Introduction

Phytophthora root and stem rot is a common pathogen in North Carolina due to our frequent wet weather. Because the pathogen is soilborne, the disease can significantly impact yields in fields where disease is severe. Although it is favored by wet soils, symptoms are often present under hot, dry conditions that typically occur in late summer.

Pathogen

Phytophthora root and stem rot of soybean is caused by Phytophthora sojae. P. sojae is not a true fungus, but belongs to a group of fungal like organisms called oomycetes (water molds). P. sojae reproduces asexually via the production of sporgangia and zoospores. Zoospores are capable of swimming short distances towards plant roots to germinate and cause new infections. P. sojae is capable of producing oospores that act as survival structures that can remain in soil or crop residues and cause new infections.

Symptoms

Although symptoms often appear later in the growing season, infections occur early in the season. Seeds, seedlings, and plants in all stages of growth are susceptible to disease when the soil and environmental conditions favor pathogen development. Disease is more severe under warm, wet conditions.

Infections begin in the roots and then spreads upward to the lower nodes of the plant. Plants may be stunted and chlorotic early in the disease cycle. The roots and lower stem turn a dark brown color, the leaves of the plant turn yellow and necrotic. The wilted leaves remain attached to the plant.

Life Cycle and Favorable Conditions for Disease

Oospores can survive for several years in a dormant state and can survive freezing and long periods of cold temperatures. Oospores germinate to directly infect plants or form sporangia that invade roots under high soil moisture conditions. As P. sojae colonizes the plant, the vascular system and lower stem tissues can occur on the lower stems. The pathogen survives as oospores in crop residue.

Management

Cultural practices that improve soil drainage is important to limit the spread of P. sojae. Because infections often take place early in the growing season, use of a seed treatment or soil-applied fungicide may be beneficial to reducing infections by P. sojae. Chemistries containing mefenoxam or metalaxyl are both effective to reducing disease caused by P. sojae.

Useful Resources

The NC State University Plant Disease and Insect Clinic provides diagnostics and control recommendations.

The Extension Plant Pathology portal provides information on crop disease management.

The North Carolina Agricultural Chemicals Manual provides pesticide information for common diseases of North Carolina. The manual recommendations do not replace those described on the pesticide label, and the label must be followed.

Phytophthora root and stem rot of soybean. The Plant Health Instructor.

Acknowledgement

This factsheet was prepared by the NC State University Field Crops and Tobacco Pathology Lab in 2020.

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