A look at 2023 soybean diseases

With Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and Soybean Check-off and Horatio Lopez-Nicora, Ohio State University plant pathologist

Dusty: Through the growing season and this fall you have been in the lab looking at samples submitted with different soybean diseases. How did the 2023 season start out and progress from a disease standpoint?

Horacio: It was a pretty strange season. We started with very cool soils for those who planted at the end of April. Then we transitioned to that dry period, but it was a very strange drought. It was dry in the very top layer during that drought and we experienced very high evapotranspiration, but the soil underneath that dry layer was still moist. Seeds were germinating and growing a root, but taking a lot of time to emerge. We saw the roots growing out of any seed treatment effect, making those plants more vulnerable to a myriad of pathogens that we normally have in our field. After that drought, in some areas we had approximately 3 weeks of nonstop rain and very little sunlight. There were conditions for multiple diseases to become very severe this season. We started receiving a lot of samples at the beginning of the season where we isolated Phytophthora and Pythium from areas that retained a lot of water, as well as Rhizoctonia and Fusarium. Rhizoctonia is a fungus that can produce survival structures so when we have a dry season, it’s essentially killing every other microorganism, but Rhizoctonia can survive by producing these sclerotia. When we have rain again, it is the first one to germinate and infect the plant. We did recieve a lot of these pathogens and we saw a lot of the symptoms in the fields with reduction in stands and some plants being killed by these pathogens.

Later in the season, when plants started flowering, we saw the presence of sudden death syndrome or SDS very early, even in some areas before flowering. That’s very weird. Conditions were very favorable in some places. SDS infects the plant earlier in the season and it will be more severe when we plant in cool soils at the end of April or beginning of May. The fungus will affect the root system and eventually the fungus will produce toxins that will be translocated upward when we’re hitting that reproductive stage of the plant and we will start seeing those visible symptoms of SDS. The veins remain very green on the leaves but in between the veins we have the yellow chlorosis and death of tissue. We call this interveinal chlorosis or necrosis. It is very typical of SDS. It’s the fungus in the root system producing the toxin going up and defoliating the plant.

My PhD student, Jenna Moore, is looking more into SDS in Ohio. We have been collecting SDS samples from all over Ohio and growing it in a pure culture. We’re asking questions about what species of fungus we have that can produce SDS in Ohio and we’re testing them against different products to seek treatments that are used to reduce the damage of SDS. We’re evaluating if this fungus is developing resistance to these products at the same time and we’re screening to see if we have some of these isolates from different parts of Ohio that are more aggressive than others. We’re doing this in the greenhouse. We also have field trials in our different research stations at Ohio State where we are inoculating the plots with the fungus that causes SDS and evaluating different seed treatments and looking at those products to see how efficient they are so we can have better recommendations for growers.

Dusty: What about some of the white mold issues we started seeing late in the season?

Horacio: White mold normally will infect at flowering and some of the symptomology will be very easily observed during that time. The infection itself takes place at flowering and most of the time when we consider a fungicide application for white mold, that’s one of the things to take into consideration. We’re not going to go out there and do our normal R3 application because it’s going to be too late for white mold. We really need to target between R1 and R3. This year in particular we should have been getting out there early. We have been seeing white mold where we haven’t in other years. And, we have areas such as northeast Ohio where we consistently have it and know that we’re going to get white mold. White mold fungus will also produce a survival structure that can be increased in the soil as more plants are affected and we can see buildup of the inoculum in the field, but white mold really is driven by conditions. Unfortunately, everything that we do to enhance and increase yield also promotes the severity and incidence of this particular disease.

Dusty: Frogeye leaf spot is another soybean disease of concern. It seems to be more of a problem in northern Ohio than in the past. Has that been a problem in 2023?

Horatio: We have seen in late summer frogeye leaf spot in the northern part of the state, north of I-70. Last year in in some of our fungicide plots we noticed for the first time the presence frogeye in our northern plots. Most of the time we have been seeing it in Ohio south of I-70, where it is a pretty common disease. Now we are seeing it more in the northern part of the state. This is something very important to consider because when frogeye leaf spot hits that window between R3 and R5, that’s the window that we want to take care of it by using a fungicide application. That’s the window where the fungus will actually damage our yield. Anything that we see after R5 in terms of frogeye, we really don’t recommend any further applications because of the very low or no yield reduction. That R3 to R5 window is the critical time where we want to be able to manage and control frogeye.

Dusty: Looking to next year, what should we plan for based on the soybean disease issues of 2023?

Horatio: If you had pockets in the fields with SDS — which is a soil borne pathogen that lives in the soil —  we need to identify areas where it actually lives to help identify some management for next year. Now we can start thinking about some effective seed treatment or some varieties that have are really good resistant package for that particular disease if you are planting soybeans there next year. We can also think about how there may be an interaction between that particular fungus and the presence of soybean cyst nematode and maybe consider sampling that field for soybeans cyst nematode after harvest. If there were problems with Rhizoctonia, Fusarium or Phytophthora this year, we’re not going to get rid of those, but if we know we have it at a level that could cost significant stand reduction, consider high levels of resistance for that particular field next year.