can hope for is that heavy rainfall events do not occur during harvest. 
  2. What we can control– injury caused by birds, diseases and insects.  
    Any injury can predispose berries to invasion from a variety of fungi, yeasts and bacteria that can result in bunch rots. Management of: birds (through use of netting and/or scare devices); diseases (through effective use of fungicides); and insects, particularly grape berry moth (through well timed insecticide applications) are important components in the reduction of berry injury levels.

Fruit flies, acetic acid bacteria and yeasts– Fungicides used for grape disease management are effective against filamentous fungi (e.g., Botrytis, powdery mildew, etc.) but not effective against yeasts and bacteria. Therefore, fungicides are not directly effective in sour rot management.
However, research conducted at Cornell in the Finger Lakes Region did show that applications of an antimicrobial material and insecticide applications against fruit flies are directly effective. Specifically, the most effective treatment regime consisted of weekly applications of Mustang Maxx insecticide (a.i. – zeta-cypermethrin) and OxiDate 2.0 (an antimicrobial, a.i. – includes hydrogen dioxide and peroxyacetic acid) starting when fruit reached 15 Brix and before any sour rot symptoms were evident.  This regime (insecticide and antimicrobial) provided an average of 69% control of sour rot. However, the insecticide alone treatments in 2015 & 2016, still provided 57% and 40% control, respectively.

It is important to mention that in 2018 in a Finger Lakes, NY vineyard a local population of fruit flies have developed resistance to Mustang Maxx, malathion and Assail. I cannot overemphasize the importance of rotating different classes of insecticides  (i.e., different modes of actions/different IRAC numbers) for fruit fly management in order to avoid the development of insecticide resistance. There are a number of registered insecticides with different modes of action and short preharvest intervals (PHI) which are effective against fruit flies. These include: Assail 30 SG (IRAC 4A, 3 days PHI); Delegate WG and Entrust SC (IRAC 5, 7 days PHI); Malathion 5EC or 57% or 8 Aquamul (IRAC 1B, 3 days PHI); and Mustang Maxx (IRAC 3A, 1 Day PHI). Greg Loeb and Hans Walter- Peterson (Cornell) suggest using a variety of different classes of insecticides in a season (refer to articles – Managing Fruit Flies for Sour Rot in 2019 and Suggested Fruit Fly Insecticide Program for 2019 under Additional Links).  

Management of Sour Rot in the Winemaking Process
Like it or not, winemakers may be forced to deal with volatile acidity issues due to sour rot. Since I am not an enologist, I will suggest 2 articles below which provide information for dealing with this problem. In addition, winemakers can also contact Molly Kelly, Enology Extension Educator, Penn State at (e-mail: mxk1171@psu.edu, phone: 814-865-6840) for assistance.

Managing Sour Rotted Fruit in the Cellar. Denise Gardner. Updated: May 5, 2016.
https://extension.psu.edu/managing-sour-rotted-fruit-in-the-cellar

Sour Rot Stinks: Some Strategies for managing Volatile Acidity. Chris Gerling.  Veraison to Harvest. Statewide Vineyard Crop Development, Update #5. Sept. 2018.
https://grapesandwine.cals.cornell.edu/sites/grapesandwine.cals.cornell.edu/files/shared/Veraison-To-Harvest-2018-Issue-5.pdf

Additional Links

For more comprehensive information concerning Sour Rot research and management of fruit flies, I highly recommend checking out the links below.

Defining and Developing Management Strategies for Sour Rot. Megan Hall, Gregory Loeb, and Wayne Wilcox. Appellation Cornell – Research News from Cornell’s Viticulture and Enology Program, Research Focus 2017-3.
https://grapesandwine.cals.cornell.edu/sites/grapesandwine.cals.cornell.edu/files/shared/Research%20Focus%202017-3.pdf

Managing Fruit Flies for Sour Rot in 2019. Greg Loeb and Hans Walter-Peterson.   Lake Erie Regional Grape Program Newsletter, September 2019, pages 6-8. https://nygpadmin.cce.cornell.edu/pdf/newsletter_notes/pdf116_pdf.pdf

Suggested Fruit Fly Insecticide Program for 2019. Hans Walter-Peterson and Greg Loeb. Lake Erie Regional Grape Program Newsletter, September 2019, page 9.  https://nygpadmin.cce.cornell.edu/pdf/newsletter_notes/pdf116_pdf.pdf

Testing soil microbial stimulators for enhancing vine health

By Justine Vanden Heuvel and Mariam Berdeja, School of Integrative Plant Science, Cornell University

What are mycorrhizae?  

Grapevines benefit from a symbiotic relationship with arbuscular mycorrhizal fungi (AMF).  Together the vine and the AMF form mycorrhizae, which play an important role in vine health, grapevine nutrition, and water relations.  A range of products – generally referred to as soil microbial stimulators – are sold with the goal of encouraging the formation of mycorrhizae. While anecdotal reports from the grape and wine industry suggest these products can provide a benefit to the vine, none have been systematically tested in Northeast vineyards.  

Arbuscular mycorrhizae penetrates the cortical cells of roots to form arbuscules (Fig. 1) to aid nutrient exchange. The hyphal coils are long, branched portions of the fungus that act as a virtual root system for the vine.  The hyphae enter the root and create vesicles for nutrient storage structures where nutrients are transferred between fungus and plant (arbuscules).

