Diagnosing Disease in Soybean Plants

Soybeans are one of the most important crops in the world. Billions of people and animals depend on them in one way or another. So when the fields aren’t producing well it makes sense to try to figure out why. There are a lot of diseases that can affect soybeans. One field can have a different problem than the field next to it. The same field can have one infection one year and something completely different the next. And to make things even more complicated plants can have more than one infection or disease problem at any given time. With so many variables how do we pinpoint the problem? Let’s talk to Clarke McGrath with Iowa State University Extension and Outreach. So, to pinpoint problems in soybean fields, the big thing is to get out and scout the field and look for the problem plants in a field whether it is in a pattern or it’s just specific areas or field wide. But the big thing is we have to look at the plant top to bottom. We look at the new growth? What is it going to tell us? Does it have disease? Insect feeding? Is it malformed from some sort of chemical application? Then we look further down the plant. In the middle of the plant, that tells you what happened a few days ago, a few weeks ago depending on how old the plant is. We go on down to the stem. We look at the stem. What do we see there? Is that going to give us any clues as to what’s going on? Sometimes we may cut the stem open late in the season to see what is inside of it. And then we look at the root system. Does it look normal? Healthy? Or does it have signs of disease or any other sort of issues that just don’t look right? And really the bottom line is, as an agronomist, you can diagnose some of these things in the field and a lot of these things they look like each other, there’s a lot of mimic out there. So, we may send it back to the plant disease clinic and let them do the final diagnosis for us. Clarke just brought in some soybean samples from the field. And our job in the clinic is to assess and try to figure out exactly what is wrong with the plants. And so one of the first things we do is just to get an overview of the plants. And so we’ll look at the plants, we’ll look at where the symptoms are at. So, on this plant you have symptoms that are on these upper leaves here. We have a different plant here with a different type of symptom. So, we have different things going on and we’re trying to figure out exactly what it is. So, when Clarke came in I asked a lot of questions and he filled out this form that has information about the recent weather in the field, it has information about what herbicides have been sprayed, other field activity that has been going on. And so we can use that information to try to sort of help hone in on what the problems are on these soybean plants. The first thing that I did was I looked through the report that he gave me. I looked at the field history. I looked at what was sprayed. I looked at sort of the environmental conditions. And that gives me a clue as to what exactly I should be looking for. The first thing I noticed there was one particular herbicide that was sprayed in this field that causes symptoms similar to what we see on this plant right here. And so, with this plant I’m pretty confident that this is a herbicide injury that is caused by some activity that was done in the field. Now, there’s four groups of organisms that can cause plant disease. The first one is fungi. Fungi can cause an assortment of symptoms. These include spots on leaves. This can also include having a reduced root system or some kind of root rotting. It can also be having some kind of lesion on the stem. How to identify these fungi, what we do is we usually take some of the diseased tissue and we take it over to a microscope and we look for some of the fungal structures or evidence of fungi under the microscope. If we don’t see some or if it’s not clear, what we can do then is plate some of these leaves out on what we call agar, which is just a nutrient for the fungus to grow and then we can actually see the fungus growing on that agar. The second one is bacteria. This is a little bit more difficult to identify than fungi. But what typically you see are spots on leaves with a yellow halo around it. And what we have to do with this is we either will cut that diseased tissue and then look at it under the microscope and actually look for the bacteria streaming from that lesion. Or the second thing we can do is just do a series of lab techniques where we quickly identify what the bacteria is. The third group of pathogens is viruses. And this is one that is actually fairly difficult to identify in the lab, much like bacteria. Virus symptoms usually are in the leaves as what you see here as some kind of distortion or mottling or some kind of a problem in the leaves. Usually you don’t see symptoms in the roots or in the stem. To identify viruses, what we do is to collect some of the diseased tissue and you can take it and do a series of lab tests that are specific for viruses to identify what virus is causing that problem. And then the last one is nematodes. And what we have here, we do see some above-ground symptoms with nematodes. Usually it’s some kind of yellowing or even smaller leaves, so stunting and yellowing of the plant. But most of the activity happens in the root system. So, what you’re looking for are either direct evidence of the nematodes and sometimes you can see that on the roots. Or the second thing you can do is process the soil and then look for the worms, identify those worms and then you know what was feeding on the roots. With so many people and animals depending on soybeans for food, it’s important to diagnose the disease correctly. That way we can take steps towards treatment and prevention.