Phytophthora damping off Click on image to view a larger version.

Stem lesions progressing from the soil line are a key symptom of infection by Phytophthora sojae. Click on image to view a larger version.

The Phytophthora fungus can infect soybean seeds, causing seed rot, or it can cause yellowing, wilting, and death of seedlings as they emerge. Infected plants are easily pulled from the ground because of rotting roots. It is difficult to distinguish Phytophthora root rot from Pythium root rot at this stage; both diseases cause root rot and seedling death. Generally, Pythium is active in cold soils, and Phytophthora active in warm soils.

Infected stems of older seedlings in the primary leaf stage may become watersoaked and eventually the leaves will turn yellow, wilt and die. As plants age, they die more slowly after infection. Plants infected before flowering will yellow, followed by wilt and death. The leaves remain attached after death.

Stem lesions are a diagnostic symptom of Phytophthora
A key diagnostic symptom of infection by Phytophthora sojae is a brown discoloration that progresses 6 to 12 inches up the stem from the soil line. Diseased root systems are smaller than healthy plants, and the taproot and lower stem are internally discolored.

The root rot phase of Phytophthora root rot is not as readily recognized as the killing stem rot phase. Although less drastic in appearance, the root rot phase can greatly reduce plant productivity. Infected plants in the root rot phase will be a lighter green, and may be stunted and exhibit uneven growth. These symptoms are cause by the diseased root system which is less efficient in supplying the plant with water and nutrients.

Phytophthora-infected soybean