Pythium Seedling Blight

April 23, 1999
As I sit in my office here in northeast Illinois, the sun is shining, but continued rain showers loom in the forecast for today as well as for the next several days. As a result, the planting season will remain on hold for many. The current weather conditions serve as a reminder about seedling blight
fungi, which can reduce stands significantly when cool, wet conditions persist through the early part of the planting season.

One genera of fungi active at this time is Pythium. Pythium species, commonly called "water molds" because of their activity in wet soils, attack both corn and soybeans. Typically, damage to corn is minimal, but some damage to small plants can occur. When Pythium infects the primary root system, very slow growth of seedlings is a common symptom. Although plant death from Pythium infections is not common in Illinois, poor early season stand development may occur until the plant has an adequate secondary root system that can replace roots lost to the fungus. Infected roots turn brownish in color and are soft and watery; the outer root layers may peel off. Although root tips are a common infection point, brownish areas scattered along the entire root may also be visible.

If the mesocotyl (the attachment of the primary roots to the developing seedling) is infected, the entire primary root system may be lost; this can result in seedling death unless the plant has produced adequate secondary roots. Wilting of the entire plant and discoloration of leaves are common symptoms of Pythium infection.

On soybeans, Pythium typically causes a seed decay or preemergence damping-off (death) of seedlings. Seed rot of a few plants to large areas of fields is not uncommon, depending on soil moisture levels. Preemergence damping-off symptoms include a rotting of the cotyledons or the seed itself shortly after germination or a soft-rotting of the developing stem (the hypocotyl) before the plant's emergence. After emergence, damping-off symptoms include a soft-rotting of the stem and death of the seedling.

Control of Pythium is based on cultural practices and seed treatments. Cultural practices include not planting too early in wet soils, particularly no-till or conservation-till fields with a heavy residue cover. Pythium is also influenced by soil temperatures and is more active when soils are about 55° to 62°F. Thus delaying planting until soils warm will help reduce loss to Pythium. However, do not delay past the normal planting time for your area.

Pythium losses can also be reduced with the addition of a seed treatment. Fungicides with specific activity against Pythium (and Phytophthora on soybeans) include metalaxyl (Allegiance) and mefanoxam (Apron XL).

Keep in mind there are no sources of resistance to Pythium in corn or soybeans. You must base disease management on improving conditions for the seed and emerging seedling and on using the appropriate seed treatment. Infected seedlings may recover but commonly do not perform as well as healthy plants.--Joe Toman

Author: Joe Toman