Soybean root diseases and wet soils

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Recent heavy rainfalls have delayed soybean planting, and put more pressure on growers to plant when conditions are less than ideal, but it’s a good idea to consider the effect of planting into wet soils on soybean seeds from a perspective of seed and seedling diseases.

During wet years, fields of newly-planted soybeans and soybean seedlings can be subject to significant losses from root-decaying fungi such as Pythium, Phytophthora,  Rhizoctonia and Fusarium.

Although each pathogen has an ideal temperature range for germination of spores and development of disease, one thing they all have in common is a liking for wet soils. Soils may be soggy due to poor drainage resulting from heavy clay, or compaction, or saturated from heavy rainfall. Soils may remain wet for extended periods of time, especially in low areas of fields where water collects. Fields that are under no-till or reduced tillage, tend to stay wet longer and warm up more slowly than fields under conventional tillage.

Pythium
spp. are common soil-borne pathogens that can infect seeds, seedlings and roots of soybeans at cool soil temperatures (40-59oF). Pythium can cause delayed emergence and poor stands, resulting in significant plant losses. Pythium belongs to a class of fungi called oomycetes, whose dead spots on roots where infection is mild, to dead root tips and loss of the taproot. Severely infected seedlings are stunted, chlorotic and may die. Symptoms caused by another oomycete, Phytophthora sojae, are similar. P. sojae is most severe in flooded or poorly drained soils, but its development is favored by warmer soil temperatures (68-77°F). A row of dead plants may occur in a low area or in patches in the field. Plants pull up easily from the ground, because there are almost no roots attached.

Rhizoctonia root and stem rot also tends to occur in patches, and is more common on seedlings and young plants. A reddish-brown lesion develops on the root and may extend upward to the stem. Plants look chlorotic due to lack of roots, but the chlorosis is sometimes mistaken for nitrogen deficiency. Wet soils, high organic matter and temperatures of 77 to 84°F are optimum for development of the disease, although symptoms become much more visible when plants are under drought stress later in the season.

Several species of Fusarium cause root rots that produce sunken, light to dark brown lesions and loss of lateral roots. The damage may go undetected during scouting, and the disease may only be evident when a loss of yield is noted. In a limited survey of soybean root and stem disease in Michigan last year, the most commonly isolated fungus from field samples collected was F.  oxysporum, but we have not yet determined how significant a role it may be playing in affecting soybean yield.

Regardless of the disease, good drainage is important for management. Take what measures you can to improve drainage. If your field has a history of seedling disease, fungicidal seed treatments may be helpful. Consult the field crops guide for a list of seed treatments and the fungi they are labeled to control. A few seed treatments are labeled for Phytophthora, but look carefully at the label for the rate to use. Use Phytophthora- resistant varieties if this disease has been a problem in the past. Try to delay planting into wet fields until planting conditions improve.

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