Plant Health Initiative

Seedling Diseases - Scouting

Soft, mushy root and stem rot are diagnostic damping-off symptoms. Identification of the causal pathogen requires more information, including soil factors and plant variety. Photo credit: X.B. Yang, Iowa State University

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Scouting for soybean seedling diseases
Identification of soybean seedling diseases
X.B. Yang, Iowa State University

Several fungi can cause stand establishment problems either before or after emergence. When digging in the seed bed, look for rotted or mushy seed. As seedlings emerge, scout for decay on roots and stems. Plant death can be rapid. Often, there can be an uneven distribution of mortality in the field, or the poor stands may be limited to especially wet areas of the field.

In addition to plant symptoms, gather the following information on the field conditions:

• general soil temperatures
• general soil moisture - saturated, wet, dry
• variety resistance or tolerance to Phytophthora
• plant growth stage

Together with plant symptoms, this information can be used to determine the casual agents.

• If the infected plants have good resistance to Phytophthora, the chances are high that Pythium is the cause when temperatures are cool.
• Seedlings become more resistant to Pythium after V2 growth stage. If you continue to observe dying plants after V2 growth stage, the chance that Phytophthora is the cause increases.
• Phytophthora sojae and Pythium species infect most readily in very wet to saturated soil conditions
• Rhizoctonia solani and Fusarium prefer drier soils.
• Many Pythium species prefer cool soil (50 to 60°F). Pythium is often the first pathogen that causes problems after planting.
• Symptoms of Pythium and Phytophthora infection are difficult to distinguish from each other. A rule of thumb often used is that if damping-off occurs in warm soil (70-80° F), it is more likely to be caused by Phytophthora. If it occurs in cool soil (50-60° F), it is more likely to be caused by Pythium.
• Rhizoctonia tends to be most active in warm soils over 74°F

It may not be necessary to distinguish between Pythium and Phytophthora diseases because the active ingredient in seed treatments to used control them are the same. However, if a variety with Phytophthora resistance genes (Rps1k) has Phytophthora damping-off, it indicates the possibility of the occurrence of new races in the field. If you want to determine the causal agent, you can send a sample to your state Plant Disease Clinic.