Fig 1. Arbuscular mycorrhizal fungal structures in the fine roots of grapevine. Samples were collected 5 months after inoculation. A: Arbuscules, HC: Hyphal coils, V: Vesicles, 40x magnified.

Greenhouse experiment

In order to screen products for further testing in vineyards, we initiated a greenhouse trial in 2019 using potted vines of Cabernet Sauvignon (own-rooted) and the rootstock 3309C. We decided to use only products that contained the species Glomus, as it has been shown to improve AMF formation on other crops.  (Note that many biofertilizers are for sale that do not contain Glomus). In the experiment, five commercial biofertilizers were compared to a control (Table 1).  Five months following application, the vines were destructively harvested to determine whether the biofertilizers had resulted in the formation of AMF and whether vine growth or nutrient acquisition was improved with the treatments. 

Product numberNameContained species 
1Big Foot ConcentrateGlomus intraradicesGlomus mossaeGlomus aggregatumGlomus etunicatumN,P,KHumic acidsSoftwood biocharWorm castings 
2BioOrganicGlomus mosseaeGlomus clarumGlomus aggregatumGlomus intraradicesGlomus deserticolaGlomus etunicatumGlomus monosporusGigaspora margaritaParaglomus brasilianum 
3MycoGrow SolubleGlomus intraradicesGlomus mosseaeGlomus aggregatumGlomus etunicatumbGlomus deserticolaGlomus monosporumGlomus clarumRhizopogon villosulusRhizopogon luteoulusRhizopogon amylopogonRhizopogon fulviglebaPisolithus tinctoriusSuillus granulatusLaccaria bicolor 
4MycoApply Endo GranularGlomus mossaeGlomus intraradicesGlomus aggregatumGlomus etunicatumClay 
5MycoApply All PurposeGlomus mossaeGlomus intraradicesGlomus aggregatumGlomus etunicatumRhizopogon villosullusRhizopogon luteolusRhizopogon amylopogonRhizopogon fulviglebaPisolithus tinctorius

Biofertilizers increased colonization by AMF

All five products tested increased the proportion of roots that were colonized by AMF (Fig. 2), although the Cabernet Sauvignon roots responded more strongly to the products than the 3309C roots. 

Fig 2. Significance differences were found among the treatments for AMF structures in cv. Cabernet Sauvignon and rootstock 3309C

Biofertilizers increased dry weight of vine organs

In general, the biofertilizers increased the dry weight of shoots, roots, and trunk in the vines (Fig. 3) likely as a result of increased nutrient content in the leaves (data not shown).  Most micro and macronutrients were increased in concentration in the treated vines. 

Fig 3. AMF increased shoot trunk and root biomass for all the treatments in cv. Cabernet Sauvignon and rootstock 3309C

Summary

All five of the products tested warranted further testing in the vineyard.  In a complimentary vineyard trial funded by the New York Farm Viability Institute, the products have also demonstrated their ability to form AMF in field-grown vines as well, although whether those AMF structures are increased long-term without repeated applications is unknown.

Acknowledgements

We thank the Pennsylvania Wine Marketing & Research Board for funding this research. 

Mid and Late-Season Grape Disease Control

By: Bryan Hed, Plant Pathology Research Technologist, Erie County

2018 was a disastrous season for many grape growers in Pennsylvania. Excessive rainfall occurred almost everywhere below Interstate 90 and some growers have told me it was their worst crop, ever.  Now, looking at various NEWA weather station locations across PA, it’s been shaping up to be another wet season in a lot of places, yet again. May rainfall was heavy in all but the Lake Erie region, with 6-9 inches of precipitation recorded across most of the state. Conditions lightened up a bit in June but were still wetter than average in most places (even in the Lake Erie region). But now, conditions in July actually appear to be drying up in a few (but not all) locations, giving some growers a break in terms of fungal disease management. 

Hopefully, most premium wine grape growers have already applied fruit-zone leaf removal to open their fruit to better sunlight and aeration and better pesticide penetration. The benefits of this practice cannot be overemphasized, and in our wet, humid climate, it is one of the most effective cultural treatments we know of for reducing the susceptibility of the crop to disease of all kinds (especially bunch/sour rots), and improving coverage, and therefore efficacy, of fruit protection sprays. If you haven’t yet applied this treatment, it is not too late, though the benefits of leaf removal may be reduced the later it is applied. There is also a greater danger of sunburn on your fruit the later it is applied, and for that reason you may want to confine your leaf removal at this time to the east or north side of the trellis (depending on row orientation), especially in areas where very hot mid/late summer temperatures are expected. 

And with that, let’s talk about diseases and their control for the remainder of the season. Much of this information has already been covered in previous blogs in previous years, and I have borrowed some information from those blogs here (no need to reinvent the wheel). 

Downy mildew

As you know all too well, wet years are ideal for downy mildew. At about this time, the fruit of most grape varieties are resistant to this disease, but cluster stems may remain susceptible for a couple of weeks after fruit are resistant, and leaves will remain susceptible all season. If the weather remains wet or wet weather returns, downy mildew can be a serious threat to grape canopies and ripening, until harvest. Continue scouting for the distinctive white ‘downy’ sporulation on the undersides of leaves. Growers of susceptible varieties need to keep closely monitoring their vineyards for active sporulation and use that information in combination with the DMCast model on NEWA (http://www.newa.cornell.edu/) to determine if and when infection periods occurred.

