Dormancy in Grapes

By Dr. Mike Campbell, Director, Lake Erie Regional Grape Research and Extension Center

As winter sets in plant growth in the vineyard stops. The onset of short days and cooler temperatures results in a state of no growth and reduced metabolic activity termed dormancy. The apparent dormant state of perennial plants is composed of multiple stages that have major significance to the plant’s ability to withstand environmental stress, particularly the cold of winter in Pennsylvania, or the drought found in deserts or Mediterranean climates. Grapes, as a perennial plant, enter a developmental state in the fall called ecodormancy. Ecodormancy is brought on by shorter days and cooler temperatures and is characterized by reduced growth that can be reversed by altering growing conditions. Ecodormant vines slowly transition to a state of endodormancy through a process called acclimation (Figure 1).  

Figure 1: The stages of dormancy in a perennial plant. The process of acclimation involves shorter days and gradually cooling temperatures. Deacclimation results from reaching the time and cool temperatures to meet the chilling requirement. 

Endodormancy, sometimes referred to as deep dormancy, is a state where there is an internal suppression of growth. This internal suppression includes an increase in the plant hormone abscisic acid, an inducer of dormancy, as well as cryoprotectants that provide cold hardiness. Once placed into endodormancy exposing the vines to optimal conditions of temperature and light will not result in growth. If autumn weather involves gradual cooling conditions, and the vines are in excellent health, the plants will reach a maximum depth of endodormancy and a state of maximum cold hardiness. The depth of endodormancy is genetically determined and different varieties of grapes reach a specific maximum cold hardiness.  This is one reason why certain varieties of grape, particularly varieties of Vitis vinifera L., have limited use in Pennsylvania and the Lake Erie region; the winters reach a cold temperature that is below the threshold of endodormancy where freeze damage occurs (Figure 2). 

Figure 2: Comparison of the maximum cold hardiness between chardonnay and Concord grapes illustrating the impact of deep dormancy on freeze tolerance.    

Once a vine enters endodormancy a combination of time and cold temperatures (chilling hours) is required to remove the internal mechanism that prevents growth. The chilling requirement is not necessarily temperatures below freezing, and winters in Pennsylvania usually provide more than enough chilling hours to terminate endodormancy in grapes. In very warm climates, where chilling hours are not met for termination of endodormancy, application of chemicals such as hydrogen cyanamide are used to initiate bud growth. In the northern United States, including Pennsylvania, chilling requirements to break endodormancy, and initiate growth if warm weather occurs, are often met by mid-winter.  This creates a challenge for growers. Once endodormancy terminates vines are in a condition of ecodormancy, which is characterized by a condition where growth is largely suppressed by environmental conditions such as air and soil temperature. Ecodormancy vines also begin to lose cryoprotectants found in the plant tissues resulting in a reduced level of cold hardiness. (Figure 3). Thus, if endodormancy is terminated early in winter, and that termination is followed by a spell of warm weather, vines will begin to grow. Bud break in vines leaves them susceptible to freeze damage. This means that cold damage to grape vines can occur if the temperature falls below the maximum for cold hardiness in endodormancy but also at higher temperatures when vines have left endodormancy and lost cryoprotectants.

Figure 3: Lack of chilling requirements can result in prolonged endodormancy and delayed bud break (A). If chilling requirements are met warm winter weather can result in premature termination of endodormancy and early bud break increasing the risk of freeze damange in early spring (B).

Changes in climate have implications to the process of dormancy in grapes. The concept of a warmer climate suggests that maximum cold temperature will be on average higher. While it would take a significant amount of global warming to result in a climate in Pennsylvania where chilling requirements are not met, as in the warmer Mediterranean regions where grapes are grown now, there other challenges that climate change presents to growers. Higher temperatures bode well for grape varieties that have a higher sensitivity to cold damage in endodormancy. Increasing average winter temperatures through climate change may mean the ability to successfully grow more varieties sensitive to cold in Pennsylvania. However, there is another more insidious downside to climate change and that is the impact of changes on chilling requirement. A warming climate will also result in an increase in warm spells mid-winter, which will result in earlier termination of endodormancy, increasing risks of vines damage from late winter and spring frosts. Growers can expect new challenges as climate change impacts the dormancy cycle in grape varieties growing in our region. 

Citations

Davenport J, Keller M, Mils L. 2008. How cold can you go? Frost and winter protection for Grape. HortScience 43:1966-1969. 

