Downy Mildew in Sunflower
Downy mildew occurs in sunflower when 2-3 inches of rain occur within two to three weeks of planting. The only control for downy mildew has been to use Apron treated seed.
Downy mildew is common in many fields, especially where heavy rains has occurred shortly after planting. Plants with systemic downy mildew are recognized by the stunted plants, yellow symptoms on the main veins and a downy white growth on the lower surface of the main veins. Occasional plants with systemic infection do not result in yield losses since nearby plants compensate in yield. When large areas of plants have systemic infection, then yield loss occurs.
Some fields have very few systemically infected plants, but may have plants showing secondary spread. Symptoms of secondary spread are small yellow spots on the leaves with a downy white growth on the lower leaf surface opposite the yellow areas. Occasionally, there may be symptoms on a main vein, but the plant will not be stunted. In most cases, secondary infection does not cause yield loss.
In 1998 the mildew fungus was found to have developed resistance to Apron in many parts of eastern North Dakota, Western Minnesota and northeastern South Dakota. About one third of the fields had a high percentage of Apron resistant strains, another third had a moderate percentage of resistant strains, and the last third had a low percentage of resistant strains.
Research is in progress to identify a suitable substitute seed treatment for Apron.