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Providing Growers Local Information on QoI Fungicide Resistance to Guide Fungicide Selection for Cucurbit Powdery Mildew 2003

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Project Leaders:

Meg McGrath, Dept. of Plant Pathology, Riverhead

Carol MacNeil, Vegetable Specialist, CCE Ontario, Wayne, Yates and Steuben Counties

John P. Gibbons, CCE,Vegetable Extension Program in Ontario, Wayne, Yates and Steuben Counties, and NEWA, NYS IPM Program

Alan Erb, Vegetable Specialist, Lake Plains Vegetable Program

Cooperators:

Ted Blomgren, Vegetable Specialist, Capital District Vegetable Program

Chuck Bornt, Vegetable Specialist, Capital District Vegetable Program

Abby Seaman, Area Vegetable IPM Specialist, WNY

John Mishanec, Area Vegetable IPM Specialist, ENY

Rick and Laura Pedersen, Pedersen Farms, Stanley

Curt Petzoldt and Jim Engel, Vegetable Systems Project, NYS IPM Program, Geneva

Additional cucurbit growers in other participating county

Type of grant: Monitoring, forecasting, and economic thresholds

Project locations: Ontario County, Suffolk County and Lake Plains Counties

Abstract:

Systemic fungicides are an important tool for managing powdery mildew, the most common disease of cucurbit crops throughout the world. Unfortunately, most systemic fungicides are at risk for resistance development because they have single-site mode of action. Thus modification of one gene in the pathogen may be enough to enable the pathogen to resist the action of the fungicide. The cucurbit powdery mildew fungus has demonstrated a high potential for developing resistance. First detections of resistance to QoI (strobilurin) fungicides in North America occurred in 2002 and included New York. Although resistance has developed, QoI fungicides will continue to be valuable for managing powdery mildew and resistance to DMI fungicide until resistant pathogen strains become common. To use QoIs wisely, growers need to know the proportion of the pathogen population that is QoI resistant before the first application and how much the population changes with QoI use. Through this project, QoI resistant strains were found to be uncommon at the start of disease development. However, where they occurred (Suffolk County), their frequency increased dramatically after QoI and DMI fungicides were used. QoI resistant strains were common in most pumpkin fields examined in September, including 1 of 2 organic production fields examined and a field where neither QoI nor DMI fungicides were used. This information, along with recommendations on how to modify fungicide programs, was provided to growers during the growing season through newsletter articles. Thus growers were able to avoid unnecessary applications of an expensive fungicide during the second half of the epidemic when QoI resistant strains were sufficently common that QoI fungicides were unlikely to have been effective. Monitoring needs to be continued in the future to determine where QoI resistant strains are sufficiently uncommon that this group of fungicides will be effective. When resistance first developed to DMI fungicides, for several years these resistant strains were uncommon when powdery mildew started to develop, thus these fungicides continued to provide some control.

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