Validating a Novel Action Threshold for Sour Rot in Grapes

Commodities

• Fruits: grapes

Practices

• Education and Training: decision support system, on-farm/ranch research
• Farm Business Management: risk management
• Pest Management: disease vectors, weather monitoring

Project Focus: Wine grapes are an important crop in New York state, but growers face threats from diseases throughout the growing season that decrease marketable crop. Sour rot is a devastating disease for wine grapes in New York when conditions are favorable. It reduces marketable yield, alters the chemical composition of harvested grapes and negatively affects wine quality. Sour rot is a complex and unpredictable late season bunch rot that can be especially problematic in grape varieties with tightly packed bunches and thin-skinned fruit. According to a 2019 New York grower survey, several recent years have had severe sour rot out breaks due to weather conditions, resulting in $3,000/acre worth of lost fruit from sales per grower. Sour rot is characterized by a complex interaction of microbes, including yeasts, acetic acid bacteria and lactic acid bacteria. The complexity of the disease, and its unpredictability between years, has resulted in a lack of research and few agreed-upon management strategies compared to other bunch rots affecting grapes. Vinegar flies (Drosophila melanogaster and D. suzukii) are an important vector of sour rot, and because of this, microbial pesticides are ineffective when applied alone. Previous research has shown that weekly applications of insecticides, targeting vinegar flies, and antimicrobial pesticides significantly decrease sour rot occurrence, but these materials are expensive for growers and resistance has been identified in 3 of the main insecticide classes labeled for use in grapes. Due to the intermittent nature of the disease, calendar-based spraying to control sour rot is environmentally unsustainable, costly and further compounds the risk of insecticide resistance, threatening the only viable option for control growers currently have.

Solution and approach: Using eight years of historic data we have developed a weather-based tool that accurately predicts the sour rot severity within a particular year. We have translated this tool into a within-season action threshold to indicate when spraying is necessary. The action threshold for risk is based on late season temperature and humidity conditions in a rolling 14-day window in which the model calculates daily risk values. One year of plot trial data from the Cornell AgriTech University farm showed that model-indicated spraying resulted in 3 fewer applications, and resulted in equivalent sour rot severity as the grower standard. However, further validation is necessary before it can be recommended to growers. We aim to 1) conduct a second year of plot trials of the weather-risk model to indicate spray timing, 2) validate model-timed sprays in 10 commercial vineyards in susceptible varieties in Finger Lakes and Long Island and 3) deploy an accessible online application in collaboration with Cornell IPM/NEWA that will be promoted to growers.

1. We have developed a novel weather-based tool that assess sour rot risk in grapes with high accuracy in historic observational data. This weather-informed risk model was adapted to an action threshold and performed as well as the grower standard while decreasing the number of pesticide applications by 60% in plot trials of an early season cultivar in 2025. Before it can be broadly recommended for adoption by growers, however, it must be validated in commercial vineyards.
2. Test the efficacy of a weather-informed sour rot risk model that indicates spray timing we aim to collaborate with 10 commercial vineyards in the Finger Lakes and Long Island in New York State. In 2026 early ripening susceptible cultivars will be tested and in 2027 late-ripening susceptible cultivars will be evaluated.
3. We will assess the efficacy of this action threshold to control sour rot in early and late grape ripening cultivars compared to the grower standard calendar spray approach for control.
4. Reducing pesticide applications to targeted sour rot risk windows will save growers money and time. It will also decrease the pesticide load in vineyards, protecting the watershed, soil health, grower health and slowing the development of insecticide resistance of important insect vectors.