Key soybean diseases to watch

By Stephanie Karhoff, Ohio State University

As soybeans move through pod fill, scouting for mid-season diseases allows growers to evaluate variety performance and make future management decisions. Key diseases to be on the watch for include sudden death syndrome (SDS), Sclerotinia white mold, Phytophthora root and stem rot, frogeye leaf spot, and a new concern for Ohio soybean growers, red crown rot.

SDS is caused by a soilborne fungus that infects soybean roots early in the season. However, foliar symptoms typically do not become noticeable until flowering and pod fill. Leaves develop yellow spots between veins that expand into brown, interveinal necrosis while the veins remain green. In severe cases, leaf tissue drops away, leaving petioles attached to the stem. Fields with a history of soybean cyst nematode (SCN) often experience greater SDS severity. Fortunately, growers can take advantage of continued support from the Ohio Soybean Council and submit up to two soil samples to the OSU Soybean Pathology and Nematology Lab for free SCN testing.

Above-ground symptoms of SDS can look almost identical to red crown rot and brown stem rot, so accurate diagnosis requires digging up plants and taking a closer look at the stem and roots. Red crown rot produces reddish discoloration at the stem crown and may form small, red fungal reproductive structures near the soil line. Unlike SDS, severely affected plants often die with leaves remaining attached to the stem. The easiest way to distinguish between SDS and brown stem rot is to split the lower stem lengthwise. SDS-infected plants have a white, healthy-looking stem pith (center), while brown stem rot causes a characteristic brown discoloration of the pith.

Also be on the lookout for Sclerotinia white mold and Phytophthora root and stem rot. When scouting for white mold, look for patches of prematurely dead plants, especially in low-lying or heavily shaded areas of the field. Infected plants often develop a bleached, gray-white appearance on stems, with fluffy white fungal growth and hard, black survival structures (sclerotia) present on or inside stems. Fungicides are most effective for white mold when applied during flowering before symptoms are present. Phytophthora root and stem rot symptoms later in the season range from dark brown stem lesions extending upward from the soil line to wilting or prematurely dying plants.

Growers should also watch for frogeye leaf spot, a foliar disease favored by warm, humid weather. Symptoms begin as small, circular lesions with gray centers and reddish-brown to purple margins and are often most noticeable in the upper canopy. Frogeye leaf spot can be easily mistaken for paraquat injury and other disorders so be sure to check spray application records and assess whether symptoms follow a regular pattern or are more sporadic.

When considering fungicide applications, remember that SDS, brown stem rot, red crown rot, and Phytophthora root and stem rots cannot be managed with foliar fungicides. Fungicides are targeted toward foliar diseases such as frogeye leaf spot and generally provide the greatest benefit when applied around the R3 growth stage (beginning pod) and when disease pressure is present. In low-disease environments, yield responses are often inconsistent and may not cover application costs.