Early symptoms of Downy Mildew occur on young plants but the disease does not develop in a field until the late vegetative or early reproductive stages. Initially small, irregular, pale yellow spots appear on the upper leaf surfaces. Later, they become grey-brown with a yellowish margin. On the lower side of the leaves, the spots have a grey fuzzy appearance due to the presence of the pathogen. Symptoms frequently occur at low levels throughout the crop canopy. When pods are infected, an encrusted mass of fungal-like growth is visible inside the pods. The infected seed has a dull white appearance and is partially or completely encrusted with the fungus. Lesion size and shape depend on the age of the leaf. Older lesions may turn greyish brown to dark brown with yellow or green margins. Severely infected leaves turn from yellow to brown and drop prematurely.
Downy mildew is caused by the fungus-like organism, Peronospora manshurica. It overwinters in the field as thick-walled resting spores in leaf debris and less often on seeds. The disease is most common after flowering begins. Young leaves are most susceptible and infected leaves are often seen on the top of plants. The lesions on the older soybean plants may increase in number and decrease in size on older leaves. The disease is favoured by moderate temperatures (20-22°C) and high humidity. The downy mildew fungus overwinters in the field as thick-walled resting spores (oospores) in leaf debris and on seeds. The development of the disease depends mostly on the climate. When humidity drops, the downy mildew pathogen will suffer and future disease spread will be avoided. In the occurrence of high humidity and consistent rainfall, downy mildew will continue to develop.
To this day, we are not aware of any biological control method available against this pathogen. If you know of any successful method to reduce the incidence or the gravity of the symptoms, please contact us.
Always consider an integrated approach with preventive measures together with biological treatments, if available. Apply fungicides like metalaxyl, oxadixyl together with mancozeb, maneb or zineb for seed treatment.