You are investing a lot of money in the soybean seed you buy today and you expect the variety to yield in return. But to realize the yield you expect, you first have to the give each and every seed a fighting chance to emerge and produce a healthy seedling that will mature into a pod-bearing plant. And soil-borne pathogens are your nemesis.
When considering all the stresses that soybeans incur during the season it is remarkable how well they do in Illinois. From the day of germination until yellowing, soil-borne pathogens are nibbling on plant roots and attacking the plant. Pathogens like sudden death syndrome (SDS) and charcoal rot aren’t classified as seedling diseases, but still infect seedlings even though symptoms don’t appear until the reproductive stages.
The soil is full of organisms, some good and some bad. We want a lot of the good ones to promote soil health and must contend with the bad ones. The most common seedling disease culprits Illinois producers are dealing with include Phytophthora sojae, Rhizoctonia solani, Fusarium and Pythium. Some pathogens cause seed rots while others attack the roots and lead to pre- or post-emergence death (damping off). Remember it’s difficult to diagnose the cause of seed rot and seedling disease in the field, so don’t hesitate to send infected seedlings to the Disease Diagnostic Clinic at the University of Illinois to confirm.
Thin stands or large areas with poor emergence are the most common symptoms. Plants that are infected but don’t succumb can survive, albeit in a weakened condition with compromised roots, and remain small the whole season. Most of your production fields have a mixture of these pathogens; however, it is not easy to predict what fields or years will experience significant losses. And sometimes it is only some acres in a field that are affected.
Since your fields already are infested, the incidence and severity of seedling diseases is governed primarily by the soil temperature and moisture at planting time and during the early growth stages. Early planted fields, high moisture, low temperatures and poor growing conditions are all factors that create a favorable environment for these diseases and make soybeans more vulnerable.
Your first action is to plant into good soil conditions and your second action is to buy varieties with seedling vigor, resistance or tolerance to these diseases. With the exception of Phytophthora sojae, resistant varieties aren’t available for seedling diseases, but remember some varieties may be less susceptible than others. Therefore, it is good to plant multiple varieties on your farm with varying maturities to reduce the impact of seedling diseases on farm production.
So, your third course of action is seed treatments and many have excellent activity against these pathogens. Today most seed treatments combine two or three fungicides to target fungi and fungal-like organisms like Pythium, Phytophthora and Fusarium. However, depending on the conditions and the pathogen spectrum in a particular field, some seed treatments are more effective than others. If seed treatments are used, it is advisable to compare treatments from multiple seed companies. The pathogen spectrum on your farm is unique, and what works well for a neighbor may not work as well for you.
While seed treatments may provide quicker emergence, increased stand and vigor and higher soybean yield, their use doesn’t guarantee higher yields. But your first goal is to get a good stand and then let your other agronomics carry it to the finish line.
Professor of Plant Pathology
Southern Illinois University