If you see active, white sporulation on the undersides of leaves, the downy mildew pathogen is capable of spreading quickly under wet conditions. Even humid nights that result in heavy dews by morning, can continue to fuel downy mildew development. Once out of control, it can strip vines of their leaves and effectively end fruit ripening for this year and shoot ripening for next year’s crop. It could also mean your grapevines will go into winter dormancy at less than optimal hardiness and more vulnerable to damage by severe cold, leading to another bout with crown gall and trunk renewal to have to deal with for years to come. All these issues are connected, and this is definitely a disease you want to keep under very tight control, especially on Vitis vinifera. 

If you find yourself trying to control this disease well into the ripening period, be aware that your list of chemical control options will start to dwindle as we get within 30 (Ranman, Reason), then 21 (Ziram), then 14 (Revus, Revus Top, Zampro) days of harvest, until in the end you’ll be left with Captan, copper, and phosphorous acid products (0 day pre-harvest interval), which have their shortcomings, discussed below.

Another reason to keep this disease well under control is that products like Ranman, Reason, Revus/Revus Top, and Zampro, all contain chemistries that are prone to the development of resistance, and should not be used to put down an epidemic, which will speed up the resistance development process. Even phosphorous acid products, which are less prone to resistance development, can be lost to resistance through repeated applications on a heavily diseased vineyard. I know this is probably the last thing on your mind when your vineyard is under an attack of epidemic proportions, but still another good reason to keep downy mildew well in hand. 

Conversely, Captan or copper fungicides would be least risky in terms of the development of resistance and can be an effective means of controlling downy mildew late into the growing season. Just be aware that formulations of Captan have seasonal limits, so plan ahead if you can. There are also some insecticides that should not be applied with Captan. Also, keep in mind the risk for injury by copper applications, and that copper injury will be exacerbated by application under slow drying conditions and application to wet canopies (for example, don’t make applications to dew covered canopies in the early morning). It’s also important to consider that copper is poisonous to yeasts and that excessive copper residues at harvest can interfere with fermentation, and wine stability and quality. Unfortunately, it’s impossible to predict how high residues will be on fruit at harvest; that’s going to depend on the copper formulation (fortunately some of the newer coppers utilize lower copper concentrations), rate of material used, spray coverage, and amount of rainfall from application to harvest. I am not aware of any information that establishes a nice, clean cut-off date or pre-harvest interval for avoiding excessive copper residues at harvest. There is also some evidence that late Captan sprays can cause problems in the winemaking process, in terms of delaying fermentation and negative effects on wine quality but the consequences seem less severe and irreversible.  For more on this, consider this online article by Dr. Annemiek Schilder, former fruit pathologist at Michigan State University.

If you are protecting a non-bearing, young vineyard from downy mildew (you’re not selling/harvesting a crop), you can continue to use mancozeb products to control downy mildew past the 66-day pre-harvest interval. You can also consider using mancozeb after harvest to keep canopies clean of downy mildew and ‘firing on all cylinders’ until that first frost. The longer your vines can continue to produce and store carbohydrates after harvest, the better prepared they’ll be to withstand winter cold. 

Fluffy, white downy mildew sporulation on the underside of a grape leaf

Powdery mildew

Good control of powdery mildew is also very challenging in wet years when humidity levels remain ‘through the roof’ and cloud cover occurs for extended periods of time. Now that we are largely past the fruit protection period, our focus is on keeping leaves clean, especially on V. vinifera, for about 6-8 more weeks. I say this for many of the same reasons expounded in the section about downy mildew (ensure optimal ripening of fruit and shoots/canes, ensure optimal cold hardiness, more effectively and more easily manage fungicide resistance, etc). But there is another very important reason, demonstrated by some excellent research conducted by Wayne Wilcox, Dave Gadoury and graduate students at Cornell University, who showed that controlling powdery mildew up to about Labor Day can also go a long way to reducing overwintering inoculum and disease pressure the following spring. Why Labor Day? When powdery mildew infected leaves die by that first hard frost in fall, the mildew on those leaves stops developing and also dies…unless it has had time to form fully mature, winter resistant structures called chasmothecia. In other words, if the chasmothecia in a powdery mildew colony on a leaf, do not have time to fully mature before the leaf dies, they will not be tough enough to survive the dormant period (winter) and will not contribute to the bank of primary inoculum that infection periods draw upon the following spring. Knowing this, a grower can get a better handle on the ‘size’ of the powdery mildew problems he/she will potentially face next spring and the spring after that, and so on. If, for example, you had heavy powdery mildew development earlier in this season (on clusters and/or leaves), expect to have to deal with powdery mildew early next season and you’ll have to take appropriate action during early shoot growth stages with preventive fungicide sprays. Once again, this is particularly important if you are growing Vitis vinifera and much less important for growers of native varieties like Concord and Niagara. 

Greyish-white colonies of powdery mildew growing across the upper surface of grape leaves

Botrytis bunch rot control

If you’re growing bunch rot susceptible wine grape varieties, you have already applied a Botrytis specific fungicide at full bloom and probably pre-closure (?) This is because Botrytis infections can occur during bloom and early fruit development under wet conditions (which most of us have had). These Botrytis infections of the clusters usually remain dormant, or ‘latent’, and do not result in active rot of the fruit…until after veraison, when injury to berries or high humidity, or some other factor (research has not completely determined all the factors involved) may lead to activation of a percentage of these infections and cause clusters to rot. 

