: The disease is caused by a soil borne fungus. The plant is susceptible very early in the root development. Cool and water saturated soil is most conducive for infection. This is particularly the case for flat ‘slow to drain’ soils that hold water. The fungus can live in soils for 8 to 10 years. Wild sunflower and some weeds like marsh elder are also hosts. The spores are wind-blown so a field with no sunflower history is not risk free of the potential for this disease.
Damage: Seedling infection seldom exceeds 25 percent of the field, although there have been exceptions. The infected plants die off very early and do not compete for nutrients with neighboring plants. If the infection is sporadic throughout the field, the compensating ability of sunflower will compensate and the yield impact will be minimal. However, if all or a majority of the plants in a part of the field are infected then the damage will be significant.
Economic Thresholds: This has not been developed. The infected plants are easily recognizable early after emergence. An infection of greater than 15% may be problematic depending on plant population and if the diseased plants are concentrated in one or more parts of the field.
Scouting Method: Not developed.
Management: Most seed companies have hybrids that are resistant/immune to all known races of downy mildew. Although there are many races, USDA ARS field surveys have found 3 or 4 common races. The pathogen is capable of mutating into new races. To extend the immunity of hybrid resistance, fungicide seed treatments are highly recommended in combination with hybrid resistance. Currently Azoxystrobin (Dynasty®) in the Cruiser DM Pak® and Fenamidone (Reason®) in the Idol® with an insecticide are labeled for downy mildew suppression. Delaying planting in cool wet soils until the soil warms up is a management strategy as well.
Research: USDA ARS is constantly monitoring the pathogen for new races. USDA ARS is also identifying new resistant genes. Resistance is governed by one gene.
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Source: NDSU Extension Bulletin 25 Sunflower Production Handbook, NDSU Extension Service, September 2007.