To determine when to quite making fungicide applications to corn and soybean fields, farmers need to be thinking about the minimum amount of time that must pass between the last pesticide application and harvesting of crop, or the grazing or cutting for feed.
"With the buzz of airplanes still being heard out in the countryside and September upon us, we need to remind growers about the pre-harvest intervals for fungicides applied to corn and soybean fields," says Daren Mueller, an Iowa State University Extension specialist in plant pathology. "In July in Kansas, some harvested wheat was embargoed when fungicide levels were questioned until tests could be completed to determine residue levels."
Pre-harvest Interval is the minimum amount of time that must pass between the last pesticide application and the harvesting of the crop, or the grazing or cutting of the crop for livestock feed. Typically, PHIs for fungicides applied to field crops range between 21-30 days.
Some triazoles and triazole + QoI fungicides have restrictions based on growth stages instead of a specific number of days; fungicides cannot be applied later than soybean growth stage R5 for soybean or after silking for corn. Mueller provides the following guidelines for pre-harvest restrictions for fungicides.