In varieties with very compact clusters, the pre-closure application may be extremely important as it may be your last opportunity to get protective fungicide residues onto the interior surfaces of clusters. Along with the bloom spray, this spray will also help to reduce ‘latent’ Botrytis infections that continue to accumulate throughout the ‘green’ berry development period. The pre-closure spray may also be a good opportunity to clean clusters of bloom trash (dead cap and stamen tissue that got stuck in the clusters after bloom). Bloom trash provides a substrate for Botrytis and serves as a focal point for bunch rots to develop later in the season, from inside clusters. The compactness of clusters plays an important role in not only the retention of bloom trash (the tighter the cluster, the more bloom trash retained) but also the effect of retained bloom trash on cluster rot; as compactness increases, the enhancement of bunch rot by retained bloom trash increases. 

Another bunch rot control measure is leaf removal around clusters, which we’ve already discussed above. It is an expensive operation to add to your production costs and is most cost-effectively applied by machine (machinery costs aside). We have found that it can be mechanized most effectively if vines are trained to a vertical shoot positioned (VSP) or some other two-dimensional trellis system with a relatively focused and narrow cluster zone. One additional benefit of leaf removal that I haven’t mentioned yet is the fact that it can also reduce bloom trash retained in clusters: when comparing clusters of vines treated with and without leaf removal, we noted a significant reduction in bloom trash where leaves were removed, regardless of timing or method (by hand or machine). 

Our next fungicide application for Botrytis is made just before or at veraison. As fruit begin to soften and skins become thinner and more easily penetrated by fungal pathogens like Botrytis, an application at this time, to rot prone varieties, is a good way to stave off bunch rot development. After veraison, fruit also becomes more susceptible and more likely to become injured by birds, insects, excess moisture/humidity, and overcrowding of berries in tight clusters. Botrytis fungicides can protect intact fruit surfaces and may help to reduce the spread of Botrytis rot on fruit, even after they have become injured. 

Lastly, an application about 2-3 weeks after veraison, especially under wet weather conditions, can reduce further rot development during the last stretch of ripening. Keep in mind that Botrytis fungicides control Botrytis and will not provide protection against sour rot organisms that often destroy the fruit of overly compact clusters, despite the application of a full Botrytis fungicide program.

And speaking of sour rot…

In case you haven’t already heard, there is some relatively new information on sour rot control that I would like to impart. It’s been included in previous blogs as well and that information was presented earlier this year at the Mid Atlantic Fruit and Vegetable Convention in Hershey PA. However, it bears repeating it here. It originated from work conducted by Dr. Megan Hall, a former graduate student of Wayne Wilcox at Cornell University, and it demonstrates how additional pesticide applications during the latter stages of ripening (beginning around 15 brix) can significantly reduce the development of sour rot, which for many premium wine grape growers in PA, is public enemy no. 1 at harvest. Her incredibly thorough work has shown a close connection between fruit flies and sour rot development. It turns out that the presence of the flies is important to the accumulation/generation of acetic acid in rotting fruit. Treatments composed of weekly, tank-mix applications of an insecticide (to control the flies) and an antimicrobial (to kill bacteria) have been found to reduce sour rots by 50-80% over unsprayed vines. So far, the best results appear to occur when weekly sprays are initiated before sour rot symptoms are observed (preventive sprays before about 15 brix). This exciting work should provide yet another effective option for sour rot control in the wet, humid parts of the eastern U.S. and we are looking forward to hearing more about this rot control option in the near future.

Lastly, don’t forget how important good canopy and fertility management is to the efficacy of your expensive Botrytis fungicide and sour rot pesticide applications. It’s always a good idea to make sure your shoots are well tucked and spaced within the catch wires, and summer pruning has removed shoots ends that may block sprays from thoroughly penetrating the fruit zone, just before you make each Botrytis fungicide application. We like to wait as long as possible to trim shoot tips because of the effect on lateral growth stimulation, but make sure excessively long shoots have not flopped over to block spray penetration into the fruit zone. Limiting shoot growth after veraison with good canopy and fertility management will also limit the supply of new green tissue that is hyper susceptible to powdery and downy mildew and will contribute to more effective late-season management of these diseases as well.   

For further reading on this and many other disease management topics, refer to the 2019 New York and Pennsylvania Pest Management Guidelines for Grapes. If you don’t have a copy, you can get one through Cornell University Press. Every commercial grape production operation should have one! 

Early Season Update

Dr. Michela Centinari, Assistant Professor of Viticulture, Department of Plant Science

Another growing season has started for many Pennsylvania grape growers. Unfortunately, but not surprisingly, we are seeing and hearing of situations of vine winter injury across the State. This past winter, the lowest temperatures occurred at the end of January and during the first two days in February, with values around -5 °F (-20.6 °C) here in State College (central PA) and even lower temperatures were recorded at other locations.The injury seemed to have mainly affected Vitisviniferavarieties with reports of bud kill up to almost 100% for the most cold-sensitive varieties and, in some cases, trunk splitting.Growers also noticed uneven /nonuniform budburst which is typical of winter-injured vines.  We ask that more growers share their experiences with us; in particular, we would like to know if growers made any pruning adjustments and what the results are/have been. 

