Issue 8 of the Bulletin (May 16, 2003) discussed the major pathogens, symptoms, and management considerations associated with soybean seed and seedling diseases in Illinois. Common pathogens that cause soybean seed and seedling diseases in the state are Pythium, Phytophthora, Rhizoctonia, and Fusarium. Information and photos of symptoms for diseases caused by these pathogens can be found at http://cropdisease.cropsci.uiuc.edu.
The recent wet and cool weather conditions in Illinois may have set the stage for widespread damage from seed and seedling rot of soybean caused by Phytophthora and Pythium. Reports of replanting due to stand loss from seed and seedling diseases have started to come in, and we expect more of this to occur soon.
In much of Illinois, the soil conditions are close to ideal for damage to soybeans caused by Phytophthora and Pythium. These two pathogens cause very similar seed rot and pre- and postemergence damping-off of seedlings. The seedlings with damping-off symptoms usually have brown to tan tissue that is soft and rotting. Pythium is usually favored by cool, saturated soil conditions, and Phytophthora by warmer and intermittently wet soil conditionsconditions that exist across much of Illinois.
Pre and post-emergence damping-off of soybean caused by Phytophthora sojae. Similar damage and symptoms are caused by Pythium.
Pythium can be managed by using seed treatments and by improving drainage if possible. After planting, not much can be done to manage these diseases, but consider noting which fields have problems with seed and seedling diseases for implementation of management strategies in future. Seed treatments containing metalaxyl (for example, Allegiance) or mefenoxam (for example, Apron XL) can be effective for control of Pythium and Phytophthora for about 2 to 3 weeks after planting.
Phytophthora sojae can attack and kill soybeans from planting to harvest. In addition to improving drainage in fields where possible and using the seed treatments noted above, resistance is used to manage Phytophthora rot of soybean. Soybean varieties with specific resistance to Phytophthora should be planted. The major resistance genes Rps1c or Rps1k should be effective in most fields. However, we have recently confirmed that these resistance genes are no longer effective in some parts of Illinois. This research, funded by the Illinois Soybean Checkoff, has shown that races (pathotypes) of Phytophthora occur in some parts of Illinois that kill soybeans with Rps1a, Rps1c, and Rps1k. Many of the Phytopthora isolates from Illinois soybean fields can defeat Rps1a, and a smaller number can defeat 1c. A smaller number can defeat all of the common resistance genes (Rps 1a, 1c, and 1k) available in commercial varieties for Illinois. We know that these aggressive isolates are causing damage, but they do not seem to be widespread in Illinois. We still do not know how much damage they are causing. Results from this research will help with selection of soybean varieties with Phytophthora resistance and can assist breeders developing soybean varieties with Phytophthora resistance for Illinois.--Dean Malvick