Ferguson J, Moyer M, Mills L, Hoogenbom G, Keller M. 2014. Modeling dormant bud cold hardiness and budbreak in 23 Vitis genotypes reveals variation by region of origin. Am. J. Enol. Vit. 65:59-71. 

Horvath D, Anderson J, Chao W, Foley M. 2003. Knowing when to grow: signals regulating bud dormancy. Trend in Plant Science 8:534-540. 

Kalberer S, Wisniewski M, Arora R. 2006 Deacclimation and reacclimation of cold-hardy plants: Current understanding and emerging concepts. Plant Science 171(1)3-16. 

Or E, Vilozny I, Eyal Y, Ogrodovith A. 2000. The transduction of the signal for grape bud dormancy breaking induced by hydrogen cyanamide may involve the SNF-like protein kinase GDBRPK. Plant Molec Biol 43(4):483-494. 

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

Non-Saccharomyces Yeast in Winemaking

By Dr. Molly Kelly, Enology Extension Educator, Department of Food Science

As harvest begins you may want to consider utilizing non-Saccharomyces yeast strains. These strains have been shown to improve aromas, lend complexity and improve mouthfeel. In this post, we will provide an overview of their use and provide information concerning commercial strains available.

The Use of non-Saccharomyces Yeast in Winemaking 

The use of S. cerevisiae as a starter culture is the most widespread practice in winemaking. There is, however, an interest in conducting uninoculated fermentations. An uninoculated fermentation is often referred to as a “spontaneous” or “native” fermentation involving the sequential action of various non-Saccharomyces and indigenous Saccharomyces yeasts. The use of “mixed starter” of non-Saccharomyces with strains of Saccharomyces cerevisiae provides an alternative to both “spontaneous” as well as inoculated fermentations. The possible benefits include added complexity, enhanced ability to secrete enzymes important in production of aroma compounds and a fuller palate structure. 

The ability of non-Saccharomyces yeasts to produce wines with distinct flavor profiles has increased interest in the use of such yeasts in mixed starters. In addition, research demonstrating the ability of non-Saccharomyces yeasts to lower alcohol content of wines, control wine spoilage and improve additional wine properties have been reported. 

Grape musts naturally contain a mixture of yeast species and therefore fermentation is not a “single species” fermentation. During crush, the non-Saccharomyces yeasts on the grapes, cellar equipment and in the environment may come in contact with the must. Cellar surfaces play a smaller role than grapes as a source of non-Saccharomyces yeasts. 

It is believed that a selected and inoculated strain of S. cerevisiae will suppress any “indigenous” non- Saccharomyces species and dominate the fermentation process. However, several studies have shown that non-Saccharomyces yeasts can persist during fermentations inoculated with pure cultures of S. cerevisiae

Non-Saccharomyces yeasts are thought to be sensitive to sulfur dioxide (SO2), poor fermenters of grape must and intolerant to ethanol. It is generally accepted that non- Saccharomyces yeasts, not initially inhibited by SO2, would not survive during fermentation due to combined toxicity of SO2 and alcohol. However, research has shown high numbers (106-108 cells/ml) and persistence of non-Saccharomyces yeasts in some wine fermentations, suggesting their potential role in winemaking. 

There are two general practices of inoculation when using non-Saccharomyces yeasts in mixed starters. The first, co-inoculation, involves the inoculation of the selected non-Saccharomyces yeasts at high cell concentration along with S. cerevisiae. The second practice, sequential inoculation, allows initial inoculation of selected non-Saccharomycesyeasts at high levels which are allowed to ferment on their own for a given amount of time before S. cerevisiae is added to complete the fermentation. Although both viable, potential interactions between yeast could determine which strategy is more appropriate. Today, there are available many non-Saccharomyces strains compatible with Saccharomyces strains for the improvement of wine primary aroma. See below.

 The most frequently studied species include: Torulaspora delbrueckiiMetschnikowia pulcherrimaCandida zemplininaHanseniaspora species and Lachancea thermotolerans). 

These yeasts are usually poor fermenters, therefore S. cerevisiae (either indigenous or inoculated) is necessary to take the fermentation to completion. Typically, non-Saccharomyces yeasts have been used in sequential fermentation where they grow or ferment prior to inoculation with S. cerevisiae.

Why use non-Saccharomyces yeast? 

