Field conditions in much of Illinois are prime for damage to soybeans caused by Phytophthora and Pythium. These pathogens have often been called "water molds." Pythium is most favored by cool, saturated soil conditions, and Phytophthora is highly favored by intermittently wet soil conditions--conditions that are occurring across much of Illinois. This article will cover basic aspects of Pythium and Phytophthora diseases of soybean and their management, and will briefly discuss some new research on Phytophthora rot of soybean in Illinois.|
Phytophthora and Pythium cause very similar damage to soybean seed and seedlings. In fact, they are virtually indistinguishable without laboratory tests. Both pathogens cause seed rot, preemergence damping-off, and post-emergence damping-off, all often associated with tan-brown, soft, rotted tissue. They can cause serious disease damage under favorable environmental conditions. For more information and photos of symptoms, please see the soybean section of a new University of Illinois Field Crop Disease website that is under development (http://cropdisease.cropsci.uiuc.edu).
Pythium is a widespread pathogen that affects soybean and corn. There are a number of different Pythium species that affect these crops. Last week in the Bulletin (May 24, 2002), Pythium was briefly discussed as a pathogen of corn. It is often considered to be a minor problem compared to Pythium diseases of soybean; however, we received reports from Illinois in the last week of scattered wilted, rotting corn seedlings that have symptoms typical of Pythium. If conditions are favorable for Pythium infection of corn, they almost certainly are favorable for Pythium infection of soybean. After planting, not much can be done to manage Pythium diseases, but it would be worth the effort to take notes on which fields are having a problem with seed and seedling diseases and consider improving drainage and using seed treatments in those same areas in the future. Soybean varieties are not available with resistance to Pythium. Seed treatments containing metalaxyl (e.g., Apron or Allegiance) or mefenoxam (e.g., ApronMaxx or Apron XL) are effective for control of Pythium for perhaps up to about 2+ weeks after planting.
Phytophthora (species sojae) does not cause disease of corn but can cause devastating disease on soybean. And unlike Pythium, which causes most damage to seed and seedlings, Phytophthora can attack and kill soybeans from planting to harvest. In addition to improving drainage in fields where possible, there are two keys to managing Phytophthora rot of soybean. First, plant good-quality soybean seed that has specific resistance to Phytophthora. Look for major resistance genes such as Rps1a, Rps1c, or Rps1k. (Note: Rps stands for resistance to Phytophthora sojae.) Unfortunately, in several midwestern states and in parts of Illinois, the Rps genes are losing their efficacy (more on this in the following paragraphs). Second, fungicidal seed treatments are recommended as good insurance, especially in areas that have previously had problems with seed and seedling rot. The same seed treatments previously noted that are effective for Pythium are also effective for control of Phytophthora (e.g., metalaxyl and mefenoxam).
A new research project on Phytoph-thora rot of soybean in Illinois was initiated recently, with funding provided by the Illinois Soybean Checkoff. More information was needed to determine if Phytophthora populations in Illinois are developing the ability to kill plants with available resistance (Rps) genes as they are in nearby states. The project is still in progress, but this will serve as a brief project summary to date. With the help of seed company representatives and regional extension educators, more than 200 soil samples, mostly from soybean fields with a history of Phyto-phthora rot or similar seedling health problems, have been collected and tested. The soils originated from more than 23 counties in Illinois, representing north-central, northeast, west-central, east-central, southern, and southeastern regions of the state.
We obtained isolates of P. sojae from many of the soil samples and have tested these isolates against commercial soybean varieties with Rps genes 1a, 1c, and 1k. We found (as expected) that many of the isolates from Illinois can defeat Rps1a and a smaller number can defeat 1c. Unfortunately, in our preliminary work, we also found a few aggressive isolates of Phyto-phthora that can defeat all of the resistance genes (Rps 1a, 1c, and 1k) that are commonly available in commercial varieties sold in Illinois. Although we know these aggressive isolates exist in Illinois, we do not know how much damage they may be causing. The aggressive isolates do not seem to be widespread in the state, and Rps 1c and 1k are still effective in most areas of Illinois. We will continue this research and will determine races of Phytophthora in Illinois. The results will help with selection of soybean varieties with appropriate Phytophthora resistance for Illinois and will be of value for breeders developing soybean varieties with Phytophthora resistance best suited for Illinois.
I would like to obtain additional soil samples from selected areas of Illinois where Phytophthora has been a problem. I especially would like to obtain soil samples from northeastern counties (Kankakee, Iroquois, Will, and Ford). Please contact me if you know of fields anywhere in Illinois where Phytophthora rot or seed or seedling rot of soybean has been a problem, and we can discuss whether it would be appropriate to collect soil samples from those areas for Phytophthora research.--Dean Malvick