Grapes: The Sour Rot Situation – Facts for Fancy Fruit

Grapes: The Sour Rot Situation


Grape harvest is underway in the southern part of the state, and early varieties have been harvested in central Indiana. The recent storms and heavy rains could not have come at a worse time, especially for varieties near harvest, and tight clustered varieties such as Vignoles, Seyval Valvin muscat, etc. Excess rain causes fruit to expand as water enters the berry. Unfortunately, in many cases, the berry cannot hold all the water and something has to give. That something is usually the berry skin, which cracks under the pressure. In tight clustered varieties, the berries may squeeze each other until they burst or separate from the pedicel. Once split, the berries are open to insect pests, and various spoilage organisms such as bacteria, fungi and yeasts. The resulting “sour rot” can be a major concern for fruit and wine quality. The most obvious symptom of sour rot is a pre-harvest decay accompanied by the smell of vinegar (aka acetic acid, or what winemakers call volatile acidity or VA). The berries usually turn a tan color, soften, and eventually break down and disintegrate. The decayed berries seldom have any noticeable fungal growth or fruiting bodies on the surface like we see with Phomopsis, Botrytis or black rot. We see sour rot almost every year, but especially in years where heavy rains occur near harvest.

There has been some excellent research done in recent years by Dr. Wendy McFadden-Smith at OMAFRA in Ontario, and Dr. Wayne Wilcox, fruit pathologist at Cornell University. Here are some details on sour rot from the annual “Grape Disease Control” document by Dr. Wilcox. Real “sour rot” is caused by a combination of yeasts and bacteria. While filamentous fungi, such as Botrytis, may be associated with the rotting berries, they don’t appear to be the cause. The vinegar in rotted berries is produced by acetic acid forming bacteria, mostly species of Acetobacter and Gluconobacter. Yeasts are also involved including various wild types as well as the good Saccharomyces types. Apparently the yeasts convert the sugar to ethanol then the bacteria convert the ethanol to acetic acid in a microorganismal “tag team effort.” Both the yeasts and bacteria need some type of physical injury or wound to infect the plant, so birds, rain cracking, compression in tight clusters, etc. is all involved in the process.

Some interesting findings from Dr. McFadden-Smith’s group is that sour rot does not become a problem until berries reach about 15 Brix. Additionally they found, not surprising, that temperature has a major effect on the rate of development, with rot developing rapidly at higher temperatures (68-77˚F in their case), much more moderately at cooler temperatures (59-68˚F) and hardly at all when temperatures remained in the 50s. Tough luck for those of us in Indiana. We are unlikely to see temperatures in the 50s over the next few weeks as we progress through harvest.

What is most interesting is that another group of organisms has been shown to be a key part of the sour rot equation. Research done by Megan Hall, a student in the Wilcox lab has shown that fruit flies of the Drosophila genus are a key component of the disease cycle. These can be both the normal D. melanogaster species, or the new Spotted wing variety, D. suzukii. Megan ran a few experiments where she inoculated berries in petri dishes with standard wine yeast and acetic acid bacteria, both with and without fruit flies being introduced into the dishes. Each day for five days she measured ethanol and acetic acid accumulation. What she found was that after day 4 ethanol concentration increased rapidly in the dishes without flies, but there was little additional accumulation where flies were present. Acetic acid, on the other hand, was very low in inoculated berries without flies, but ten times greater in treatments with flies. Apparently the ethanol is not being converted to acetic acid unless the flies are present, even though the bacteria are present. There is obviously more to figure out, and I’m sure Megan is working hard on the project again this year. In the meantime, we’ll just have to be content in knowing that fruit flies are definitely a cause of sour rot development, not just being attracted to the rotting fruit.

So what can you do? As with most diseases there are several approaches. First and most obvious is to minimize berry injury such as cracking. Unfortunately we can’t stop the rain, but other injuries such as bird pecking, berry moth, etc can be managed. Another strategy is to provide a canopy microclimate that is not conducive to disease development. Shoot thinning and positioning, leaf removal, nutrient management, and training system can all play a role. Lastly, are more directly at this stage of the season is minimizing the populations of yeasts, bacteria, and fruit flies. So let’s focus on that approach.

A number of fungicides and antimicrobial sprays were tested in Ontario. Most did not have any effect on sour rot development. This is not too surprising since fungicides are designed to control filamentous fungi, not yeasts and certainly not bacteria. More recently work in New York has shown some benefit from Oxidate (hydrogen dioxide and peroxyetic acid) and Fracture (a naturally occurring seed protein from lupines, Banda de Lupinus albus doce BLAD). Both of these products are labeled for grapes and have short PHIs (0 and 1 day respectively).

What has worked best in New York is controlling fruit flies. In New York trials, they applied insecticides and antimicrobials weekly starting at 15 Brix, or as a rescue treatment at the start of symptoms. In just a single year of study so far, they got pretty good results with Oxidate, and reasonably good control with Fracture when applied with an insecticide weekly beginning at 15 Brix. The results were less impressive when applied at the beginning of symptoms, with little additional control over the insecticide alone. A key finding was that the insecticide alone had a significant effect. Severity of rot was 43% lower when insecticides were included across all their treatments, and they saw a 50% reduction in disease severity from the insecticide alone, without antimicrobials.

So what should you do? We really don’t have a solid recommendation yet, but I think at the very least we should be applying insecticides to manage fruit flies. Based on the findings in New York, that should at least slow the spread. And because we have a significant threat from SWD in our region, these applications make sense. Addition of an antimicrobial such as Oxidate may be effective, but we really don’t have enough data to make a recommendation. But that is an option that growers can try.

We have gained quite a bit of experience over the past two years since SWD showed up, especially with trials in red raspberries. We’ve found that SWD are not easy to control in berries. But grapes are much less attractive hosts, so I think we have had reasonably good success with Mustang Maxx, Delegate, and malathion in rotation. (See articles on SWD by Rick Foster in recent issues.).

I’ve heard from several growers since I first posted this article last week. Thanks for the updates. I’ve gotten some good questions. One is, “If there are only a few berries split and rotting, and the fruit maturity still a week or two away from optimal, can I just wait and let them dry out as the rest of the fruit ripen?” The answer is “yes” that is a good strategy. I walked my plots this weekend and, despite recent insecticide applications, there was a plethora of bees, wasps, flies, and beetles happily feeding on the damaged clusters. So, with a little luck from the weather, the “wait and see” approach is certainly a viable option. On the other hand, if more rain causes additional splitting, all that insect activity is sure to spread the pathogens around, leading to more rot. So, it’s a judgement call. Let’s hope for a significant break from this tropical moisture pattern we’ve been in.


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