UMD Researcher Helps Region’s Winegrape Growers Combat Fruit Rots with Less Fungicide

Mengjun Hu Identified the Fungus Responsible for Late Season Rots and Developed a Risk Model to Guide Farmers in Preventing the Disease

Courtesy Mengjun Hu

Image Credit: By bagging bunches of grapes at different times in the growing season, Menjium Hu determined when the fruit is most susceptible to the ripe rot infection.

February 4, 2025 Kimbra Cutlip

Maryland and Virginia winegrape growers face challenges that traditional wine regions like the Mediterranean don’t: warm, humid summers that invite fungus to thrive. Late in the growing season, fungal infections can devastate a field of beautiful grapes after months of careful tending. To combat the problem, farmers often begin spraying fungicide early in the spring, when grape vines flower, and continue regularly until nearing harvest in the fall. But still, many grapes succumbed to infection and rotted on the vine, especially the popular varieties like Cabernet Savignion and Chardonnay.

But recent work by UMD plant pathologist Mengjun Hu has helped farmers pinpoint the pathogen responsible for a significant portion of late season rots and identified the most effective window for applying fungicide. He and his team developed a disease risk model that is reducing fungicide applications, saving farmers money and improving wine grape harvests in Maryland and Virginia.

“In my opinion, Mengjun’s work has made Cabernet Sauvignon viable again in the Mid-Atlantic,” said Lucie Morton, an independent viticulturist and leading consultant to the North American wine industry. “Cabernet Sauvignon is one of the most susceptible grapes to Ripe Rot, and before this work, I had clients who lost entire seasons because they didn’t know what to spray for or when to spray.”

As Maryland vineyards expanded from roughly 299 acres in 2002 to 1,200 acres today, late season rots had become a significant economic problem, but the major pathogens responsible were unclear. “Before I began working with wine grapes, ‘Late Season Rots’ was just a general term,” Hu said. “We didn’t know what specific pathogens were causing it, and growers were spraying broad spectrum fungicides and sort of guessing at when it was most effective.”

Bunch of purple grapes with many berries whithered. — After months of cultivating a crop, late season rot can devastate grapes just as they are nearing harvest.

To help bring more clarity to the issue, Hu and his team collected over 400 samples of fungus from grapes affected by late-season rots and identified them down to the genus or species level. That work revealed that a disease called Ripe rot was the main culprit, and identified the species of fungi most responsible for driving infections.

Then Hu and his former PhD student Scott Cosseboom (now a senior research associate at Cornell) conducted experiments in two Maryland vineyards to determine when grapes are most susceptible to infection with Ripe rot. The team placed bags over bunches of grapes at different stages in development --from flowering, to BB size, pea-size berries, through color change, and pre-harvest stage-- to protect them from fungus in the environment. It was commonly believed that the flowering stage provided a gateway for fungus to enter the plant, and as fruits grew and ripened, increasing sugar levels caused the fungus to take off. To combat this, farmers applied fungicide during flowering, which usually happens in June, and reapplied every 10 to 14 days until harvest. That meant 10 to 12 applications in a single season.

Hu’s experiments showed otherwise. Grapes were most susceptible to Ripe rot when they were left unprotected right after they changed color, only six weeks or so before harvest in mid-autumn. That dramatically reduced the need for fungicide throughout the earlier part of the growing season.

But Hu wanted to give farmers even more specific information. His team conducted a greenhouse experiment, manipulating moisture and temperature, and inoculating grapes with fungus under different combinations of these conditions. He discovered that leaf wetting time and temperature proved to be important factors affecting the risk of Ripe rot.

Then they combined the greenhouse data (i.e. environmental risk models) with the three years of vineyard data (i.e. phenology susceptibility model) to create a predictive model that could pinpoint the highest risk for Ripe rot infection. Based on three-year field trials, that model reduced a farmer’s need to apply fungicide during that vulnerable six weeks from color change to harvest by 30%.

Working with Steve Purvins, a local grape grower and programmer, Hu’s team incorporated the ripe rot model into GrapeCast, a disease alert system that allows farmers to make fungicide application decisions based on their specific climate conditions and the developmental stage of their grapes. GrapeCast is managed by the Maryland Grape Growers association.

Reducing fungicide application not only saves farmers money, but it lowers the risk that fungus will develop resistance to these treatments, and it protects farm workers and the environment from the risk of excessive antifungal agents.

Hu is also testing other pathogens associated with late season rots to determine how to manage them. So far, he and his team have found that some pathogens can only cause secondary infections. They can’t infect grapes on their own, but they get a foothold once something else like a bird or a storm damages a grape’s skin or the vine’s bark. They also found a cryptic and previously undescribed fungal pathogen Aspergillus uvarum that can cause primary infection.

Most recently, his team has been addressing the issues from another angle, determining the effects of different pruning strategies, managing a vine’s canopy as a way to reduce ripe rot and other fruit rots. Together, this knowledge could lead to integrated pest management (IPM) that further reduces the need for chemical fungicides. He has also been adding pictures of specific fruit rots and management and treatment information into an App that he and his colleagues previously developed called MyIPM. The app is designed to deliver complex integrated pest management information when farmers need it.