By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and soybean check-off
Scouting for symptoms of early season diseases in a field is similar to scouting for early season insect problems. “Symptoms are representative of problems in the field,” said Dr. Horacio Lopez-Nicora, OSU Extension Soybean Pathologist and Nematologist. “The symptoms will be in pockets and clustered together or aggregated together. The pockets will be delayed in emergence or have stand reduction. Those pockets may be insects or very wet areas that have abiotic physiological damage or disease, or they may be a combination.”
A disease will only impact the plants if all three parts of the disease triangle occur. Those parts include the disease being present, having the correct environmental conditions, and having a susceptible host.
“Most of the time, a farmer will know historically that the field has been affected by certain disease organisms,” said Lopez-Nicora. “Places in the field where water is retained and there is poor drainage are places that have problems with water molds such as phytophthora and pythium. Within these pathogens there are specific species, especially with Pythium we have different species that can affect plants at different temperatures.”
The environmental conditions must be conducive to diseases. “Air temperature, standing water in the case of water molds, cold soils in the case of fusarium or rhizoctonia can negatively affect the crop,” said Lopez-Nicora.
Controlling the level of susceptibility of the host plant is an area of management that farmers have more control over. “Selecting a variety of soybean seed that is resistant to the pathogens and also using a seed treatment will help to alleviate the risk. We have been noticing with early planting dates that these plants take a while to emerge. We have seen that some plants are taking two weeks to come up,” said Lopez-Nicora. “In some of our trials we have found damage that is looks similar to damage caused by fungal like pathogens such as oomycetes. Most of the time that looks like discoloration of the tissue or the seed may even be completely rotten. The seed may germinate but not fully emerge. We call this pre-emergent damping off. This is only known by digging up those seedlings and getting them out. There are also some seedlings that will have just recently emerge but then are affected by the organism and we call that post-emergent damping off.”
Many pathogens can attack young seedlings.
“Depending on what organism we are talking about, the symptomology will be slightly different,” said Lopez-Nicora. “The water molds will tend to rot the tissue. Pythium will rot the cortical area but leave the pith of the seedling intact. Fusarium will create a very brown to reddish discoloration similar to rhizoctonia. These can be difficult to tell apart without experience and training. In our lab we can help identify them.”
“Digging the plants is easy if you target the areas where a disease is suspected, or the crop is not emerging normally. In the lab we can surface sterilize the plants and isolate the pathogen if one is present in the plant and correctly identify it,” said Lopez-Nicora. “The same concepts apply to seed and seedling diseases as insect pests when it comes to planting conditions. As we get into the warmer season plants will emerge faster and the seed treatment products will be more effective.”
If there are concerns about a disease still being present in a replant situation, Dr Lopez-Nicora refers back to the disease triangle. The disease pathogen may still be present in the soil, but the environmental conditions have hopefully improved, and the treatment on the seed should also help protect the crop from the pathogen.