Plant Health Initiative

Phytophthora - Symptoms

The Phytophthora fungus can infect and kill plants at all stages of growth, from seedlings to older stands. Infected stands may survive but are less productive than healthy stands.

Soybean plant infected with Phytophthora sojae. Click on image to view a larger version. Photo credit: Anne Dorrance, Ohio State University

Figure 2. Brown lesions that begin in the roots and progress up the stem from the soil line are diagnostic for Phytophthora Click on image to view a larger version. Photo credit: University of Wisconsin

Damping-off phase

Soybean seeds can be infected before emergence, causing them to rot. Phytophthora often kills young seedlings shortly after emergence. Plants infected at this stage will show typical "damping-off" disease symptoms.

However, several other fungi can cause damping off at this stage, too, such as Pythium and Rhizoctonia. Generally, damping off by Pythium occurs when soils are still cold, while Phytophthora fungi are active when soils warm to 60°F or higher. Laboratory tests can distinguish seed and seedling rot caused by Phytophthora sojae from Pythium or other seed-rotting fungi at this stage.

Symptoms of infection at later growth stages may include yellowing, wilting and death of plants, or stunting and yellowing with no death.

The symptoms will develop 5 to 14 days after soil has been saturated and are often confused with flood damage, or can occur along with flood damage. Flooding injury to soybeans destroys the cortical cells of the roots but leaves the root stele (center) intact. Phytophthora grows in all parts of the soybean roots and turns the entire root tissue a tan to dark brown color.

Stem lesions are a diagnostic symptom

A key diagnostic symptom of Phytophthora stem rot is a brown lesion that develops from the roots and progresses up the stem from the soil line. The key difference between Phytophthora stem rot and other stem cankers is that there will be little to no roots on a plant that was attacked by P. sojae, and the infection occurs from the base of the plant. Other pathogens that cause stem canker do not cause root rot.

On occasion, the Phytophthora lesion may develop on only one side of the soybean stem. Cut the stems open to determine if the internal tissues are colonized from the soil line to the top of the lesion.

Root rot phase

The root rot phase is not as readily recognized as the killing stem rot phase. Plants will be a lighter green, and may be stunted and exhibit uneven growth. Stems of older plants may become watersoaked and eventually the leaves will turn yellow, wilt, and die. These symptoms are the secondary effects of an impaired root system that is less efficient in supplying the plant with water and nutrients.