Take these steps to reduce fungicide resistance risks
Brenna Aegerter, a University of California Cooperative Extension farm adviser in San Joaquin County, and Jim Mueller, a field scientist for Dow AgroSciences based in Brentwood, Calif., provide these tips to reduce the risk of powdery mildew developing resistance to registered fungicides.
Treat early. Applications should begin at the very first sign of disease or even before you see symptoms, if conditions are favorable.
“Powdery mildew has such an explosive disease potential that you have to spray preventatively,” Mueller says.
During the early season, he recommends sulfur at short application intervals of seven to 10 days.
“Begin chemical fungicides, such as Rally, at the first signs of disease or when the forecast model indicates conditions are favorable for disease development,” Mueller says.
He recommends continuing applications on intervals of 14 days or less.
Coverage is critical. Obtain the best coverage possible, and this typically means spraying by ground.
During 2007, many of the performance issues were related to late-season aerial applications to plants with heavy crop canopies and high inoculum levels, Mueller says.
Tankmixes and rotations: Unfortunately, tomato growers only have a few different classes of chemicals that control powdery mildew. To prolong those that work, rotate among different Fungicide Resistance Action Committee groups.
DMIs, such as Rally, belong to Group 3 whereas strobilurins, such as Quadris, Flint and Cabrio, belong to Group 11.
In particular, Group 11 fungicide applications should not be made consecutively, and their use within the season should be minimized.
Tankmixing with a multi-site materialGroup Mmaterial is another way to reduce the risk of resistance to Group 3 and 11 fungicides. Dow AgroScience recommends tankmixing all Rally applications with sulfur, a Group M. In addition, Dow recommends tankmixing Rally with sulfur unless environmental conditions may cause crop injury.
Late-season treatments: Once the disease is established in a field, Group 3 and 11 products are less effective. Their continued use under these situations will increase the risk of resistance developing. Once the disease is established, use contact materials, such as potassium bicarbonate (Kalligreen, Armicarb and Milstop, among others) or sulfur (Microthiol and Thiolux). Maximize control with contact fungicides by obtaining good coverage.