Wheat Root Rot

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This week, we received several samples of stunted wheat from Robeson, Lenoir, and Beaufort. There are more reports of wheat “going backward” in the Tidewater area. Contrary to my earlier message, we have NOT found downy mildew. The problem we have confirmed is a root rot caused by a related soilborne pathogen. This is Pythium root rot. Pythium is a fungus-like organism called an oomycete, and it is widespread in soil and plant debris. What has brought on the root rot problems is frequent excessive rain and prolonged standing water.

The wheat in this March 29 photo was badly damaged by Pythium root rot at the Lenoir County OVT. Root systems rotted over the winter, and were unable to take up nitrogen at top-dress. A few plants had intact roots so they could absorb the nitrogen and start jointing. This is why we started noticing the stunting after top-dress. The level of damage depended on position in the field, with lower areas hardest hit.

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We have tested and ruled out wheat soilborne mosaic virus and wheat spindle-streak mosaic virus in all of these samples.

Should growers abandon fields with widespread stunting of this type? It depends on how widespread and severe the damage is. No chemical remedy is available, and additional nutrients will not help a crop with severe stunting of roots and aboveground parts.

For the future:

Pythium root rot:

Main symptoms are severe stunting, with missing or poorly tillered plants, and brown rotten areas at the bases of the most stunted tillers. The first true leaf is often noticeably shorter than a normal, healthy first leaf. There is no known variety resistance. The pathogen remains in and on crop residues and can persist for years. Straw on the soil surface or only slightly incorporated will favor Pythium root rot. Waterlogged soil is the chief reason for a severe outbreak. Thus, good drainage is a key control measure. High-quality seed should be used in fields where drainage is problematic, and seed treatments containing metalaxyl or mefanoxam may be advisable. Seed age can significantly impact a seed’s sensitivity to Pythium root rot. Older seeds or seeds that have been coated with seed treatment for an extended period of time are more prone to infection, because pesticides tend to degrade the integrity of the seed coat and cause the seeds to ‘leak’ more exudates when they are germinating. The longer the seeds have been treated, the more susceptible they become. In general, seeds coated for 1 year or more could be more prone to Pythium root rot.