Two of the most common root diseases observed in Iowa soybean fields in July are Rhizoctonia root rot and Phytophthora root rot. In the spring, both Rhizoctonia and Phytophthora can cause seedling blights but now these fungi are causing soybean root rot. The ISU Plant Disease Clinic has received a few samples of Rhizoctonia root rot. The Phytophthora root rot also starts to show up in some soybean fields.
In a normal year, Rhizoctonia root rot is found in latter June and July. The number of samples decreases, and the extent of the disease also decreases, as soybeans grow into the reproductive stage. However, the Phytophthora root rot will continue to cause damage to soybeans, especially if there are frequent and periodical rains in late July and August.
Rhizoctonia root rot is caused by the fungus, Rhizoctonia solani. The disease generally is first noticed by the presence of wilted and dead plants in early summer. In most cases, diseased plants are scattered in fields as a single plant or group of dead plants in a row or in circular areas. Often they are noticeable on hillsides as uneven growth of the plants.
Reddish-brown lesion of Rhizoctonia root rot.
Typically, the most common symptom of Rhizoctonia root rot is reddish-brown lesions on lower stems at the soil line. The reddish-brown color is most evident immediately after a plant is removed from the soil. If conditions favor disease development, lesions may enlarge down to the root system and develop into sunken, reddish brown dry rot (see photo). Fields that had Rhizoctonia seedling blights in the spring have a high chance for root rot in early summer.
When the weather becomes relatively dry and windy, plants with extensively damaged stems and roots may wilt and die. That is why we see more Rhizoctonia rot on hillsides. Severely affected plants may remain stunted or less vigorous during the rest of the growing season. If the pathogen only causes minor root rot, plants may recover from infections by growing new roots. Use cultivation to mound soil around the base of the plant to promote root growth above the diseased portion of the plant. There are no resistant varieties available and rotation is not effective to control this disease.
Plants killed by Phytophthora.
Unlike Rhizoctonia, plants with Phytophthora root rot are most common in low, poorly drained areas, headrows where soil is compacted, and heavy clay soils. Symptoms of the disease also may appear on higher ground during a wet season. Use of no-till may increase the disease.
The disease pattern varies within a field. It may be circular, corresponding to poorly drained areas, or it may occur as a dead or dying individual plant or groups of plants in a row (see photograph on page 113 of this year's ICM newsletter). Severely affected fields have uneven appearance both in color and plant height.
Brown discoloration by Phytophthora root rot.
Typical symptoms of Phytophthora root rot in mid-July include yellowing followed by wilted leaves that remain attached to the plant (see top photo). Another diagnostic symptom is a brown discoloration of the stem and lower branches. Unlike Rhizoctonia, the lesion of Phytophthora extends from the soil line upward (see bottom photo). The tap root is dark brown and the entire root system may be rotted and diseased plants are easy to pull out.
The disease can be effectively managed by using resistant varieties. If you find the disease and have not used any resistant varieties, select varieties with genes resistant to major Iowa Phytophthora races, races 1, 3, and 4, such as the Rps-1k gene. If the fungi defeat your resistant varieties, consider using other tolerant varieties.
This article originally appeared on pages 128-129 of the IC-476(18) -- July 15, 1996 issue.