Understanding and managing the water molds in soybeans

By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and Soybean Checkoff

When farmers scout soybean fields for diseases, the use of fungicides is a common tool for disease management; however, there are some diseases that are not a true fungus, so a fungicide will not be effective at controlling them. The Field Leader had the opportunity to visit with Horacio Lopez-Nicora, PhD, Soybean Pathologist and Nematologist at The Ohio State University.  He shared the difference between some of the common diseases we deal with and those that are not a true fungus, as well as some thoughts on managing these plant stressors.

Field Leader: As we go through the growing season, what are some of the plant diseases you have been seeing or getting reports about that have your attention?

Lopez-Nicora: The oomycetes are water mold pathogens. They look like fungus. They grow like fungus. But they are not true fungi. This is very important to understand because any fungicide will not take care of them. It has to be products that are very specific for this group, the oomycetes or the water molds, such as Pythium and Phytophthora, are the ones that thrive in conditions with saturated soil. We see them frequently when we get a lot of rain in poorly drained fields with heavy soil.

Those are fields that are very prone to these pathogens thriving. Phytophthora sojae specifically. We have benefited from the amazing work that Dr. Ann Dorrance has done for several years at Ohio State. Her work showed that Phytophthora soje is an important pathogen in soybeans, and that some of the resistant sources are being overcome by the Phytophthora populations we have in Ohio.

Field Leader: What should growers be aware of and looking for?

Lopez-Nicora: It has two phases when it poses the greatest threat to the crop. One is when it is very early, at the seedling stage, even at the seed and pre-emergent stage. We call this the damping-off stage. When this happens, the plant can die before coming out of the ground, or very soon after emerging. These are called pre-emergence or post-emergence damping-off.

Pythium and the other water molds can cause these problems as well. With Phytophthora, if the combination of heavy rains, wet conditions, and above all warm soil temperatures occurs, it can come back and cause adult plants to die.

Field Leader: What should growers look for specifically when scouting?

Lopez-Nicora: If you walk the fields and see dead plants, get down and look near the ground in the transition zone between the root and the stem, in the crown area. You may notice discoloration that is chocolate-brown in color. It can also be a reddish color. This is very characteristic of Phytophthora. These plants normally die, and you can pull the plant out and see the root rotting and the disease coming up the stem as well. This is why it has the name Phytophthora stem and root rot.  

We are observing Phytophthora in our research trials. We have also been experiencing the same in commercial fields. We have received samples from growers in the lab for diagnosis, and we saw a lot of dumping off early in the season. Now we see a lot of symptomatic adult plants with Phytophthora.

Field Leader: What parts of Ohio seem to be most frequent for reports of Phytophthora this year?

Lopez-Nicora: The western part of the state and northwest Ohio. Northwest Ohio is an area where we have heavy soil, and it is not uncommon to have fields with standing water or very wet conditions. These pathogens have the ability to swim and require that water to move. They have little flagella, and they can move and swim in the water and reach the root systems and infect them.

One of the ways to reduce Phytophthora is to reduce the environmental conditions favorable for the disease. Reducing soil compaction, having good drainage, and tiling to get rid of the water is a great way to alleviate some of these saturated soils and help reduce the disease.