Since winter injury is a reoccurring issue for the eastern US, during certain years, we have covered topics related to vine cold hardiness, injury assessment, and pruning techniques for winter-injured vines at Extension meetings. Also, we have posted an announcement that focused on Pruning strategies for cold climate viticultureon the Penn State Viticulture and Enology Facebook page in January 2019, just before the “Arctic Vortex” event hit our region. Please do not hesitate to contact us if you have questions on how to manage cold-injured vines.

We heard from several PA growers in southern and central PA that budburst occurred earlier this year, a week to 10 days is what has been typically reported, than in 2018. This was also true for the hybrid varieties grown at the Penn State research farm at Rock Springs (central PA). I checked the growing degree days (GDD), a widely used index of heat accumulation, data calculated by the Network for Environment and Weather Applications (NEWA Cornell) for weather stations located in North East, Erie (northwestern PA), Biglerville (south-central PA), and Reading (southeast PA). Although historic data are not available, I compared the average GDD accumulated from January 1 to May 15 for 2013-2017 to those accumulated for the same period in 2018 and 2019 (Figures 1, 2 and 3).

Figure1. Cumulative growing degree days (GDD) at North East, Erie (northwestern PA)

Trends across locations/regions 

Not surprisingly, it was cooler in Erie compared to south-central and southeastern PA between January to Mid-May, not just in 2019 but for each year analyzed. In 2019, approximately 158 GDD accumulated between January 1 to May 15 in Erie, while GDD were at least double in south-central and southeast PA. Differences in temperatures across regions and locations explain why budburst typically occurs much earlier in southeast PA compared to the northwestern part of the state.

Difference between years 

In Erie, the GDD accumulated between January to mid-May 2019 (red line) were slightly lower than those for the same period in 2018 (blue line) and for the 2013-2017 average (black line). Also, note that there was no accumulation of GDD for a few days in May 2019 due to cool temperatures (Figure 1). The trend, however, was opposite in south-central and southeast PA, at least at the locations reported in this post. April was warmer (higher GDD) in 2019 compared to 2018 and the 2013-2017 average. While warmer spring temperatures favor earlier budburst they also increase the chance of freeze injury to green, tender plant tissues (Figure 4). 

Figure 4.Spring freeze (frost) injury on Concord, Rock Springs (PA)

At several locations across PA, temperatures were below freezing in the early morning of April 29 and some varieties were close to or already passed budburst. Below freezing temperature does not necessarily mean freeze injury as many factors affect the temperature at which the plant tissue is damaged or killed.  However, the freeze event on April 29 did cause freeze damage to vines at several locations, while others avoided the damage by using frost protection methods, such as frost dragons.  Some of the varieties grown at the Penn State research vineyard at Rock Springs, chiefly Marquette and young LaCrescent vines, sustained freeze injury. It is too early to estimate crop losses, but at least we are seeing some secondary shoot development (Figure 5).

How to recognize a secondary from a primary shoot

A relatively easy way, especially for caned pruned vines, is to check the angle of projection from the cane. Primary shoots typically grow with an angle of 45°, while secondary grow at an angle of 90° (figure 5).

Figure 5. Primary shoot damage after spring freeze event on April 29. Note a secondary shoot developing at 90° from the cane.

You can learn more about the basics of spring freeze injury and methods of protection at https://extension.psu.edu/understanding-and-preventing-spring-frost-and-freeze-damage

Looking ahead

It is almost time for some early season canopy management practice. Please check the following articles if you need information on shoot thinning or early leaf removal: 

Early season grapevine canopy management, Part I: Shoot thinning

Early season grapevine canopy management, Part II: Early leaf removal 

2019 PA Wine Marketing & Research Board Symposium: Presentation Summaries

On March 5, 2019, Penn State researchers and Extension personnel presented research findings and provided five-minute overviews of upcoming studies at the 2019 Wine Marketing & Research Board Symposium, held in conjunction with the Pennsylvania Winery Association Annual Conference.

In this post, we have included short summaries of what each presenter discussed during their session along with a PDF/access to their presentation.

Research presentations

Under-vine cover crops: Can they mitigate vine vigor and control weeds while maintaining vine productivity?

Presented by Michela Centinari, Assistant Professor of Viticulture, Suzanne Fleishman, Ph.D. Candidate, and Kathy Kelley, Professor of Horticultural Marketing and Business Management 

Michela, Suzanne, and Kathy discussed research conducted at Penn State related to the use of under-vine cover crops as a management practice alternative to herbicide or soil cultivation. Michela reviewed potential benefits of under-vine cover crops, such as reduction of excessive vegetative growth, weed suppression, and reduced soil erosion. She showed how the selection of cover crop species depends on the production goals of a vineyard, climate, vine age, and rootstock. Suzanne presented results from her research project. She is investigating above- and belowground effects of competition between a red fescue cover crop and Noiret grapevines, comparing responses between vines grafted to 101-14 Mgt vs Riparia rootstocks. Surveys will be administered to Pennsylvania grape growers and wine consumers in the Mid-Atlantic region. Growers will be asked to respond to questions about interest in using cover crops and benefits that could encourage their use. The consumer survey will focus on learning whether cover crops use would impact their purchasing decision and if they would be willing to pay a price premium for a bottle of wine to offset additional production costs.

Presentation PDF file

Impact of two frost avoidance strategies that delay budburst on grape productivity, chemical and sensory wine quality.

