With the recent rains, we are seeing and hearing about increased reports of gray mold (Botrytis bunch rot) and sour rot. Both diseases are considered late season issues and despite the dry growing season we experienced this year, the recent precipitation was enough to increase berry moisture and cause some clusters to begin to rot in our experimental vineyards at the West Madison Agricultural Research Station.

Botrytis cinerea, the pathogen that causes gray mold is easily diagnosed due to the observable grey, fuzzy growth on the clusters (Figure 1). Tight-clustered varieties are highly susceptible to Botrytis infections. Additionally, injured berries from weather or insect-feeding are very susceptible to Botrytis. Unfortunately, once infections begin on ripening clusters, there are minimal intervention options. Preventative fungicide applications for Botrytis should begin at bloom and additional applications can be made around bunch closure and veraison. However, with lower disease pressure this season, applications of fungicides may have been sparse this year, allowing the pathogen to survive in blossoms and remain inactive until fruits began to ripen and conducive conditions (i.e., rain) developed.

Sour rot is one disease you can often smell before you see it if not scouting frequently. If you have ever walked through your vineyard and smelled vinegar, you’ve likely come across sour rot. Sour rot appears this time of year as discolored, shriveled berries (Figure 2) that are often slimy in appearance. Unlike Botrytis, there is no fuzzy fungal growth associated with sour rot infections. Sour rot is associated with wounding, fruit flies, bacteria, and yeasts. Sour rot favors warmer temperatures (68 – 77⁰ F) and rain. Like Botrytis, once infections occur, chemical control methods at this point in the season are limited. The best solution is to remove infected fruit from the vineyard entirely. Do not leave fruit on the vineyard floor as insects will feed on the infected clusters and continue to spread the disease around the vineyard. Furthermore, infected cluster removal will alleviate some of the insect pressure. Sour rot prevention should begin following veraison, and managing fruit flies is critical to controlling this disease.

While many fungicide options are not feasible at this time in the season due to pre-harvest intervals exceeding target harvest dates, there are still some measures you can take to minimize the impact of late season rots. Remove infected clusters and damaged clusters as these can serve as sources of inoculum. Additionally, minimizing damage to fruits is important as they are more sensitive to wounding this time of year; wasp, birds, and rain splitting can also result in wounding. Finally, leaving fruits on the vine longer will put them at greater risk for infection when conditions are conducive, so harvest dates may need to be adjusted.

Continue to be on the lookout for these pathogens and finish the 2023 growing season strong!

This article was posted in Disease, Grapes and tagged botrytis, Botrytis cinerea, cluster rot, cluster rots, Courtney Cameron, disease, Grapes, Leslie Holland, sour rot.