UMD Researcher Helps Winegrowers Fight Infections With Less Fungicide
Work Developing Risk Model May Have Saved Cabernet Sauvignon as Viable Mid-Atlantic Crop
A UMD researcher conducted experiments in Maryland vineyards to determine when grapes are most susceptible to infection with the fungal disease ripe rot (below). The work resulted in a disease risk model that is reducing fungicide applications and saving money for farmers in Maryland and Virginia.
Photos by Mengjun Hu
Raise a glass to Maryland and Virginia winegrowers, who face a daunting challenge that that their peers in traditional wine regions like the Mediterranean or Northern California can mostly ignore: warm, humid summers that invite fungus to thrive.
Late in the growing season, fungal infections can invade and devastate a field of beautiful grapes after months of careful tending. To combat the problem, farmers often begin spraying chemical fungicide early in the spring, when grape vines flower, and continue regularly until nearing harvest in the fall. It still doesn’t prevent many grapes from succumbing to infection and rotting on the vine, especially the popular varieties like cabernet sauvignon and chardonnay.
But recent work by plant pathologist Mengjun Hu in the College of Agriculture and Natural Resources 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 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. “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 today, along with more than 80 wineries, late-season rots had become a significant economic problem. “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,” said Hu, an associate professor in the Department of Plant Science and Landscape Architecture.
To help bring more clarity to the issue, he 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 student Scott Cosseboom Ph.D. ’22 (now a senior research associate at Cornell) conducted experiments in two Maryland vineyards to determine when grapes are most susceptible to infection with the disease. The team placed bags over bunches of grapes at different stages in development—from flowering to 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 the biggest risk didn’t occur during flowering. 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 the amount of time leaves remained wet and temperatures proved to be important factors affecting risk.
Then they combined the environmental risk models based on greenhouse data with the three years of vineyard data 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 Southern Maryland 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 contact with 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 infection on its own.
Most recently, his team has been addressing the issues from another angle, determining the effects of different pruning strategies, and managing a vine’s canopy to reduce ripe rot and other fruit rots.
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, designed to deliver complex integrated pest management information when farmers need it.
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