What do these “native” yeasts have to offer winemakers and how can they be utilized in a planned and controlled manner to produce desired wine styles? Recently, consumer and market demand for lower ethanol wines has driven some research to develop various approaches to produce these wines.  Several studies have reported lower ethanol yields when using non-Saccharomyces yeast. In addition, some non-Saccharomyces yeast can utilize sugars with the production of desirable esters and other flavor/aroma compounds with the added advantage of only minimal production of ethanol. 

The variety of flavor/aroma compounds produced by different non-Saccharomyces yeasts is well documented. The compounds produced by different Saccharomyces yeasts include: terpenoids, esters, higher alcohols, glycerol, acetaldehyde, acetic acid and succinic acid. Wine color may also be affected by non-Saccharomyces yeast. Conversely, the improper use of non-Saccharomyces yeasts may result in serious fermentation issues including: stuck/sluggish fermentation, high levels of acetic acid and ethyl acetate, as well as lack of reproducibility etc. The goal of the winemaker is to emphasize the positive impact of non-Saccharomyces wine yeasts while minimizing its negative impact. 

Conclusion 

There are potential benefits of the use of non-Saccharomyces yeast in wine production; the abundance of grape yeast biodiversity presents many opportunities to explore their use. Strain selection is of key importance, as not all strains within a species will necessarily show the same desirable characteristics. The goals of many researchers have included: efficient sugar utilization, enhanced production of volatile esters, enhanced liberation of grape terpenoids to improve wine flavor and other sensory properties. These goals can be met by selected non-Saccharomyces wine yeast and their proper use in the winery. 

Please see links below to information summarized by PSU student Tyler Chandross-Cohen on commercial non-Saccharomyces stains.

Please contact me or your Scott labs representative with any questions. 

BIODIVA YEAST: This Yeast was initially sold in a pre-blended kit, partnered with a specific S. cerevisiae strain, but now is isolated for winemakers who can match it with a compatible S. cerevisiae of their choosing for both red and white wines. This isolated yeast makes developing wine more customizable. After creating your own blend, the resulting wines will have more intense aromas, mouthfeel and complexity. The S. cerevisiae strains compatible with Biodiva are 43, BDX, ICV D254, L2056, QA23, and VRB.

 FLAVIA YEAST: This yeast is a pure culture of Metschnikowia pulcherrima, which is selected for its ability to produce aroma and flavor revealing enzymes. Flavia is best used with creating aromatic whites and rosés. Flavia will enhance the aroma and flavor profiles of wines optimizing varietal characteristics while bringing freshness and volume in the mouth. 

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

A Visit at the 17th Australian Wine Industry Technical Conference

By Dr. Helene Hopfer, Assistant Professor of Food Science, Department of Food Science

In late July of 2019, I was fortunate to be able to participate at the 17th AWITC in Adelaide, Australia. I was invited to speak about our sensory regionality study on commercial Riesling and Vidal blanc wines from Pennsylvania. 

https://twitter.com/The_AWITC/status/1153522535956283392

Last year, Dr. Kathy Kelley wrote about her sabbatical leave in Australia, and provided an excellent overview into Australia’s wine industry, therefore, this blog post will focus on the presentations and posters at the conference. 

TheAustralian Wine Industry Technical Conference & Exhibition (AWITC)is happening every three years, organized by The Australian Wine Research Institute (AWRI)and the Australian Society of Viticulture & Oenology (ASVO). Combining plenary sessions, workshops, poster presentations and a large trade exhibition, the AWITC attracts a large audience (over 1,200 participants this year) primarily from the Australian wine industry. Over 4 days, every aspect of grape growing and wine making, from vineyard to grape vine to enology and wine consumers is covered, providing scientific stimulation and lots of discussion for the industry. Intended to present the latest research findings while at the same time being approachable and transferrable for industry members, the AWITC hosts a wide variety of speakers (academics, industry members, governmental speakers, as well as forward-thinking leaders from other industries). Proceedings and video webcasts of all talks will be made available online on the website, where also all past proceedings are made public. Lots of participants also live-tweeted from the conference, so many impressions can also be found on the official event twitter handle @The_AWITC.

