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Symptoms and Signs

Phytophthora root and stem rot is caused by the oomycete plant pathogen Phytophthora sojae. This water mold can infect seeds, seedlings, and plants in all reproductive stages of growth when soil conditions favor pathogen development. Symptoms usually become apparent one to two weeks after heavy rains and are most common on soils that are poorly drained.

Seed rot and damping-off. Phytophthora sojae causes seed decay, and pre- and post-emergence damping-off (Figure 2) of soybeans under wet and warm soil conditions. The optimum temperature for disease development is 25 to 30°C (77 to 86°F). Fields with extensive seed rot and pre-emergence damping-off often require replanting. A light brown soft rot may develop on roots or the hypocotyl as seedlings emerge from the soil. As the roots and or hypocotyls become colonized, the seedlings may die.

Figure 2

Root and stem rot. The severity of the infection on soybean plants in the vegetative and reproductive stages of growth is directly related to the level of resistance in the plant. In highly susceptible cultivars, P. sojae colonization begins in the roots, then spreads several nodes up the stem. The roots and stem turn a chocolate brown color, the leaves of the plant turn yellow, and the whole plant turns a reddish-orange to orange-brown color. Occasionally a lesion will only occur on one side of the plant, but it is continuous from below the soil line up the plant. The yellowed, wilted leaves cling to the plant as it dies (Figure 3). In highly susceptible cultivars, virtually every plant in the field may be killed during the course of the production season; cultivars with moderate to low levels of partial resistance may suffer between 20 and 50% stand loss. For cultivars with higher levels of partial resistance, the stem rot phase does not develop. Roots are still colonized and are light brown in color, and in some cases the plants may be stunted. There are usually no visible symptoms, other than reduced yields, under field conditions for cultivars with high levels of partial resistance.

Figure 3

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