Brown stem rot (BSR) is a major disease of soybeans in Wisconsin and in the Midwest. Incidence of brown stem rot has greatly increased over the past few years and the disease is found in all soybean-producing areas of Wisconsin. The increase in brown stem rot seen in recent years has been attributed to an expansion of soybean acres and shorter rotations.

While some soybean diseases are related to stressful growing conditions, brown stem rot is most severe when conditions are optimal for soybean growth. BSR is frequently associated with fields that do not respond to management practices and have difficulty achieving yields of 50 bushels/acre. 

Brown stem rot is caused by the fungus Phialophora gregata which lives in soil and soybean residue. The pathogen infects through the roots and invades the vascular system of soybean plants. The infection eventually impedes the movement of water and mineral nutrients needed for growth. 

Late-season soybean field with symptoms of brown stem rot

Yield losses to BSR of 10-30% are common. BSR may reduce both seed number and seed size. Severely diseased plants may also lodge more easily than noninfected plants. Yield loss is greatest in cultivars with a longer relative maturity and if both foliar symptoms and stem symptoms are present.  Brown stem rot will have a greater negative effect on yield if temperatures are cool (64-75F) in early August, followed by hot, dry weather during late pod-fill.

Scouting for Brown Stem Rot 
Recognition of the disease is a major problem. Browning of the internal stem and root systems of infected plants are present as early as the V4 growth stage, and intensify as the plant progresses into reproductive stages. However, there is no external evidence of BSR at this time and signs of early infection will go unnoticed unless the stems are cut open and examined.

In late July and early August (R4 to R5 growth stage), the stem browning may be accompanied by a sudden interveinal chlorosis and necrosis of the leaves, wilting, and defoliation. Symptoms of brown stem rot are often confused with early crop maturity or the effect of dry soils. Excessive leaf retention at maturity is another clue that BSR may be present. Farmers and agricultural consultants should know the symptoms and controls of this important soybean disease.

Disease Cycle
The brown stem rot fungus appears to be widely distributed throughout Wisconsin, but it only increases to economic levels when soybeans are continuously or frequently grown in a field. The fungus survives in infested debris in the soil, but drops to low populations if crops are rotated. Soybean is the only host for the brown stem rot fungus.

The brown stem rot fungus produces spores that germinate, invade soybean roots, and progress into the vascular systems of soybean seedlings. The fungus remains restricted to the lower stem until the plant starts pod formation. At that time, the pathogen advances up the stem, especially when cool air temperatures (60-75 F) prevail. The height of internal stem discoloration depends on the susceptibility of the variety and the duration of the cool temperature.

Different types, or pathotypes, of P. gregata appear to vary in their pathogenicity. Two types have been distinguished genetically and in the severity of disease they cause. Pathotype "A" (defoliating pathotype) causes more severe internal stem symptoms and defoliation than the other pathotype, called Pathotype "B" or "nondefoliating". Different ratios of the pathotypes may be present in each field.

Infection by the fungus occurs over a wide range of soil conditions. Greater yield reduction often occurs when dry soil conditions prevail during at least part of the growing season. Vascular infection coupled with low soil moisture increases the water deficiency in diseased plants, leading to reduced pod fill and smaller seeds. However, yield losses may be reduced during years of high rainfall or optimal irrigation. The appearance and severity of foliar symptoms are also favored by dry soil conditions.