Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
We have seen several cases of pre- and post-emergent damping off in corn caused by Pythium in recent weeks. Mild to moderately infected seedlings often are stunted and grow more slowly than healthy seedlings in the same field. The mesocotyls of infected seedlings (Figure 1) are often rotted (Figure 2) instead of being healthy, firm, and white in color. The mesocotyl serves as a pipeline to the developing seedling, helping to move essential carbon and resources from the endosperm to the developing plant tissues. Severely infected seedlings may die before or after emerging from the soil. In some cases, the entire root system can be infected and appear yellow to brown. Often the outer tissues of the roots easily slough off and are water-soaked. Affected seedlings tend to occur in areas of the field that hold more water, such as low lying areas, turn rows, and other compacted areas of the field. Seedlings can survive mild to moderate Pythium infections if the root systems have developed adequately.
Figure 1. The mesocotyl of a corn plant, which can be rotted when damping off organisms infect corn seedlings. The mesocotyl acts like a straw, helping traffic carbon from the seed endosperm to the developing plant.
Pythium overwinters for many years, in the absence of corn, as oospores. Wet, cool conditions allow for the oospores to germinate and eventually produce small spores with the capacity to swim in soil water (zoospores). Root exudates (amino acids, carbohydrates, secondary metabolites) attract the motile zoospores to seedling roots, and under the appropriate conditions, infection may occur. Damaged seed also exudes compounds that can attract zoospores, and wounding to the seed coat provides a route of entry for the fungus. Different Pythium species can infect corn at different temperatures. The key factor for Pythium is excessive water – when soil is saturated with water for an extended period of time, regardless of the temperature, you may see Pythium associated damping off in fields where the pathogen is present. A final thing to keep in mind is that there are Pythium that are saprophytic, meaning that they live on dead or decaying material. Therefore, if you have seedlings that are dying from some other factor (anoxia, chemical damage, insect damage) you could still find Pythium on the plants. Many times, multiple factors are involved in seedling rots and damping off.
Figure 2. Corn seedlings with rotted mesocotyls
The potential exists for development of Pythium in early-planted corn when wet weather conditions persist and corn does not germinate and grow quickly. Seed treatments with oomycete specific ingredients may provide some protection for 10 to 14 days after planting. Other control measures should be aimed at improving seedling emergence. In particular, avoid planting too early when soils are wet and cold, especially in no-till or conservation-tillage fields because heavy crop residues retain additional moisture.