The year’s first incidence of Asian soybean rust in a commercial field was discovered in southern Alabama’s Baldwin County. The July 12 finding – in a 60-acre field of late Group 4s – occurred about a mile from a sentinel plot where the disease was found on June 28.
“Overall, the incidence was at less than 20 percent. That’s low severity,” said Ed Sikora, Extension plant pathologist with Auburn University on Thursday. “A colleague, Arsenio Gutierrez – who also found rust in the sentinel plot a few weeks ago – found rust in the commercial beans. I’d been in the field a week previously and didn’t find anything along the edge. Arsenio has really good eyes. He wasn’t too far into the field when he found it. The rust was fairly noticeable.”
About a week prior to picking up rust in the nearby sentinel plot, Extension suggested the producer make a fungicide application. He did. Sikora suspects the disease “deposited in that field and the sentinel plot at about the same time. The symptoms we’re seeing are a result of infection before spraying.”
The infected field will soon have a second triazole/strobilurin application.
The plant pathologist said soybean producers in the southern half of Alabama should spray a fungicide “if they’re in the later reproductive stages: R-4/R-5. In the area of the state where the rust was found (near Mobile), there’s only one early-planted field – and that’s exactly where the rust was found. The majority of the soybeans in the area are ahead of bloom. Last week, I was in fields there and most beans were in the second trifoliate stage, early vegetative. There’s no reason to spray before bloom.
“For Alabama growers, scout your fields at least twice a week. If you’re beyond bloom, have your fungicides ready to go. If you’re at the R-3 stage, a fungicide application would be helpful even without rust. We’re beginning to find other diseases. Yesterday, I saw frogeye, mildew, and what’s likely Cercospora so a fungicide, even in the absence of rust, will give control of these diseases and keep yields up.”
Alan Blaine, Mississippi Extension soybean specialist, agrees with Sikora. “Growers shouldn’t focus on rust to the exclusion of plant health issues and other diseases. Where we have good yield potential, we make money spraying fungicides. All the Delta states have data supporting that. Many producers seem to be forgetting a strobilurin-based program helps yield. It’s a money-making deal. ‘No rust, no spray,’ is the wrong approach.”
The week of July 18, Blaine said all Group 5s in the Extension verification program will be sprayed with a fungicide. “We’re not spraying them because of rust. We’re spraying them just because it’s time.”
The Mississippi soybean crop has great yield potential. When an irrigated field hits R-3, “I say hit it with a fungicide,” said Blaine. “I’m going to hit my beans like there’s no rust out there. The worst-case scenario is I spray the field with a strobilurin. Then, if rust does show up, I can come back with a triazole. Using that approach, a grower is potentially paying for another trip in the airplane. And, by that second spraying, we may need to treat for stink bugs anyway – just add to the mix.”
Billy Moore, Mississippi Extension plant pathologist emeritus, is involved in scouting/planning for Asian soybean rust in the state.
“Since yesterday, I’ve put over 700 miles on my vehicle,” said Moore on Thursday. “We’re looking hard for this disease.”
Over the last few years, Mississippi has been the earliest planted soybean state in the country. Currently, most of the state’s crop – “maybe 70 percent,” said Moore -- is past R-3, closer to R-4.
“The one factor that concerns me is the weather conditions have become more favorable for rust. I’ve been in and out of showers all day long, and it’s cooled down a little.
“However, right now, the amount of rust spores available is extremely low. And the air current isn’t blowing them into Mississippi. There’s been one exception to that: about two days before Dennis moved in, the weather modeling folks said rust could have moved into southeast Mississippi. Beyond those two days, though, nothing of much concern.”
Blaine echoed Moore’s beliefs. “As of 8 p.m. on July 14, we haven’t found any rust in Mississippi. It may show up tomorrow. It won’t surprise me if we find rust in the next 10 days. Chances are Dennis brought something here. But the next question you have to ask is: how much did he bring?”
Given how far along Mississippi’s crop is and having watched the progression of rust since it was found in Florida in mid-February, “I don’t think we’ve got much to worry about,” said Blaine. “I don’t see it building to massive levels, particularly since it hasn’t moved very far north.”
Some Mississippi soybeans are “done today. In three weeks, half our crop will be done at R-6. Any rust that comes in after that will have minimal impact. By August 15, (the vast majority) of the crop will be ready.”
With rumors and recommendations to spray for soybean rust coming from many directions, Blaine said producers often hear conflicting advice. “The producer has to make the final call. But if he hears something that doesn’t sound quite right, call us. There are some wild recommendations being made. Don’t believe everything you hear. Before spraying for rust, make sure you’ve heard the full story.”