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

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