When should fungicide be applied? Should I spray before or after a rainfall?

Randy S. Sanderlin, Bollich, Patricia A.  |  8/3/2006 11:29:09 PM

Fungicide application, using an air-blast sprayer, for control of pecan scab disease. Photograph by R. A. Melanson.

Fungicides control disease by preventing infection; they do not “cure” disease that is already present. Pecan scab disease requires rainfall or dense fog for infection to occur. Thus, to prevent infection with the fungal pathogen of pecan scab disease, a fungicide should be on the trees before rainfall occurs. Trying to apply fungicide to stop disease development after a rainfall is usually not successful because once the pathogen becomes established in the pecan tissue, a fungicide will not stop it from producing lesions on the tissue. There is often concern about rainfall washing off fungicides, and this can cause a reluctance to spray ahead of predicted rainfall. However, fungicides are not easily washed off once they have dried, and many fungicides are absorbed into the pecan tissue and not affected by rain. Fungicides need to be applied on a regular basis; as new pecan tissue develops, it is not protected by previous applications and fungicides will lose their activity over time. Depending on rainfall frequency and amount as well as other environmental factors and host susceptibility, fungicides to protect against pecan scab disease generally need to be applied every two to four weeks from early spring through the summer.

More information can be found in the Fungicide Application Recommendations for Pecan Disease Control on the Plant Pathology page of the Pecan Station website.

Question answered by Dr. Randy Sanderlin, Pecan Research-Extension Station plant pathologist.

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