Reduce impact of root rot diseases

A menagerie of soybean root rot diseases are quietly overwintering in your soils, patiently waiting to dine on tender, succulent shoots in early summer.

Reduce impact of root rot diseases

A menagerie of soybean root rot diseases are quietly overwintering in your soils, patiently waiting to dine on tender, succulent shoots in early summer.

Thankfully, you have ways to minimize or prevent these fungal diseases from claiming your crop. You can plant resistant varieties in well-drained locations. You can select the appropriate chemical and coat seeds to protect them.

Operating under the “forewarned is forearmed” premise, plant pathologist Dean Malvick, University of Minnesota, offers this overview of soybean root rots that he sees across the state and how best to reduce their impact on your crop.

Rhizoctonia root rot. This pathogen infects seeds and roots, causing younger plants to die in groups in a row or in patches. It is identifiable by its red-brown color on the root or stem. Research at Waseca looked at various seed treatments, including Apron and some experimental products. Researchers infected plants with rhizoctonia root rot and evaluated eight treatments. They noted increased plant stand with several treatments, but yield did not significantly increase.

Phytophthora rot. “This is new in the sense that there are new races,” Malvick says. “With the right weather, they reach their potential.” This disease kills soybean plants at all growth stages. Outbreaks are generally associated with water-saturated soils.

A common symptom is dark brown discoloration on the outside of lower stems. It can look similar to stem canker at this stage in mid- to late-season beans. In the early part of the season with seedlings, it looks very similar to infections caused by pythium.

The best seeds treatments are Apron XL, Allegiance and other forms of Metalaxyl. If you’re looking for resistant varieties, choose ones with Rps genes (resistance to Phytophthora sojae) that are effective against specific races. For example, Rps 1k and 1c are effective in many areas, but where they are not, Rps 3 or 6 may be.

“It’s a long, drawn-out process to identify races and which varieties hold up,” Malvick adds. “It takes time and money. However, it can be done.”

Pythium seed and seedling rot. Cool, wet soils provide a good home and environment for this fungus to infect and kill seeds and seedlings. It’s tough to identify since it cannot be distinguished from phytophthora damage to seeds or seedlings in the field. Your best bet: Treat seed with Apron XL, Allegiance or other forms of Metalaxyl.

Charcoal rot. Usually this pathogen is found in drier fields and in hot, drought-like conditions in other parts of the Midwest. But it has found a home in Minnesota in some fields. Growers can identify it by its gray lines and pepper-like specks in the plant roots and stem.

“You usually don’t see the symptoms until late season when plants are stressed,” Malvick says. At the present, there isn’t a good management strategy for the disease in Minnesota.

Fusarium root rot. “These pathogens affect the whole plant and rot the roots,” Malvick says. “These pathogens significantly reduce root size and function.”

The pathogens live a long time in the soil and affect corn, too. Crop rotation does not deter them, and fungicide seed treatments are not effective.

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This article published in the February, 2011 edition of THE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.

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