Presented by Michela Centinari, Assistant professor of Viticulture 

Crop losses and delays in fruit ripening caused by spring freeze damage represent an enormous challenge for wine grape producers around the world. This multi-year study aims to compare the effectiveness of two frost avoidance strategy (application of a food grade vegetable oil-based adjuvant and delayed winter pruning) on delaying the onset of budburst, thus reducing the risk of spring freeze damage. Our objectives are to: i) evaluate if the delay in budburst impacts grape production and fruit maturity at harvest, as well as chemical and sensory wine properties; ii) elucidate the mechanism of action of the vegetable oil-based adjuvant through an examination of bud respiration and potential phytotoxic effects; and iii) assess the impact of the two frost avoidance strategies on carbohydrate reserve storage and bud freeze tolerance during the dormant season. 

Presentation PDF file

Toward the development of a varietal plan for Pennsylvania wine grape growers

Presented by Claudia Schmidt, Assistant Professor of Agricultural Economics, and Michela Centinari, Assistant Professor of Viticulture

Claudia Schmidt is a new Assistant Professor of Agricultural Economics with an extension appointment at Penn State. Claudia used the opportunity of the symposium to introduce herself to the industry. In her presentation, she first gave an overview on what and where Pennsylvanians buy their wines and spirits. She then talked about the research needed to develop a varietal plan for the Pennsylvania grape and wine industry to match existing and future grape production and variety suitability with anticipated consumer demand. The immediate next steps on her research agenda are to develop a  baseline survey of grape production in Pennsylvania and, in collaboration with Michela Centinari, region specific cost of production of grapes.

Presentation PDF file

Survey for grapevine leafroll viruses in Pennsylvania: How common is it, and how is it effecting production and quality?

Presented by Bryan Hed, Research Technologist

This is a continuing project funded by the PA Wine Marketing and Research Board, that has focused on the determination of the incidence of grapevine leafroll associated virus 1 and 3 (the two most economically important and widely distributed of the leafroll viruses) in commercial vineyard blocks of Cabernet franc, Pinot noir, Chardonnay, Riesling, and Chambourcin, across the Commonwealth. Over two years, the survey has shown that grapevine leafroll associated viruses 1 and/or 3, were present in about a third of the vineyard blocks examined. Infection of grapevines by grapevine leafroll-associated viruses can have serious consequences on yield, vigor, cold hardiness, and most notably fruit/wine quality. Bryan also discussed a second phase of the project, anticipated to continue for at least another two years within 6 vineyard blocks of Cabernet franc, identified in the survey. In these vineyards, we plan to plot the spread of these viruses, examine and report their effects on grapevine vegetative growth, yield, and fruit chemistry, and characterize the influence of inter- and intra-seasonal weather conditions on virus-infected grapevine performance.

Presentation PDF file

Integrating the new pest, spotted lanternfly, to your grape pest management program.

Presented by Heather Leach, Extension Associate

Spotted lanternfly (SLF) is a new invasive planthopper in the Northeast U.S. that threatens grape production. Heather covered the basic biology, identification, and current distribution of SLF. She also presented on the economic impact of SLF in the grape industry and ways to manage SLF in your vineyard. SLF can feed heavily on vines causing sap depletion in the fall which has resulted in death of vines, or failure of vines to set fruit in the following year. While biological controls such as pathogens and natural enemies along with trapping and behaviorally based methods are being researched, our current management strategy relies on using insecticides sprayed in the vineyard. Heather showed results from the 2018 insecticide trials conducted against SLF, with efficacy from several products including bifenthrin, dinotefuran, thiamethoxam, carbaryl, and zeta-cypermethrin. You can read more about the results from this trial here: https://extension.psu.edu/updated-insecticide-recommendations-for-spotted-lanternfly-on-grape

Presentation PDF file

Five-minute research project overviews

Impact of spotted lanternfly on Pennsylvania wine quality.

Presented by Molly Kelly, Extension Enologist 

The Spotted Lanternfly (SLF) presents a severe problem both due to direct damage to grapevines as well as their potential to impact wine quality. Insects are known to produce or sequester toxic alkaloid compounds. The objectives of this study include characterizing the chemical compounds in SLF and production of  wines with varying degrees of SLF infestation. We can then provide winegrowers with recommendations for production of wine from infested fruit. Toxicity studies will be conducted to determine the levels of toxic compounds in finished wine, if any, using a mouse bioassay.

Presentation PDF file

Exploring the microbial populations and wild yeast diversity in a Chambourcin wine model system

Presented by Chun Tang Feng, M.S. Candidate, and Josephine Wee, Assistant Professor of Food Science  

In Dr. Josephine Wee’s lab, we are interested in the microbial population and diversity associated with winemaking. When it comes to wine fermentation, not only are commercial yeasts involved in this process, but also many indigenous yeasts. Our research goal is to isolate the wild yeasts and assess their feasibility of wine fermentation. We are expecting to explore the unique yeast strains from local PA which are able to make a positive impact on wine flavor. 

Prezi Presentation 

Rotundone as a potential impact compound for Pennsylvania wines

Presented by Jessica Gaby, Post-Doctoral Scholar and John Hayes, Associate Professor of Food Science  

This study will examine Pennsylvania consumers’ perceptions of rotundone with the goal of determining whether a rotundone-heavy wine would do well on the local market.  This will be examined from several different perspectives, including sensory testing of rotundone olfactory thresholds, liking and rejection thresholds for rotundone in red wine, and PA consumer focus groups.  The ultimate aim of the study is to determine the ideal concentration of rotundone in a locally-produced wine that would appeal to PA consumers.

