Downy mildew is a very common foliar disease of soybeans, but it seldom causes serious yield loss. The pathogen may also infect seed and reduce seed quality. Diseased plants are usually widespread within a field. Seedlings that are infected from oospores on the seed can develop large chlorotic areas on the first and second pairs of true leaves. The disease is more common in late vegetative and reproductive growth stages. Lesions occur on upper surface of leaves as irregularly shaped, pale green to light yellow spots that enlarge into pale to bright yellow spots.Older lesions turn brown with yellow-green margins. Young leaves are more susceptible than older leaves, so disease is often found in the upper canopy. Lesion size varies with the age of the leaf affected. On the underside of the leaf, fuzzy, gray tufts may be seen growing from each lesion, particularly when humidity is high or leaves are wet, for example, early in the morning. Infected pods show no external symptoms, but the inside of the pod and seed may be covered with a dried, whitish fungal mass that appears crusty and contains spores. Infected seed can be smaller appear dull white and have cracks in the seed coat.
Peronospora manshurica survives in leaves and on the surface of seed. Extended periods of leaf wetness are favorable for movement of the pathogen. High humidity and moderate temperatures favor infection. The increased resistance of older leaves and higher temperatures mid-season usually stop disease development before extensive damage occurs.
The best time to scout this disease is R3 through R6; after frequent rains. Disease is often found in the upper canopy. Fuzzy growth often can be seen in the mooring when dew is still on leaves.
Variety selection: Many sources of resistance are available. However, many races of the pathogen have been identified, and varieties that are resistant to all known races have not yet been developed.
Crop rotation and tillage: Crop rotation and burial of infested crop residue using conservation tillage practices can reduce pathogen levels.
Photo by Daren Mueller