TECHNICAL DOCUMENT ON DOWNY MILDEW OF SOYBEANS

Downy mildew is a common problem in soybean [Glycine max (L.) Merill] which grown in field conditions but is rarely seen in greenhouses. Soybean downy mildew, caused by fungus-like organism Peronospora manshurica (Class Oomycetes; Phylum Oomycota), is a widely spread disease and can survive in crop residues and on the seed surfaces. P. manshurica has zoospores (flagellate motile spores) and needs water or high humidity for infection. Spores are carried onto plants by wind and rain and infection can spread quickly through a field during periods of cool, wet, or high humid weather. P. manshurica produces saclike sporangia carried on sporangiophores emerge from the stomata of the lower surface of the leaves. On the upper surface of the leaves, they will have pale green to light yellow spots. As the spots enlarge, they become pale yellow and of irregular size and shape and may enlarge to lesions. In moist weather, a pale blue to gray downy growth of the mildew fungus appears on the lower surface of the leaves, directly opposite the lesions on the upper side of the leaves. The disease rarely causes serious damage to soybean, but some reports revealed that the disease reduced seed size and quality if present early in the season and at high levels. The objective of this technical document is to study soybean downy mildew: symptoms and signs, causal agent, epidemiology, disease development, disease cycle, ecology and disease management.