The disease was prevalent in late summer, but the damage level was low. The fungus can be seedborne and seeds from infected plants may have fungal colonies on their coats. In Iowa, this disease has not been considered a major production problem and most varieties used in the state are not resistant to this disease. However, seeds infected with downy mildew can be mistaken for white mold-infected seeds, and such a misidentification occurred many times during the 1997 planting season. The pathogen also can survive in crop residues.
Aphids are the vectors that spread the disease during a growing season.
The dry summer this year may have favored the development of this disease. However, this discoloration should not be considered as a reliable indicator of SMV because other stresses can cause such mottling. Seed testing is necessary to determine the presence of the infection. Phomopsis species and Anthracnose species were frequently isolated from premature, dying plants after mid-August of this year. Typical symptoms were bright yellow leaves on the top of the plant followed by death of tissues from the top down.