Southern rust popping up on some Iowa corn crops, Iowa State testing finds

[00:00:00]All right, let's switch gears now and turn to an agricultural story here on your [music] Wednesday.

[00:00:04]Have you ever heard of the fungal disease southern rust? Well, for the last several years, it's been hitting southern corn crops hard, and it's also been known to make its way north into our area. Now, so far this summer, the lab at Iowa State University Extension had not seen any signs of the disease in Iowa until now. And that is why Virgil Schmidt, Extension Field Agronomist with ISU Extension and Outreach, joins us right here in The Current. Virgil, thank you so much for being back with us. The rumor mill admittedly has been going pretty crazy this summer. And we should point out people could be coming from a place of expertise with this because Iowa had a lot of southern rust last year. What happened in 2025?

[00:00:44]>> Well, in 2025, well, we had the worst outbreak that we've had in over 30 years. Uh there were people who basically said it cost me 20, 30, or more bushels per acre uh if they had susceptible hybrids. And so, that is fresh on everybody's mind.

[00:01:01]>> Mhm.

[00:01:02]And that brings us to this year. We're again, up until this week, we didn't see any signs of southern rust in Iowa, but for those that don't know about it, can you tell us what is this fungal disease?

[00:01:12]What does it do to corn crops?

[00:01:15]>> Well, uh you know, first of all, it's a disease that does not overwinter in Iowa. In fact, it cannot overwinter in the US.

[00:01:22]Uh it uh comes from from Mexico up on southern winds on an annual basis. And as you suggested earlier, it typically starts in the southern states, usually Texas. Uh and then uh if we have southerly winds, it it moves in our direction.

[00:01:37]Um and uh what it does basically is it really starts to limit uh the ability of the plants to conduct photosynthesis. If the uh plants are infected early, such as at tasseling time, uh yields can be uh down 30 to even 50% on susceptible hybrids if they do not have a fungicide applied.

[00:02:00]>> So, what have you guys been seeing in your lab in just the last few days?

[00:02:04]>> Yeah, um well, we've been getting a lot of samples uh for uh you know, several weeks now, and uh everyone was negative until one that uh was confirmed to be positive on Monday afternoon, and uh and then that information was released on Tuesday.

[00:02:21]Uh also on Tuesday, we got some samples from uh Muscatine and Louisa counties, as well as Mahaska County that I think are probably going to to be positive here.

[00:02:32]Now, in a normal year, uh we find a little, uh but it typically does not uh really go anywhere because we typically do not have the right weather for the disease to really do well. In 2025, we had that weather.

[00:02:49]>> Okay, can you refresh us? What is that weather? I mean, what's the favorable conditions for southern rust?

[00:02:54]>> Yeah.

[00:02:55]Sure. Well, I think first of all, we have to have a susceptible hybrid. Uh last year, we saw a lot of difference between hybrids uh simply because some were and some were not.

[00:03:06]Um and so, we have to have a susceptible hybrid, or else it's not going to be an issue. Uh and if we have that susceptible hybrid, we have to have basically four different things that occur simultaneously.

[00:03:17]Uh we have to have a stormy weather pattern. Uh we have to have greater than uh 8 hours of leaf wetness on an average over extended period of time, more than 90% relative humidity, and uh somewhere about 77° to 88° F, and we had that last year, which is what made what happened last year possible. Uh if we look at the 6 to 10 and the 8 to 14 day outlooks, uh they're calling for warmer than normal and drier than normal. And like I said, usually it's not a problem because this time of year it tends to be fairly warm, but also tends to be fairly dry. Last year was an exception.

[00:04:01]>> So, then knowing where we are right now, you know, end-ish of July, we're about six, maybe seven weeks out from this year's corn harvest, uh does our current corn maturity level impact any of these effects of southern rust?

[00:04:14]>> Uh it sure is. The later the infection, as I indicated earlier, the less is the the yield loss. Uh basically, we are now in what is called R2, which is the blister stage. Uh we have a a few fields that are in R3, which is the milk stage.

[00:04:31]Once the plants get beyond the milk stage, uh they're far enough along uh that uh if the disease even hits really hard, it's really not going to have that much of a negative influence on yield to uh really make a a fungicide application worthwhile. Uh now, typically, uh we move from uh from milk to dough, which is the next stage, uh on about August 7th.

[00:04:58]Uh and you know, that's just 2 weeks and about 2 days from now.

[00:05:02]Uh and so, you know, we really kind of got a a short time window to be really looking at it and being concerned.

[00:05:10]And if the weather outlooks are correct, um with it being warmer than normal and drier than normal, um I expect that uh basically, it's going to be pretty much a non-issue when we're at the season's end. We will see if that is correct.

[00:05:24]>> Season's end is coming up a lot faster than I think it is. Virgil, thank you so much for your time today. I know you're incredibly busy. I really appreciate this.

[00:05:34]>> As always, my pleasure.

[00:05:36]>> If you guys at home want to share this story, we have our online version with Virgil up at wqad.com.

[00:05:41]Just click on the as seen on TV tab at the top of the home page.