The conference started out with a traditional welcome by a local Aboriginal leader from the Adelaide Plains people. Providing a Welcome to his people’s land and an invitation to learn and work collaboratively, his inspiring speech was a great kick-off to the event, followed by the official opening by the Australian Minister for Primary Industries and Regional Development. 

https://twitter.com/The_AWITC/status/1153080623080173568

In the first two sessions, the supply and demand for Australian wine and its future were evaluated. Following the official outlook from Wine Australia, Warren Randall provided a thought-provoking talk on China very soon becoming the number 1 wine-consuming nation in the world. Although individual wine consumption for Chinese is estimated to reach 1.6 L per person per year (compare to US consumers averaging to 3.1 L per person per year), the sheer number of Chinese middle-class consumers leads to an estimated additional need of 850 million L within the next 5 years. This additional need equates to 1.2 m tones of grape, about 71% of Australians total annual production! The Chinese will remain to be a net importer, particularly for quality wine – the question is though whether Australia will be able to satisfy this demand, especially with the severe drought many Australian grape-growing regions face.

https://twitter.com/RetallackViti/status/1153099001769496576 

The subsequent talks reiterated the importance of China as a major Australian wine importer as well as for Australian wine tourism: Brent Hill from the South Australian Tourism Commission presented compelling research showing that wine tourism improves brand recall and sales, independent of winery size. For example, international marketing campaigns in combination with direct flights to Adelaide led to tripling visits from China to wineries in South Australia. Wine tourism also aligns nicely with consumers’ demands for personalized products that align with their values. Health and Well-being are driving consumer preferences and will continue to do so, as presented by Shane Tremble from the Endeavor Drinks Group, a major alcoholic beverage retailer in Australia. 

The afternoon session was dedicated to diversity, equity, and inclusivity in the wine industry. Our own unconscious biases create barriers to enter the wine industry, especially for talents from underrepresented groups. Diversity, equity, and inclusivity is not just about social justice, but is a real business loss, especially as the wine consumer base becomes more and more diverse. How can we make sure to meet the needs of our consumers if we don’t really understand them and their needs?  

https://twitter.com/The_AWITC/status/1153159464062611456

A large portion of the meeting was dedicated to different aspects of climate change and how the wine industry will be able to continue doing business. A representative from a major insurance company presented on her company’s strategy to climate change, and managing risks associated with a changing climate – from special loans for businesses to lower their carbon footprint and greenhouse emissions to ways to manage physical risks such as flooding and bush fires, this presentation was eye-opening. Tools already available for growers, such as high-resolution weather data, provide action-able data for e.g., harvesting or irrigation. Clonal selection, vine training systems and better suited varieties and rootstocks are another tool in the toolbox to adapt to climate change, particularly to higher temperatures and increased incidences of drought, as demonstrated by Dr. Cornelis van Leeuwen from Bordeaux Sciences Agro.  

https://twitter.com/The_AWITC/status/1153198270383980544

Ending with the conference’s gala dinner, this first day proved to be full of insights and what the future may bring. 

The next day started off with the fresh science session, including research on how changing climate will also change insect and disease pressure: Using the example of sooty mold and scale insects, Dr. Paul Cooper presented data and models that show how warmer temperatures will influence occurrence of scale insects and subsequent sooty mold. Similar scenarios could become more prevalent in PA as well, as for example late harvest insect problems could appear at an earlier stage during berry ripening (see also this blog post by Jody Timmer).

On the enology-side, several presentations were given to look at smoke-taint remediation of wines, alternatives to bentonite fining with grape seed powder, and the mechanisms underlying autolytic flavors in sparkling wines. A particular interesting, but also terrifying talk was given by Caroline Bartel from the AWRI on increasing SO2tolerance of Brettanomyces bruxellensis strains: Over the past 3 years, the AWRI has seen an increased number of Brettanomyces strains that show greater tolerance to SO2, some exceeding 1 mg/L molecular SO2

https://twitter.com/HeleneH14/status/1153471125172572160

Biosecurity is a big topic for Australian grape growers, as almost all vineyards are own-rooted, including some of the oldest productive vineyards in the world being over 100 years old! This history is however under threat, as phylloxera has arrived in Victoria and New South Walesa few years ago. Managing the biosecurity threats and best practices to protect vineyards from not just phylloxera but also grapevine viruses was the overarching theme of this session. Showing data from the Napa Valley, Dr. Monica Cooper from UC Extension highlighted the importance of clean plant material when it comes to managing grape vine diseases: in a newly planted vineyard, not enough certified disease-free material was available, and hastily organized vines, infected with red blotch virus, were planted alongside healthy vines. Within a few years, 100% of infected vines had to be removed to avoid spreading of the disease into other parts of the vineyard and adjacent vineyards. 

https://twitter.com/The_AWITC/status/1153481714116657152

The last talk in the session was given by Dr. Antonio R. Grace from the Portuguese Association for Grapevine Diversity, who argues that clonal selection of grapevines may increase efficiency but decreases resilience, complexity, and diversity.

