NPIPM:Peronospora manshurica (soybean)
Compiled by: Buyung Hadi, from the materials by: Dean Malvick and Lawrence Osborne
Downy mildew is a widespread disease on soybean that does not usually cause economic damage. However, in rare occasions, downy mildew may cause defoliation and reduce yields.
Causal Organism
Downy mildew is caused by the fungus Peronospora manshurica.
Symptoms and Signs
Initially, downy mildew infection causes the formation of small, light green spots on the leaf surface. These spots grow in size and change in color to yellow and, eventually, brown. Several spots may coalesce forming larger brown/grayish lesions with yellow borders. In early morning, especially after heavy dews, tufts of fungal growth is visible on the underside of the spots. If seedlings are infected by downy mildew, subsequent systemic infection may produce inter-veinal chlorotic leaf areas, especially near leaf tips. Seeds can be infected by downy mildew, sometimes resulting in the production of hyphae-coated seeds.
Life Cycle and Epidemiology
The fungus overwinters on infected leaf debris and within seeds. Planting downy-mildew infected seeds may introduce the inoculum to the field. In the field downy mildew can infect soybean of all ages, although the disease is more common after flowering. Young leaves are more susceptible and infected leaves are commonly seen on the top of the plants. High humidity and mild temperatures (18.3-23.9˚C or 65-75˚F) favor downy mildew development.
Management Approaches
Crop rotation with non-bean crop reduce the possibility of inoculum buildup within a field. Where downy mildew infection was severe in the previous season, plant residue should be buried to avoid possible inoculum spread from overwintering colonies in the residues. Soybean varieties resistant to downy mildew are also available.
