Pythium and Other Seedling Diseases in Crops
June 30, 2011
The extended periods of cool and wet conditions this spring are still causing the development of seedling diseases in corn, soybean, and sorghum. The most common seedling diseases identified in samples submitted to the UNL Plant and Pest Diagnostic Clinic are those caused by Pythium species.
Seedling diseases can be caused by any of several common soilborne organisms, such as Pythium, Fusarium, Rhizoctonia or plant parasitic nematodes. Seedling diseases are often difficult to diagnose because their symptoms are very similar. Sometimes diagnosis may be of limited value because management is the same for several seedling diseases. Microscopic examination and other laboratory analyses of the diseased seedlings can often identify the cause(s) of the problems. Seedling diseases also can be confused with insect injury, herbicide damage, planting problems, or environmental stresses that often have similar symptoms.
Symptoms of seedling diseases include:
- Rotted seed prior to germination
- Rotted or discolored seedlings after germination prior to emergence
- Post-emergence seedling damping off (Figures 1-3)
- Root decay
At least 14 species of Pythium have been identified that can cause seedling blight and root rot. These pathogens require excessive moisture because they produce motile swimming zoospores that infect plant roots. The pathogen overwinters in soil and infected plant debris by producing thick-walled oospores that can survive for several years in the absence of a suitable host or favorable weather conditions.
Although uncommon, Pythium also may cause stalk rot disease in corn during extended periods of wetness during the middle and later portions of the growing season. Symptoms of Pythium stalk rot can cause collapse of the lower stalks at or near the soil surface. Stalks may appear collapsed, twisted, and water-soaked and could be confused with bacterial stalk rot, but lack the characteristic foul odor.
Unfortunately, resistance is not available for diseases caused by Pythium. Although improved field drainage can help reduce seedling disease severity, the most common method for disease management is with the use of seed treatment fungicides. Oomycete fungi, such as Pythium and Phytophthora (soybean only), can be managed with seed treatment fungicides that contain the active ingredients metalaxyl or mefenoxam.
For more information on management of Phytophthora in soybean see NebGuide G1785, Management of Phytophthora Root and Stem Rot of Soybeans, which explains the use of resistant varieties and increased rates of seed treatments.
Almost all seed corn is already treated with more than one fungicide, often an insecticide, and, sometimes a nematicide. These products can provide protection against some of the pathogens that cause seedling diseases. But, in spite of their activity, diseases may still develop, such as during extended periods of inclement weather or under severe pathogen pressure. Seed treatments will only provide protection during the first few weeks immediately after planting. You can minimize the likelihood of developing seedling diseases by planting high quality seed at appropriate planting depths and soil conditions.
Tamra Jackson and Loren Giesler
Extension Plant Pathologists
Plant and Pest Diagnostic Clinic