Farmers watch for soybean diseases

Soybeans have recently responded well to the drier conditions and moderate temperatures. The crop is beginning to flower and farmers will evaluate their stands for potential mid-season diseases and make applications of any necessary fungicides.
There are three diseases that a farmer may see at this time of year. The common names match the visual symptoms, such as brown spot, frogeye, and white mold.
Brown spot is what I call the freckle disease. It often appears as tiny, angular red to brown spots on the older or lower leaves of the soybean plant. However, they can be as large as a quarter-inch in diameter.
Brown spot spreads during wet growing conditions. Leaves with numerous spots may eventually turn yellow and fall from the plant.
If enough leaves drop early from the plant, farmers may lose two to four bushels per acre. Often it is not economical to spray fungicides for brown spot unless grain prices are high.
Generally, farmers do not consider brown spot a concern if the lower leaves are free of disease symptoms by early flowering (this time of year). Seldom do later infestations justify the cost of fungicide application.
Frogeye leaf spot is a second disease that farmers may see at this time. It is named for the type of symptom that shows up on the leaf: small gray spots lined with reddish brown to purple borders that may look like dark halos to some people.
The gray spots appear brown with dark fuzzy hairs on the underneath side. Some alternative herbicides to glyphosate (Roundup) may cause spots on soybean leaves that resemble frogeye, but will not have hairs on the underneath side.
Most soybean varieties are resistant to frogeye. Farmers should have made an attempt to plant resistant varieties. However, if a susceptible variety has frogeye spots prior to early pod development (growth stage R3) a fungicide should be applied to the field.
White mold is the third disease, and farmers may consider the need for a fungicide. The fungicide should only be applied to fields that have had white mold in the past.
Visual symptoms of white mold infection will show up later in the season. Plants will at first appear wilted, then the leaves will turn brown and the stems will prematurely die about two weeks earlier than normal plants.
Stems will be covered with a thick whitish mold. Premature death of plants may cause severe yield reduction.
Fields initially get white mold when the hard black fruiting structure called sclerotia gets transferred from infected fields to healthy fields. This transfer generally occurs in seed or harvesting equipment.
Sclerotia are produced in the stems of infected plants. At harvest time the sclerotia gets mixed in seed that a farmer saves from a field or in soybean stems and debris that get carried by a combine. Sclerotia resembles mice droppings.
During cool wet weather at flowering, sclerotia in the soil will produce new spores that will land on nearby soybean flowers and start the disease process. Fields that have a history of white mold should be planted to varieties with resistant genes, and apply Aproach or Topsin M fungicides at first flower.
Fortunately, most fields in Hancock County do not have a history of white mold, but it is more of a problem in adjacent counties. Many of the newer soybean varieties have better tolerance to white mold.
Most farmers in our area have selected soybean varieties that have good tolerance to the mid-season diseases and will not need fungicides. Fungicides are expensive and farmers will only use them to protect yield when conditions tilt the scale in favor of the disease.
Individuals can obtain more information from OSU Extension fact sheets about mid-season soybean diseases at the following websites:
Brown spot:
White mold:
Lentz is extension educator for agriculture and natural resources for The Ohio State University Extension Service in Hancock County. He can be reached at 419-422-3851 or via email at
Lentz can be heard with Vaun Wickerham on weekdays at 6:35 a.m. on WFIN, at 5:43 a.m. on WKXA-FM, and at 5:28 a.m. at 106.3 The Fox.


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