No. 5/April 24, 1998
Phythium Seedling Blight of Corn and Soybeans
With corn planting beginning or under way in many areas of the state, producers should be aware of the possibility of Pythium seedling blight on both corn and soybeans. The Pythium fungus is usually the first plant pathogen active in the early spring. It thrives in cool (55 to 60 degrees F) soils and can be especially damaging to stands when frequent rainfall occurs. The Pythium fungus, along with a common soybean pathogen known as Phytophthora sojae, is called a "water mold" because both fungi release a swimming spore stage (called a zoospore) that infects plant roots. The zoospores are not produced until soils become very wet to flooded. Standing water is not needed for zoospore production; 3 to 4 hours of very wet conditions can initiate zoospore production.
Damage from Pythium to corn is usually minimal and may not even be evident. The fungus can infect both the mesocotyl and the primary, or seminal, roots. The mesocotyl is the tissue bridge between the seed and the emerging seedling. If this attachment is rotted through by Pythium, plants can die unless the permanent root system is somewhat established. If the seminal roots are infected, damage is usually not as great. Plants simply grow more slowly during the first few weeks after emergence and may not appear to thrive. Once the plant has produced adequate permanent roots, growth resumes and plants show very little, if any, additional damage.
Infected roots turn brown, become soft-rotted and water-soaked. The outer root tissues may peel off easily. Although root tips are the most commonly infected point, discolored brownish areas may appear along the entire root.
On soybeans, Pythium causes seed rot before emergence or can cause damping-off after emergence. The term "damping-off" refers to seedling losses after germination and was coined because it is most common during damp or wet weather. Seedlings may not emerge or may die shortly after emergence. Pythium can infect the cotyledons, the hypocotyl (developing stem), or the roots. Root tips are a common infection site. Soft-rotting of the stem and death of the seedling are common symptoms of Pythium and Phytophthora infection.
The recommended seed treatment for control of Pythium on corn and soybeans and seedling infections by the Phytophthora fungus in soybeans is Apron. Apron seed treatment provides protection ONLY for Pythium and seedling Phytophthora infections. Thus, it is important to identify correctly which fungus is infecting seedlings if a blight problem occurs. Numerous other fungi, including Rhizoctonia, Fusarium, and Penicillium, also can cause seed rot and seedling damage in corn or soybeans. These last fungi are not controlled by Apron, and an additional fungicide may be needed. However, because most corn seed is currently treated with Apron plus either Captan or Maxim, very few problems should occur.
For soybeans, producers will need to make a decision based upon a number of factors, including:
- Planting time. Very early planted soybeans tend to have greater seedling blight problems, especially with Pythium.
- A history of seedling blights in the field. Seedling-blight fungi are basically permanent residents in a field. Their activity is greatly dependent upon favorable weather, so a single season is not adequate to judge the potential for seedling-blight problems.
- Field is either no-till or has a heavy corn residue. Residues tend to protect the soil but also can increase the time needed for drying and warming of the soils. Because Pythium is favored by cool and wet conditions, producers may wish to delay planting a bit and to add Apron to soybean seed for these types of fields.
- Soils are "heavy" and have a tendency to retain high levels of water or to drain slowly. Because Pythium is favored by wet soils, consideration should be made for using a seed treatment of Apron where these conditions are known to occur.
These are no commercially available sources of resistance to Pythium in either corn or soybeans. Disease management decisions must be made based upon the factors and the potential for continuing cool and wet conditions at planting.
H. Walker Kirby (firstname.lastname@example.org. uiuc.edu), Extension Plant Pathology, (217)333-8414