Pythium and Phytophthora root rots of soybean

Diane Brown-Rytlewski, Michigan State University Extension, Department of Plant Pathology - July 13, 2006

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.

Despite recent dry weather, growers may still be seeing evidence of root rot diseases in soybean fields, especially those that were very wet earlier in the year. Pythium root rot and Phytophthora root rot can develop in poorly-drained or saturated soils or soil that suffers from compaction. No-till areas with fine-textured soils tend to be favorable environments for these diseases, as the soil can become more easily saturated than coarser textured soils. The swimming spores, (called zoospores) need saturated soil to move to soybean roots to cause infection. Seedlings infected die rapidly, leaving gaps in rows. Soil temperatures in the range of 50 to 59°F are favorable for Pythium; Phytophthora needs warmer soil temperatures (around 68°F) for production of zoospores. Heavy rainfall resulting in flooded fields within a week or so of planting creates the most favorable conditions for infection. The symptoms are similar for both diseases . Young plants turn yellow, collapse and die, leaving gaps in row plantings. In plants that have reached more advanced growth stages, foliage wilts and turns yellow starting at the bottom of the plant and moving up. Dead leaves may remain attached. Lower stem discoloration starts near the soil line and spreads upward. Root tips and feeder roots are infected, causing root rots. Check plants in low areas or poorly drained parts of the field for symptoms.

It’s been mentioned several times on our Field Crop CAT Alert conference calls this year that this is a year when properly tiled fields will pay off. Improving drainage to remove excess water will reduce the duration and amount of soil saturation, creating a less favorable environment for the spores to cause infection. There is no genetic resistance to Pythium root rot, but there are soybean varieties with tolerance to Phytophthora sojae that would be worth considering for next year if you have had problems with Phytophthora root rot this season.

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