Pythium Seedling Blight Potential

April 28, 2000
This winter most of Illinois experienced unusually dry soil conditions. The warmer-than-usual conditions this spring have pushed early planting of corn in many areas around the state. Over the past few weeks, however, we have experienced the typical roller-coaster weather patterns famous in Illinois. Recent cool and wet periods create ideal weather conditions for the fungi that cause seedling blights. In particular, fungi in the genera Pythium are active during periods of cool, wet weather and can reduce stands significantly.

Fungi that belong in the genera Pythium are called water-molds because they thrive in soils that are wet and cool (55° to 60°F). These fungi overwinter in the soil and in plant debris as oospores. Moisture is necessary for oospore germination, and it provides a medium for movement "swimming" of the germinated motile spores called zoospores, which infect the plant root system. Three to four hours of wet conditions can be sufficient for initiating zoospore production.

Corn

While Pythium may cause minimal damage to germinating corn, this fungus can infect a substantial portion of the developing root system, including the mesocotyl, that attaches the primary roots of the seed to the developing seedling, the primary or seminal roots. Infection of the mesocotyl can result in loss of the primary root system, causing the developing seedling to die, unless adequate secondary roots have developed. Corn plants during the first few weeks after emergence may grow more slowly and appear less healthy when only their primary roots are infected with Pythium. Root tips or the entire root system of the corn plant can become infected with Pythium, appearing brown and becoming soft-rotted and water-soaked. Often the outer tissue of the root is infected and may peel off, revealing a white stele. On severely infected plants, symptoms may include root system discoloration, along with yellowing and stunting of the aboveground plant.

Soybeans

Infection of the soybean plant by Pythium can occur before emergence, causing rot of the germinating seed and seedling or, after emergence, causing "damping-off" of the young seedling. Depending on soil moisture levels, seed rot and damping-off can occur in small areas or large sections of the field. Rotted seeds are soft and fail to germinate. Damping-off symptoms on the seedling before emergence include rotting of the cotyledons, a soft-rotting of the hypocotyl, or rotting of the root system. After emergence, the stem and seedling may die, and its tissue will appear soft and rotted.

The potential exists for development of Pythium in early-planted corn if cool, wet weather conditions persist and corn does not germinate quickly. Seed treatments may provide protection for 10 to 14 days after planting. Other control measures are based on cultural practices that improve conditions for the seed and for seedling emergence: improve drainage in low, wet areas of the field, and avoid planting too early when soils are wet and cold, especially in no-till or conservation-tillage fields with heavy crop residues that tend to retain moisture. Soybean producers need to consider which fields have a history of seedling blights or which fields contain soils high in clay and have a tendency to retain soil moisture or drain slowly. In these cases, consideration should be given to using a seed treatment. For additional information on soybean seed treatments for control of Pythium, see "Soybean Seed Treatments for 2000" by Bruce Paulsrud and Wayne Pedersen in the April 7, 2000, issue no. 2 of the Bulletin. Resistance is not an option for controlling Pythium in corn or in soybean.--Loretta Ortiz-Ribbing

Author: Loretta Ortiz-Ribbing