Managing soybean foliar, stem diseases

CARBONDALE, Ill. — Current research efforts are leading to a clearer picture of the incidences and severity of soybean foliar and stem diseases.

Jason Bond, plant pathologist and director of the Illinois Soybean Center director at Southern Illinois University Carbondale, detailed the diseases that threaten soybeans each year during the Illinois Soybean Association-hosted Soybean Summit.

“Most of the soybean foliar and stem diseases are continual threats to production, primarily because they overwinter in our fields. Maybe not in a particular field that you’re growing your crop in this year, but maybe a neighboring field,” Bond said.

“Sometimes what’s happening in fields in neighboring Kentucky and Tennessee contribute to what we deal with as these pathogens move into our area.”

Here is what Bond had to say about the common foliar diseases in Illinois soybeans fields.

On Downy Mildew

This one that I come in contact with in almost every field. If we’re having reasonable growing conditions, it’s very easy to find downy mildew. It can look pretty dramatic in the field.

I’ve never seen a case where if you sprayed a fungicide to control that pathogen that you get a response. For whatever reason, the genetics that we grow in our environment, it is not a yield-robber.

It is a parasite because it attacks our soybeans and it causes symptoms. So, you can call it a disease-causing agent, but it rarely impacts yield in our areas. Generally in other parts of the world it does cause some yield loss.

On Septoria Brown Spot

This foliar disease is another one that is present in most fields. It’s usually easy to find it as that plant starts shutting down as it approaches maturity and when that happens, a lot of things start popping out on the leaves are a pretty evident.

It is a yield-robber in some environments and in some cases we do see people spray a fungicide and see a response to controlling this pathogen.

On Cercospora Blight

Cercospora blight attacks the leaves and pods and infects the seeds and leads to a disease called purple seed stain. It’s pretty widespread. The fact that it causes seed discoloration also tells us that it is seed-borne, so it can move around the country.

So, it is a pathogen that we can actually bring to our fields if it’s not there yet. It does typically cause yield loss, depending on the variety.

One of the complexities of this pathogen is that the timing of when it arrives and it affects those leaves, you don’t immediately get symptoms. You don’t really know what’s there and then it starts showing up when the plant goes through the later reproductive stages.

So, the timing of a fungicide application is pretty difficult and it’s not real clear how much yield protection you get when you spray a fungicide.

Timing is not as easy as maybe some of the other foliar diseases. So, you don’t normally see recommendations, at least in our part of the country for spraying fungicide for cercospora blight.

On Frogeye Leaf Spot

This pathogen is a major yield-robber and is widespread in our region. It overwinters in residue in many parts of this region. What dictates if you have it or not is generally our environment.

For whatever reason, 2022 was a pretty light year for frogeye leaf spot in southern Illinois and also into Kentucky and Tennessee. Part of that season we had a real dry period in western Kentucky and in Tennessee that slowed the disease.

It was widespread in Illinois, but it was localized on its impact in terms of how much disease was there. Just because it’s in the field does not necessarily mean that yield is going to be lost. It needs a certain level of severity.

It gets the name frogeye leaf spot because of the little brown lesion that is surrounded with a little black border and is supposed kind of mirror that of a frog eye.

Once the disease pressure starts getting very severe and the leaves start getting under intense pressure they start to yellow. Once that happens, they usually start defoliating.

So, yield loss cannot only come from the pathogen affecting the tissue and causing lesions, but once we start getting to the defoliation stage that yield loss starts ramping up even more.

If you have the pathogen under the right environments with moisture and humidity, it will attack the pods and can even infect the seeds and compounding those yield losses.

On Frogeye Management

There are strobilurin fungicide-resistant populations of this pathogen and it’s pretty widespread. We can no longer manage it with a single mode of action.

Strobilurins are a great chemistry and are still very important for use in both corn and soybeans, but not just by themselves. We need two modes of action and we see some companies now even going to three modes of action.