A particular interesting session was focusing on Agricultural Technology or AgTech – robots, drones, and intelligent robot swarms! A particular impressive and eye-opening talk was given by Andrew Bate from SwarmFarm, a farmer in Queensland who now develops and sells farming robots that oppose the trend for “bigger is better”: using a swarming approach (i.e., many smaller robots that operate autonomously for maximized efficiency and adaptability), he showcased how his approach is forward-thinking and sustainable, and fueled by his own experiences as a farmer and grower. If you can check out the videos on the website! 

https://twitter.com/The_AWITC/status/1153561941664604163

In a similar inspiring manner, Everard Edwards from the CSIRO presented on low-cost drones and sensors and how to use them in the vineyard to support decision-making: for example, a go-pro camera mounted on a small cart, driving along rows, could be used for yield estimation. The technology is already there, but we are still lacking the data algorithm to make sense out of the data. 

The day was finished up with the flash poster research presentations of wine science students. From glycosylated flavor compounds locked up in grape skins, to vintage compression and the effect of very high temperatures (over 50°C/122°F) for a short time on grape berry development and tannin content, these talks showcased the breath of wine research in the various Australian research institutions. Following the evening’s theme, the next day’s fresh science included a talk on how to remediate reductive aromas in wines. Among the tested treatments (DAP addition post-inoculation, donor lees added after malo-lactic fermentation, copper addition, macro-oxygenation, and a combination of copper and macro-oxygenation) macro-oxygenation once a day of 1.5 L/min O2for 2 hours yielded the most promising results while copper addition increased the risk of reductive characters developing post-bottling. Similarly, how to easier measure total and free copper in wines and juice was the topic of Dr. Andrew Clark’s presentation. Working at the National Wine and Grape Industry Centerin Wagga Wagga, Dr. Clark developed an easy spectrophotometric method to accurately and precisely measure free and total copper in wines. 

Last, a genetic study on Chardonnay revealed that the same clones (clone 95) shows a different number of mutations depending on where it is from.  

https://twitter.com/BKPinot/status/1153820039100784641

Besides the many fascinating talks and the impressive trade show, the meeting also offered lots of opportunities to taste Australian wines. I was lucky enough to participate in a guided tasting of a type of fortified wines unique to Australia: Presented with an impressive number of Rutherglen Muscatwines of all ages and classifications, I was able to experience this special wine style, and must admit that I brought back some bottles of these “stickies”. Made from Muscat a Petit Grains Rouge grapes (literally Muscat with little red berries), very ripe grapes, accumulating very high sugar content, are fermented and fortified with grape spirit, then aged from 3 up to 20+ years in barrels. Wines undergo a solera blending, transferring wines slowly from barrel to barrel until bottling. Flavors range from floral, honey and orange peel all the way to viscous, toasted and caramel flavors. If you ever have the opportunity to taste such wines, I would strongly encourage you to do so – even if this is not your style of liking, it is for sure a worthwhile sensory experience!

https://twitter.com/HeleneH14/status/1152918484881498113

Outside of the Conference I also had the chance to visit three remarkable places: The National Wine and Grape Industry Center (NWGIC) in Wagga Wagga, University of Adelaide and the Australian Wine Research Institute (AWRI) in Adelaide, and last, but not least, Penfold’s original winery in the Adelaide Hills for a special tour and tasting of the most expensive wine in Australia, the Grange. 

Impressions from the Tasting Room at the Charles Sturt University Cellar/ The National Grape and Wine Industry Center (NGWIC) in Wagga Wagga. (top left) Modern art wine labels for their Tumbarumba Chardonnay; (top right) Big Red hoping around the vineyard outside the tasting room. (bottom) Panorama view of the Charles Sturt University Cellar Tasting Room.
Sunset in the Sydney Habor overlooking the Opera House.
Penfold’s Magill Estate Winery overlooking Adelaide, located 15 min from Adelaide downtown. (top right) Lineup of Penfold’s most known wines – St. Henri Shiraz, Bin 707 Cabernet Sauvignon, and the iconic multi-regional Grange Shiraz. (bottom left) Bottle of the first Grange made by Maz Schubert in 1951. (bottom right) Current release of Grange (vintage 2014), selling for $850/bottle. 

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!