Phytophthora Root Rot - Disease Symptoms, Development, and Management Considerations

Phytophthora root rot (PRR) is caused by the fungus Phytophthora sojae. Like Pythium, this fungus is a “water mold” and thrives in wet soil conditions. Infection by Phytophthora is favored in compacted soils and in poorly drained, fine textured and clay soils where temporary flooding may occur and drainage is poor. The disease can be found throughout most major soybean growing areas of the U.S. and Canada, but is most prevalent in areas with heavy, poorly drained soils.

Phytophthora has two distinct phases, which in effect act like different diseases:

• Phase 1: the seed rot and damping off phase
• Phase 2: the root and stem rot phase

The first phase attacks newly planted soybean seeds that have swelled from moisture intake (imbibition) or young seedlings at or prior to emergence. Seeds simply rot and disintegrate in the furrow. Seedlings have a soft decay of the lower stem, leading to yellowing, wilting and rapid death. Stem tissue becomes dark and discolored. Taproots usually are dark brown and smaller feeder roots are rotted or missing.

Symptoms of the root and stem rot phase of Phytophthora occur later in the plant’s development (e.g., July in the upper Midwest). As the fungus invades through the roots and grows into the lower stem, symptoms are manifested as a dark-brown discoloration of the lower stem that gradually moves up the plant from below the ground. This rotting from the base of the plant is a key diagnostic symptom of Phytophthora. Another key identifier is root disintegration – there will be little or no roots on affected plants. A final symptom is wilting of the plant. Beginning at the bottom of the plant and progressing upward, leaves wilt, then petioles (leafstalks) droop. Leaves of affected plants may develop a grayish cast and then yellowish spots prior to the permanent wilt of the plant.

Phytophthora sojae persists in soils as oospores, microscopic seed-like structures in crop residues. They can survive for many years in soil after residues decompose. The oospores germinate whenever the temperature and soil moisture levels are suitable for forming sporangia. Sporangia germinate directly to attack plants or indirectly to form zoospores. Zoospores swim through films of water to infect soybean roots , and the fungus subsequently grows into and among the root cells of the plant. Wet conditions and soil temperatures near 60 degrees enhance this disease. Phytophthora severity increases in low, poorly drained or compacted soils, in soils with high clay content or any unusually wet soils.

Variety selection is the key for Phytophthora stem and root rot control. Both race specific resistance and field tolerance are available in soybean varieties. At the very least, growers should avoid planting susceptible soybean varieties in fields prone to Phytophthora problems. The ability of all varieties to combat PRR can be further enhanced by the following practices:

1. Improve soil aeration and drainage.
2. Avoid activities that will compact the soil, including tillage at inopportune times.
3. Rotate crops. This will not eradicate Phytophthora, but can reduce fungus levels in the soil.
4. Ridge soil around the base of the plants during cultivation to promote root growth from the lower stem. This practice may suppress PRR when the root rot phase predominates. The ridges may have lower soil moisture, an environment less favorable for Phytophthora activity.