Presentation PDF file

Defining regional typicity of Grüner Veltliner wines

Presented by Stephanie Keller, M.S. Candidate, Michela Centinari, Assistant Professor of Viticulture, and Kathy Kelley,  

Grüner Veltliner(GV) is a relatively new grape variety to Pennsylvania, and while climatic conditions are favorable to its growth, the Pennsylvania wine industry is still becoming familiar with the varietal characteristics of GV grown and produced throughout the state.  This study focuses on defining typicity of Pennsylvania-grown GV wines.  Typicity is described as the perceived representativeness of a wine produced from a designated area, and defining typicity can improve wine marketing strategies.  This study uses multiple experimental sites across the state to create wines from a standardized vinification method.  The wines will be analyzed using both instrumental and human sensory methods.Surveys will be administered to Pennsylvania grape growers and white wine consumers in the Mid-Atlantic region.  Growers will be asked their interest in growing GV and what perceived and real barriers may impact their decision to grow the variety.  The consumer survey will focus on understating how to introduce them to a wine varietal they may be less aware of and what promotional methods may encourage them to purchase the wine. 

Presentation PDF file

Boosting polyfunctional thiols and other aroma compounds in white hybrid wines through foliar nitrogen and sulfur application?

Presented by Ryan Elias, Associate Professor of Food Science, Helene Hopfer, Assistant Professor of Food Science, Molly Kelly, Extension Enologist, and Michela Centinari, Assistant Professor of Viticulture

The quality of aromatic white wines is heavily influenced by the presence of low molecular weight, volatile compounds that often have exceedingly low aroma threshold values. Polyfunctional varietal thiols are an important category of these compounds. This project aims to provide research-based viticultural practices that could lead to increases in beneficial varietal thiols in white hybrid grapes. The expected increase in overall wine quality will be validated both by measuring the concentrations of these desirable compounds (i.e., thiols) in finished wines using instrumental analysis and by human sensory evaluation, thus providing a link between the viticultural practice of foliar spraying and the improvement of overall wine quality.  

Presentation PDF file

2018 Growing Season Recap

By: Bryan Hed, Plant Pathology Research Technologist, Erie County and Dr. Michela Centinari, Assistant Professor of Viticulture, Department of Plant Science

This past growing and harvest season has been, accordingly to many growers, one of the most challenging ever not only in Pennsylvania but in many other eastern US regions. With the 2018 season behind us, we can reflect on what we did right and what we can improve to better manage, when possible, vines under the rainfall conditions experienced in many parts of the Commonwealth. In this article, we will mainly discuss disease and vine vigor/nutrition issues related to seasonal weather conditions. Other issues growers experienced, such as Spotted Lanternfly infestations will be addressed in future blog posts. 

What was the major problem? Let’s start with the rain

In Figures 1 and 2, we reported monthly, seasonal (April 1 through October 31) precipitation and growing degree days (GDD; index of heat accumulation) collected by weather stations through the online network for environment and weather applications (http://newa.cornell.edu/) at two locations: Lake Erie Regional Grape Research and Extension Center (LERGREC) in North East (Erie County, northwestern PA) and in Reading (Berks County, southeast PA). We compared the 2018 data to the previous 19-year (1999-2017) average. 

If you look at the monthly rainfall (Figure 1; Table1) throughout the growing season, it was as if Pennsylvania was divided into two regions during July, August, and September: the southern shore of Lake Erie, and the rest of the state. The Erie lakeshore was, indeed, relatively dry as compared to the rest of the state: rainfall from April to October was only about 4 inches higher than the long-term average. However, in other parts of the state rainfall was as much as 14 inches higher than average(Figure 1A: 2018 = 43.14 inches versus 1999-2017 = 28.68 inches). Berks County in southeastern PA started out with slightly above average rainfall for April and May, followed by a slightly drier than average June, but rainfall greatly increased in the second half of the season (Figure 1A). This happened not only in Berks County, but in many regions of the Commonwealth which recorded much higher than average rainfall in July, August, and September (Table 1). 

Disease pressure

Diseases that depended for development on regular rainfall, like black rot and downy mildew, were relatively easy to control for vineyards along the Lake Erie shore. Powdery mildew was in moderate supply; not light but certainly not of hardcore, epidemic proportions. To complete the picture, we did suffer more than a little from sour rot in some of our wine grapes due to the heat and return of rains in September (5 inches). We also suffered a fair amount of fruit cracking and damage from grape berry moth near harvest that led to some serious shelling and crop loss in many area vineyards. And then, on October 11, it all came to an end. Autumn, which was technically just beginning, was being ‘run out of town on a rail’; the weather suddenly took an entirely different turn and the sun and mild weather disappeared, never looking back. 

In stark contrast, other parts of the state were dealing with way too much rain that created perfect conditions for the development of downy mildew and late-season bunch rots. Fortunately, from the rainfall data gathered from NEWA weather stations, it appears that rainfall in the early post-bloom period (second half of June – first half of July) was relatively average, with about 2.75 inches during that four-week period. This period is critical for fruit protection when the fruit of all grape varieties is most susceptible to all the major fungal diseases. However, by mid-July rainfall ramped up, and was especially abundant during the fruit ripening period; avoiding fruit rots was nearly impossible under those extremely wet conditions.

