Doug Jardine Discusses Diseases in Corn and Soybeans

(Dr. Doug Jardine) I’m Doug Jardine, I’m the extension row crops pathologist with the Kansas State University and here tonight, at the Rossville station of the Kansas River Valley Field, I discussed a number of disease topics on corn and soybeans with the attendees here. The two most important ones that we wanted to talk about here tonight were Frogeye Leaf Spot on soybeans and Southern Rust on corn. The reason those two are the most important is because there’s still time in this growing season to take action against them if you need to. Frogeye Leaf Spot is a foliar disease of soybeans. It gets a small, tannish spot with kind of a dark red ring around it. It is by far the most important foliar disease here in Kansas. We can get significant yield losses when we have a susceptible variety. Particularly, as we move towards the Missouri border in the state, the disease gets progressively worse because it likes the higher humidity and the higher rainfall on the eastern side of the state. The ideal time to be applying a fungicide for Frogeye Leaf Spot, if you have it, would be about the R5 stage of development which is just about when the pods are starting to fill. But because of the cost of fungicides, we want to make sure that we have Frogeye out there, so it’s very important to go out and scout your fields before making the decision. If you just see a stray lesion or two, you’re probably fine, but if you start to see them building in the upper and mid-canopy over a reasonable part of the field, then you need to go out and take action. We have a lot of products that work really well. The one thing about Frogeye Leaf Spot is you do not want to use a straight strobilurin mode of action product, and that would be something like straight Headline, or straight Quadris, because the fungus has the ability to develop resistances materials very quickly. You want to make sure that you use a combination product that contains at least two modes of action. The other disease we talked about out here tonight was Southern Corn Rust. Southern Corn Rust is the disease that does not overwinter in Kansas. It moves up from the south. The point I was making tonight whether it’s climate change or some other factor, but the disease has been showing up in Kansas about a month earlier than what it did five to ten years ago. Instead of coming in in late July or early August, it’s now coming in mid to late June. Still, if we get our corn planted on time, say in early April, depending on where you’re at in the state, you can probably get away without a fungicide spray. But if you didn’t get your corn planted maybe until mid-May or maybe even a little later in some places because of wet weather, you need to be out scouting. Southern Rust can be a little difficult to scout for because it will start in pockets in the field and so you have to make sure that you cover the field reasonably well in order to determine if you have it out there. There are fields in Kansas that have been sprayed this year for it. There still could be some more, especially again as we move towards the Nebraska border or up into the high plains where the corn is planted later. The good thing about Southern Rust is that some of our generic fungicides work really well. What we call generic Tebuconazoles, visit with your dealers if you’re not sure what those are. Sometimes those are only a couple of dollars an acre for the product and then plus the application cost. Some corn hybrids have some level of susceptibility but some are really susceptible. Again, the key is to get out there and scout for the disease. You can probably spray as late as the beginning of hard dough and still get an economic benefit if you have a really susceptible hybrid and it was planted late enough so that’s much different than Gray Leaf Spot where that VT, tasseling time application is by the far so. Make sure that you read the label and check for pre-harvest intervals because as we get later in the season, that’s going to reduce the number of different fungicides you can use. Some of them have a shorter pre-harvest interval of seven days and those would be the ones that you’d want to be looking for if you needed to spray for Southern Rust.

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