7/27/04 - It might look like soybean rust, but it isn't

7-27-04

It might look like soybean rust, but it isn't
Soybean foliar diseases increase in SW MN.

Heavy rainfall, high humidity and prolonged periods of wet soybean leaves have provided a good environment for development of some soybean leaf diseases. These are severe enough to defoliate lower soybean leaves in some cases. The most common foliar pathogens now are probably Bacterial blight, Septoria brown spot, and Downy mildew. I suppose that the latter disease could be confused for soybean rust. There may be a bit of bacterial pustule mixed in as well but I am not as confident on my diagnosis of the latter. There are probably many other similar appearing foliar pathogens out there as well.

All three diseases typically persist from season to season on soybean residue but can also be seed borne. Additional all can cause leaf loss. Younger leaves are more susceptible to bacterial blight and downy mildew. Older leaves are more susceptible to Brown spot. Varietal differences in severity should be expected. All three diseases are favored by high humidity (rain-splashed soil in the case of brown spot and bacterial blight) and cool to moderate temperatures. A final wrinkle, increased severities of brown spot have been linked to SCN and Fusarium, both of which seem to be in ample supply this year.

Fungicides are being promoted for soybean disease control this season. There is little MN data to quantify the economics of fungicide applications in soybeans but we do have trials out this year. Fungicides will not provide control of bacterial diseases so disease identification becomes important. Fruiting structures of fungi can sometimes be observed by placing the affected leaves in a plastic bag at room temperature overnight with a bit of moist cotton or paper towel to provide humidity. Using a good hand lens, look for dark fruiting structures in the lesions. Bacterial infections will not sporulate, but lesions may become more water-soaked. Symptoms of many soybean diseases do not appear until later in the season. The way the season is going, plastic bag manufacturers may be a good investment.

One of the rules of plant pathology is the either everything or nothing looks like the pictures. As mentioned in a previous issue, many plant disease symptoms are quite similar. They can be difficult to identify in the field and in spite of pontifications to the contrary, often require culturing and microscopic examinations for confirmation of the causal organism. I would like to once again encourage you to send any plant diseases that you are uncertain of to the University of Minnesota Plant Disease Clinic. They actually know what they are looking for and how to find it.

Delivery Address:

1519 Gortner
105 Stakman Hall
(612) 625-1275 - Phone
(612) 625-9728 - Fax

Bruce Potter
bpotter@umn.edu
Integrated Pest Management Specialist
University of Minnesota Extension Service
Southwest Research & Outreach Center
23669 130th Street
Lamberton,MN 56152
(507)752-5066 Fax(507)752-5097

URL: http://www.soybeans.umn.edu/resources/news/news_7-27-04.htm

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