Many seedling blights can infect germinating plants at this time of the year. One of the most common and earliest groups of fungi that attack corn and soybeans belongs to the genera Pythium. Fungi in the genera Pythium are called "water molds" because they thrive in soils that are wet. In addition, these fungi are earliest and very common because the various species are active over a wide range of temperatures and moisture regimes. In fact, Pythium spp. are often grouped by the temperature regimes that induce optimum infection. Cooler soils (50 to 60°F) favor three species (P. debaryanum, P. torulosum, and P. ultimum) that are more common in northern areas, particularly in early-planted fields. Several other species, including P. aphanidermatum, have higher optimum temperatures (86 to 97°F) for infection, but they can also be present in the field at temperatures as low as 60°F. These species of Pythium cause problems in more southern areas and in late-planted crops.|
Pythium fungi overwinter in the soil and in plant debris as oospores. Moisture is necessary for oospore germination and 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. Exudates from seeds and roots also induce fungal spore germination, hyphal growth, and penetration. Damaged seed encourages increased fungal attack because damaged seed leaches root exudates into the soil, attracting fungi, and the wounds provide entry for pathogen penetration.
Although Pythium may cause minimal damage to germinating corn, this fungus can infect a substantial portion of the developing root system including the mesocotyl. 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.
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, and/or rotting of the root system. After emergence, the stem and seedling may die and their tissue will appear soft and rotted.
The potential exists for development of Pythium in early-planted corn when 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. 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 seed treatments for control of Pythium, see "Seed and Seedling Rot Diseases of Corn and Soybeans" by Dean Malvik in issue no. 2, April 6, 2001, of the Bulletin. Resistance is not an option for controlling Pythium in corn or soybean.--Loretta Ortiz-Ribbing