Soilborne Pathogens Explained

Soybean yield is often limited by soilborne pathogens. The average United States soybean yield is 43.9 bushels per acre or only about 27 percent of known yield potential (160 bushels per acre). The difference between harvested yield and potential yield is lost to a plethora of environmental stresses such as heat and drought as well as a complex of poorly understood soilborne pathogens. These pathogens invade plant roots causing tissue decay, a reduction in the number of root tips, a decrease in nodule size and loss of root function.

Major yield losses are found in areas where soils have high clay content or remain saturated for long periods of time. The key to improving soybean yield is to improve root health.

One aspect of improving root health is increasing the number of root hairs. Water and nutrients are absorbed into the root just behind the root tip so increasing the number of root hairs means more water and nutrients can be absorbed and transported to the stem and leaves. In addition to anchoring the plant, healthy roots trigger faster plant growth and seed development when environmental conditions are not growth limiting.

When roots are damaged, the plant’s ability to obtain water and nutrients is greatly reduced. Removing damage to the root removes the stress and helps avoid unnecessary yield loss.

The most important pathogens associated with seedling root health include the soybean cyst nematode, Pythium species, Phytophthora sojae, Rhizoctonia solani, and Fusarium species. Iowa State University researchers have found each of these pathogens impairs root health alone and in combination with each other.

Pythium species cause seed decay, pre-emergence damping off and early post-emergence seedling death. Seed with Pythium species may decay before germination and, as they do, become soft and rotted. Phytophthora sojae also causes a soft, wet rot of seed or seedling tissue similar to that of Pythium species. Infected seedlings may die prior to emerging from the soil or shortly thereafter.

Symptoms of Rhizoctonia solani infection generally show up on seedlings as dry, dark reddish-brown lesions just above the soil line. Seedling loss from Rhizoctonia seedling blight is less common than from Pythium seed decay and Phytophthora root rot, but when present, stand loss can be severe.

The traditional soybean-corn production system creates a monoculture environment that promotes the build-up of root-infecting pathogens.

Researchers at Iowa State University report that within the soybean-corn rotation, there may have an inadvertent increase of populations of root-infecting pathogens, thereby increasing root damage. For example, there are reports from The Ohio State University that isolates of Pythium species and Rhizoctonia solani are becoming resistant to commonly used fungicide seed treatments. They suggest that if these isolates are pathogenic to both soybean and corn, it is likely the pathogen inoculum is also increasing in the field. Thus, an increased knowledge of the role of Pythium species and Rhizoctonia solani in the soybean-corn rotation is necessary for improving yield.

Improving Root Health

The first 10 to 14 days following soybean planting is the window of opportunity for Pythium and other soilborne fungi to infect the plant. Implement practices that promote quick germination and seedling emergence to prevent infection.

• Plant high quality seed and plant into a good seedbed
• Plant seed that has resistance to Phytophthora sojae. Specific resistance is controlled by major resistance genes such as Rps1k. In addition, select varieties with high partial resistance (tolerance) when possible.
• Plant where drainage is adequate, or improve drainage if possible. Avoid low areas of fields, especially if a field has a history of seedling or root rot problems.
• Consider that germination will be quickest when soil temperatures are greater than 65°F

What to Do if Seedling Diseases are Present

• If dead plants are scattered throughout the field and no significant stand reduction is apparent, then no action is needed if the disease is Pythium or Rhizoctonia damping off.
• If seedling loss is severe, consult a certified crop advisor to determine whether replanting is necessary. Replanting is not always necessary.
• Take good disease notes and use preventive measures such as seed treatment or resistant varieties to reduce the likelihood of a disease problem in the next soybean crop.

If Replanting is Necessary

If poor stands are due to diseases, fungicide seed treatments can be used in replanting to avoid further damping off, especially for Rhizoctonia and Phytophthora. If poor stands are due to a low germination rate, a seed treatment cannot improve seed germination rate or seed vigor.