There are options we can take to manage this pathogen well in advance of that beyond just reacting and applying fungicide if we see it in our fields.

When we rotate our crops to a non-host like corn, wheat or other crops, that is going to reduce the amount of inoculum in that field.

The pathogen is going to live in that residue to some degree and so that helps reduce but doesn’t eliminate the potential for those pathogens to come back.

We can also choose resistant varieties. Most companies always mark some of those that have resistance to frogeye leaf spot.

If they don’t mention it, then it’s probably not been tested or maybe they’re not really that serious or interested to tell you that it’s susceptible.

You can get some pretty good guidance from the brochures or work with a seed adviser and deploy resistant varieties. That will go a long way to help manage this disease.

There are also tolerant varieties. Look at those variety brochures and find some of their varieties that are tolerant.

When should you spray for this disease? Sometimes you’ll hear R4 or R5. If you’re producing soybean seed, seed quality is very important and you’re dealing with pathogens and controlling pathogens that will not only infect foliage, but also attack the seed, then you might have guidance from whoever you’re producing that seed for to spray at a later date.

Generally, literature and most research shows that that R3 timing is really the best, but most times when we go into fields, even in our disease hot spots, you don’t see much frogeye leaf spot at R3. It comes in a little bit later.

The pathogen is already there at R3 or sometimes even earlier, but what we’re looking for when we go out into these fields is actual symptoms, those little spots, not the pathogen.

For this pathogen to produce the spot on the leaf at R3 it would have had to affect 12 to 14 days prior to that because the spore lands on the leaf, gets into the plant, the plant doesn’t let us know that it’s sick for at least 12 days after that process takes place.

A lot of things could be happening in your field during a period of time where we’re not seeing the disease. So, it’s helpful when your plants are sick for us to know as soon as possible.

These pathogens all have these different timings from when the leaf is infected until symptoms appear. That makes it challenging.

There are no resistant soybean varieties available for helping manage stem diseases anthracnose, pod and stem blight, and charcoal rot, while there are resistant varieties for controlling brown stem rot, stem canker and phytophthora canker.

On Anthracnose

Anthracnose is widespread, especially in southern Illinois. Infection occurs in the reproductive stages, but is usually latent and not seed until the plants are mature.

If you ever get into a situation where you’re seeing anthracnose show up on green tissue on pods and leaves, that is an intense disease environment.

We typically do not see that in Illinois, and hopefully we don’t see that in the future, but this pathogen shows up way early in those reproductive stages and has a very long latent period. We typically don’t know it’s there until the plants are mature.

On Charcoal Rot

Last year was an explosive year for charcoal rot. That’s a soilborne organism that infects the roots soon after planting, but as we get into real hot and dry conditions, it just continues to grow into the vascular tissues and it exacerbates the drought and your plants are suffering in the dry conditions.

There are no resistant varieties known for charcoal rot. Fungicides seed treatments really don’t have that much efficacy against this pathogen. In-furrow fungicides have really failed to take hold on this.

This is one of those diseases that you don’t really talk about at field days too much because the only option to lessen the severity of the disease in-season is to irrigate.

This pathogen is a challenge across all crops. It affects corn, soybeans, wheat, whatever we grow in our fields is a host to this pathogen that cause charcoal rot.

On Resistant Varieties

Brown stem rot, stem canker and phytophthora canker can be managed by variety selection. The last few years, we’ve seen an uptick in stem canker from the tip of southern Illinois all the way up to the far reaches of northern Illinois.

Even though it is a pathogen that produces spores that are attacking our stems that then lead to cankers, it’s the timing of when you get the fungicide out to protect those stems at the time those spores are moving is not well defined.

Somebody might have great success and you could repeat everything they did, but if I spray a day or two different or have a different planting date, I may not have the same level of success as my neighbor. That’s how erratic this disease is in terms of trying to control it with a fungicide.

So, it’s very difficult to make a fungicide recommendation on that one even though fungicides have efficacy against the pathogen, the timing can be very difficult.