Phytophthora Root and Stem Rot (Soybeans)

Phytophthora sojae

Soybean plant infected by Phytophthora Root and Stem Rot leading to its premature death
Soybean plant infected by Phytophthora Root and Stem Rot leading to its premature death. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Phytophthora root and stem rot is a soil-dwelling pathogen and has been known to cause severe economic losses in soybean crops. It thrives in wet conditions and is often the cause of seed decay and pre/post-emergence damping off. In mature plants with no resistance, the pathogen will turn the stem brown, the leaves yellow, and ultimately kill the entire plant.
Phytophthora live in soil and soybean residue, usually persisting for several years. Infection is more likely to occur in soils that are warmer (70 to 80 degrees F) and poorly drained. It is known to infect plants throughout the season, and above-ground symptoms usually do not appear for 2 weeks after infection.
Initial symptoms in mature plants begin at the base of the stem where brown discoloration will likely appear at the soil line. Previous to this initial visible symptom, the taproot will be brown and discolored, usually decreased in overall root mass. Plants infected will likely be stunted, giving fields an uneven appearance. As the disease progresses, dark brown or reddish lesions move up the stem, eventually becoming water-soaked and rotted. Leaves begin to turn yellow. Wilting will soon follow, succeeded by death of the plant itself. In resistant varieties, symptoms may end at the wilting of some leaves, causing the plant to be stunted, yet alive.
Reddish-brown lesions progressing up the stem of a Phytophthora infection in a soybean plant
Reddish-brown lesions progressing up the stem of a Phytophthora infection in a soybean plant. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
In certain areas with continual soybeans, entire fields may be destroyed by Phytophthora sojae. Damage largely depends on the stage of the plant infected as well as its level of resistance. With the development of more resistant varieties, yield loss has ranged from 5 to 50 percent. Phytophthora is capable of attacking and killing soybeans at any stage of growth throughout the season.
The most effective mechanical means for control are through establishing proper drainage and soil structure. Tiling and tillage can be used to improve drainage. It is also helpful to modify planting dates based upon soil moisture and temperature. In heavier or no-till soils, early planting is not advised. Due to the pathogen's ability to survive for long periods of time in the soil, crop rotation will not be very effective, if at all.
Fungicides seed treatments are used to control this disease and are most effective at highest rates possible. Different chemicals used are mefenoxam and metalaxyl. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
There are varieties of soybeans that have some resistance to Phytophthora root and stem rot. It is essential that resistant varieties are used, as some stands that were not have experienced up to 100 percent yield loss.