Soybean seed and seedlings are vulnerable to a number of soil borne pathogens in the early spring. The conditions for these diseases have been quite variable this year in Ohio, with some areas very dry and cool while other locations have been warm and wet.
In previous years where we have seen alternating dry and wet conditions, Rhizoctonia is more prevalent than those diseases caused by water molds. Rhizoctonia is probably there it just gets outcompeted by the watermolds. Symptoms can range from no emergence to brick red colored lesions on the area of the stem just below the soil surface. Those seedlings that survive will produce soybeans but fewer than those that are healthy.
The watermolds, Phytophthora sojae and Pythium spp., may have a tougher time this year. The water molds survive year-to-year as oospores in the soil. To get these oospores to germinate and infect soybean seedlings, we typically need a 2-week incubation period where the soil is moist. This year, in some areas of the state it has been dry prior to planting. In addition, saturated soil conditions for 12 to 36 hours, is often needed for many of these to produce zoospores. Symptoms caused by watermolds are light to dark brown, lesions on any part of the root or cotyledon.
Other soil borne pathogens that can also infect soybean seedlings are Fusarium graminearum (the same fungus that causes head scab of wheat), Macrophomina (the charcoal rot fungus), and Fusarium virguliforme (Sudden death syndrome fungus). Symptoms caused by these pathogens can range from pink to brick red to brown and dark brown lesions, very similar to those caused by Rhizoctonia and the water molds. We are always happy to take a look at these plants collected from fields in Ohio. So, if you do have plants that are infected but not yet dead, please send them our way to Dorrance Lab; Dept. of Plant Pathology, OARDC, 1680 Madison Ave., Wooster, OH 44691.
For management of all of these seedling diseases, host resistance is one of the first strategies, but there are also seed treatments. If you have a stand issue, get the problem diagnosed correctly – sampling and recovering the pathogen is the only way to do this. Next, look at the seed treatment package. In Ohio, we have a great diversity of pathogens and a single active ingredient will not control all of the different pathogens – so some adjustments in materials will be required for that field in the future.