Other issues

In addition to high disease pressure, wet conditions led to high vegetative growth and high to excessive uptake of nutrients such as potassium (K). In addition to the timely application of canopy management practices to keep vegetative growth under control and maintain an open fruiting zone, the planting of cover crops under the vines could help limit vine vegetative growth through water and nutrient competition (For more information please refer to: Why should we care about under-trellis cover crops?. Our extension team reviewed several plant tissue analysis reports from vineyards across the state and many of them had high, and in several cases excessive, leaf petiole K concentrations. For more information on K and how to manage it in the vineyard please refer to Assessing and managing potassium concentration in the vineyard

What about heat accumulation?

The 2018 growing season in the Lake Erie region will be remembered as a hot season.  Growing degree days accumulated from May 1 to September 30 were almost 3,000 at the LERGREC located along thesouthern shore of Lake Erie (Figure 2B). In contrast, one of the coldest seasons in the last 20 years was 2003 with 2180 GDD, 800 GDD lower than 2018! In 2018 it almost seemed everything happened too fast. Concord grapes at the LERGREC went from 50% bud break to harvest in less than five months, while the growing season for Vignoles (Vitishybrid) was less than 4 months long. 

Heat accumulation was close to long-term average in Berks county (Figure 2A) and other PA regions, but with extended overcast conditions (many cloudy days!) throughout the season which might lead to moderate/low sugar accumulation in the fruit.  Additionally, the overcast conditions contribute to downy mildew, black rot, and other fruit rots. 

Tips for next season disease management

It is important to keep detailed records of where diseases were worst; those are the areas likely to develop disease first next year. Be sure to effectively scout those areas of the vineyard next season. For example, for downy mildew, that means beginning scouting by mid to late May. The downy mildew pathogen spends the winter inside infected grape tissue, especially leaves, that fall to the vineyard soil. The first downy mildew infections can occur during rainfall (at least 0.1 inches of rain and 50 °F) a few weeks prior to bloom, when vines have developed about 5-6 leaves per shoot. 

We have several very effective downy mildew fungicides, but it is important to understand the pros and cons of each one. The old standards like mancozeb (Penncozeb, Manzate, Dithane, etc) and copper formulations are effective against downy mildew, and are great for multiple, back to back applications because they pose little risk in terms of the development of resistance, but they are not as rain-fast as some of the more modern downy mildew materials like Revus, Ridomil, and Zampro, and may need to be reapplied more often under heavy and frequent rainfall conditions. And of course, with copper, there is a risk of vine injury, that is exacerbated under wet, slow drying conditions. Copper residues from late-season applications can also interfere with fermentation. On the other hand, the more rain-fast, more modern fungicides should not be used more than two or three times per season, and even though the label may permit it, we recommend you don’t make back-to-back applications of the same chemistry, among these modern materials. Also, I purposely left out mention of the strobilurins for downy mildew control (Abound, Pristine, Reason), especially for the more intensively managed wine grape areas of southern PA; downy mildew resistance to this chemistry (FRAC 11) is common and this class of fungicides should probably not be relied upon anymore for control of this disease in many parts of Pennsylvania. And then there are phosphorus acid products which have become very popular for downy mildew control. But these materials can be overused as well. They certainly are very rain-fast and effective, but they can be lost to resistance (limit their use to two or three applications per season) and they only provide about 7-10 days of protection at each application, especially under heavy disease pressure on susceptible varieties. For more information on downy mildew control please refer to  Tips for late season downy mildew control 

There are cultural measures you can take to help reduce the overwintering population of pathogens. These measures are not substitutes for a solid seasonal spray program, and they all have their price, but they can make your spray program more effective. The downy mildew and black rot pathogens predominantly overwinter on the soil surface. Strict control of grape seedlings and suckers under the row in spring can reduce opportunities for these pathogens to create ‘stepping stones’ from the soil into your canopies. However, this practice needs to be balanced with the need for renewals where crown gall and the threat of winter trunk damage are perennial issues. During dormant pruning, remove all clustersnot harvested and as much diseased/dead/old wood from the trellis as is practical. Throw this material into the row middle and chop it, or better yet remove it from the vineyard and burn it (if practical). This is especially effective against Phomopsis and black rot. Upright training systems (like vertical shoot position) reduce the probability that pathogen spores will be splashed upward from cordons and trunk, into the fruit zone during rain.

A wet season like 2018 could be the start of additional disease issues heretofore not yet encountered in prior years. For example, a disease called ripe rot(Colletotrichum sp.) may have gotten a fresh foothold in some vineyards in Pennsylvania in 2018. Ripe rot is somewhat of a ‘southern’ disease, it mainly occurs in southern PA vineyards, but it was also noticed in a vineyard in central Pennsylvania in 2018 (Figure 3).

Ripe rot is identified during the ripening period by pink or orange colored slimy spore masses that appear on infected fruit after a wetting period (Figure 3, left panel). 

Since downy mildew and late season fruit rot management was a major challenge for many growers in 2018, Grape Disease Management in Wet Seasonswill be discussed in more detail at the Mid Atlantic Fruit and Vegetable Convention in Hershey, PA on January 30, and again at our annual Grape Disease and Insect Management workshop on March 28. We hope to see you there.