Issue No. 19, Article 5/July 30, 2004
Questions Raised Regarding Downy Mildew on Soybeans in Illinois
The recent relatively cool weather with dew and/or wet conditions in many parts of Illinois has favored development of several foliar diseases of soybean. Septoria brown spot has been common, bacterial blight has been noted, and recently white mold (see the subsequent article in this edition of the Bulletin) and downy mildew have been reported.
Downy mildew has been reported more in the past week than is usual for the season. Most reports have come from the northwestern quarter of Illinois, although Ron Hines, a senior research specialist with the University of Illinois at the Dixon Springs Agricultural Center, has also reported downy mildew from far into southern Illinois. The levels of the disease appear generally to be minor in Illinois. Downy mildew has the potential to cause defoliation and reduced yields; however, yield losses are typically minimal if they occur at all. The possibility exists that this disease could be more than superficial if new races of the pathogen appear or if susceptible soybean varieties are released.
Symptoms of downy mildew on soybeans begin as indefinitely shaped, small, light green spots (not water-soaked) on the upper and lower surfaces of leaves. Small, raised tan tufts of fungal growth often develop in the lesions, especially on the undersides of leaves under wet or humid conditions. The lesions can grow together into large, irregular brown areas that may ultimately rip away from the leaf. Seed coats and the insides of pods can also become covered with a white to tan fungal growth, but infection of the outer part of the pod may not be obvious. Small spots may also develop on pods.
Symptoms of downy mildew on the top (light spots) and bottom (tan "fuzzy" spots) of soybean leaves. (Photos taken by David Faulkner of the University of Illinois Dixon Springs Agricultural Center.)
Downy mildew is caused by the fungal-like pathogen Peronospora manshurica. It overwinters on infected leaves and seeds and can be transmitted by infested seed. Common snap bean can also be infected by this pathogen. Downy mildew can occur all season. Young leaves are typically most susceptible, which partially explains why symptoms of the disease are often seen in the upper parts of plants. Downy mildew is favored by temperatures around 68°F to 72°F, as well as dew, high humidity, and frequent rain. It is also favored by susceptible soybean varieties, infested seeds, and infested residue on the soil surface. The pathogen can be spread by wind and rain, and it can also infect plants systemically from seedlings grown from infested seed.
Downy mildew can be managed with four main tactics. The first three are to select and grow resistant soybean varieties, rotate soybean with a nonbean crop, and use appropriate seed treatments if seed is infested. Fourth, it may be worth considering burying infested residue where feasible and where disease has been severe. There seems to be minimal evidence, if any, to suggest that foliar-applied fungicides would provide an economic benefit for management of downy mildew.--Dean Malvick