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Spraying by the Numbers: Fungicide Resistance Management Tables for Tree Fruit Diseases

Tree fruit growers can prevent resistance to fungicides by alternating chemicals by FRAC group, also known as "spraying by the numbers."

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Updated:

April 8, 2023

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Powdery mildew on blossoms. The powdery mildew fungus likes young tissue. As tissue begins to grow, the fungus will colonize young, green tissue as it emerges. Photo: Kari Peter, Penn State

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Resistance is more likely to develop against fungicides with a single mode of action, especially if used alone for a long time. In the orchard, resistant fungi may occur naturally in very small numbers, even before the fungicide is first used. When a fungicide is applied, it reduces the number of susceptible apple scab and brown rot fungi. The few scab and brown rot fungi that are resistant to the fungicide are able to increase in number. As the fungicide is repeatedly used, the number of resistant fungi increases. The fungicide becomes less effective as the fungus becomes more tolerant of it. These downloadable tables will help you to avoid resistance by "spraying by the numbers."

Bilingual spreadsheets developed with funding from USDA Specialty Crop Block Grant #ME44187248 project titled "Sustainable Production, Business Management, and Farm Safety Innovations for Beginning and Minority Specialty Crop Producers."

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