Travelling to the east, the first appearance is later. In cases where disease begins to appear late in the season, the general rule based on field experiments is that treatment should be made if the disease first appears 5 weeks or more before harvest. Later treatment will be of no economic benefit. Where multiple treatments are required, applications should be repeated as the disease reappears, but no closer to harvest than 5 weeks. First, randomly select the most recently fully expanded mature leaf from a representative number of plants in the field and estimate the percentage of each leaf surface covered by the powdery mildew fungus.
Good coverage is essential for control.
Fungicide should be applied in as much water as is necessary for good coverage. The calculations here are based on liquid sulfur applications, but experience has shown that sulfur dust is more effective than liquid sulfur applications because dust provides superior coverage. A few simple calculations clearly illustrate the economic benefit of timeliness in controlling powdery mildew. There is at least one sugarbeet variety that has limited resistance to powdery mildew and is available in some growing areas. Unfortunately, resistance is often incorrectly equated with immunity, and growers may incur unnecessary losses when the resistant variety is not treated when it should be.