NPIPM:Phytophthora root and stem rot on soybean
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, Dean Malvick and Lawrence Osborne
Phytophthora root and stem rot occurs on soybean growing regions worldwide. The disease was first reported in the US in 1948.
Causal Organism
Phytophthora sojae is the dominant causing pathogen of Phytophthora root and stem rot of soybean. However, another Phytophthora species than can infect soybean has been found in the Midwest. Phytophthora species are members of the Oomycetes, commonly known as water molds.
Symptoms and Signs
Seed or seedling infection may cause pre- or post-emergence damping-off. Infected tissues turn brown in color. Infection may occur throughout the season. Mid- or late-season infection causes brown lesions on the roots followed by root rot. A characteristic chocolate brown discoloration on the stem starts on the base of the plant and advances up the stem. Leaves turn yellow and wilt, typically staying attached after the plant dies. Plants are often killed in patches or in sections of rows.
Disease Impacts
Phytophthora root and stem rot decreases the yield by reducing soybean stand, weight per seed and the number of pods per plant. A field study showed 7-35% yield reduction on susceptible cultivars compared to resistant cultivars due to Phytophthora infection.
Life Cycle and Epidemiology
The causal pathogen survives in the soil and infested plant debris as spores called oospores for a long period of time. Oospores are typically produced in abundance in susceptible hosts. Oospores germinate and produce sporangia under 77-86˚F (25-30˚C) and saturated soil conditions. The sporangia produce and release zoospores that are attracted to soybean root exudates. Once zoospores reach the root surface, they germinate and send their mycelia to penetrate the root tissue to initiate infection.
Phythophthora can attack seeds and seedlings prior to or anytime after emergence. Infection occurs most readily in saturated soil conditions early in the season, but severe symptoms may not appear until late in the summer. Since soil saturation is important for the oospore germination, wet weather and compacted soil favor Phythophthora infection.
Management Approaches
Cultural Methods
Tillage promotes soil drainage, which in turn limits infection and can bury oospores deeper in the soil profile. Tillage may have very little effect on Phytophthora root and stem rot in soils with high clay content.
Since the oospores may survive for long periods of time in the soil, crop rotation may not reduce inoculum. Nevertheless, crop rotation was shown to slow population buildup of P. sojae in a given field.
Host Plant Resistance and Tolerance
Resistance against P. sojae is conditioned by Rps genes on soybean cultivars. The first Rps resistance gene was identified in the early 1950s. Subsequently eight different Rps loci have been identified to date. However, races of P. sojae able to overcome individual Rps gene have been identified in the Northern Plains.
Tolerant soybean cultivars against P. sojae are available. In tolerant cultivars, Phytophthora infection may cause limited root rot and stunting, but does not kill the plants.
Chemical Control
Seed or in-furrow treatments with metalaxyl or mefenoxam fungicides are recommended for management in fields with history of Phythophthora root and stem rot.
