Phytophthora root and stem rot of soybean
Phytophthora sojae can infect soybeans at any growth stage from seed to maturity. Early season symptoms include seed rot and pre- and postemergence damping off. The most characteristic symptom of Phytophthora root rot is a dark brown lesion on the lower stem that extends up from the taproot of the plant. The lesion often reaches as high as several nodes and will girdle soybean stems, restricting flow of nutrients and water, and stunting or killing the plant. Phytophthora root rot is more severe in no-till fields, poorly drained soils, or low-lying areas that are prone to flooding.
The oomycete pathogen survives on crop residue or in the soil as oospores. When soil temperatures reach 60°F and soil is saturated, oospores germinate and produce swimming spores, called zoospores. Warm, saturated soil, especially shortly after planting, are conducive to disease. Infection occurs via the roots, and from there the pathogen colonizes the roots and stems.
Phytophthora root rot is best managed by planting resistant varieties. There are soybean varieties available that have complete resistance to specific pathotypes (races) of P. sojae. There are numerous pathotypes, and many pathotypes can exist in a single field. Partial resistance (resistance made up of more than one gene) is available and is effective against all pathotypes. Where Phytophthora root and stem rot is a problem, seed treatments targeting oomycetes can provide some protection.
Gallery Images: C. Grau, C. Bradley, K. Black, and D. Mueller