Cercospora leaf spot in sugarbeets is the most serious foliar disease in Michigan. Defoliation from this disease can affect both tonnage and sugar content. In the last couple years, this disease has been more difficult to control in some beet growing areas. Typical fungicide rotations have included only the triazole and strobilurin class of chemistry. Recent testing results from both Michigan State University and North Dakota have confirmed that a high percentage of samples tested positive to strobilurin resistance.
There are many variables that can lead to Cercospora resistance. This would include increased use of susceptible varieties, earlier planting dates, and stretching recommended spray intervals. Chances for resistance build-up are increased if fungicides are applied late for the first leaf spot application or not controlling the disease at the end of the season. Poor spraying techniques that have inadequate coverage and failing to rotate fungicide classes can also propagate the problem. Use of strobilurin fungicides in other crops such as corn, wheat or soybeans may also be problematic.
There are several things a grower must do to help reduce fungicide resistance to strobilurin and other fungicides. If planting a highly leaf spot-susceptible variety, follow an aggressive spray program. All fungicides should be tank-mixed with a fungicide of a different mode of action and different modes of action should be rotated. Fungicides such as a strobilurin or benzimidazole (Topsin) should not be used more than once in a spray program. Spray intervals should be tightened and full fungicide rates used with a minimum of 20 gallons of water and 90 psi. The first application of fungicide should be applied just prior to first spot. Follow application spray intervals according to product labels or the BEETcast prediction model.
A more complete outline of Cercospora leaf spot resistance and management guidelines can be found on the Michigan Sugar Company website.