Many farmers throughout Alabama have been blessed this year with plenty of rainfall, but with that comes the specter of diseases, and soybeans and cotton both have been affected during the 2012 growing season.
For the most part, the state’s soybean crop looked good in late August, according to Auburn University Extension Soybean Specialist Dennis Delaney.
“Soybeans are looking pretty good, and we’ve got planting dates stretched out from early beans that are close to being harvested and late beans, including some that weren’t planted until Aug. 10, in south Alabama,” said Delaney during the recent East Alabama Crops Tour.
The annual tour took place a week prior to Hurricane Isaac making landfall in Louisiana. The storm was expected to bring heavy rainfall and wind to Alabama’s Gulf Coast region.
“A lot of our late beans have real good growth on them and have a lot of yield potential, but we’re just now getting into the critical growth stage as far as pod set and pod-fill, and that’s even more important than the vegetative stage. But anything that’s good for soybeans is also good for diseases,” says Delaney.
Sentinel plots planted throughout Alabama act as an early warning system for Asian Soybean Rust, he says.
“The disease was able to over-winter in Gulf Coast counties on kudzu and then move north as we got rainfall and as soybeans matured. We’ve got about 20 of these plots across the state, at experiment stations, and in farmer fields. We sample them about once each week as they begin blooming and become susceptible to rust,” says Delaney.
The plots are planted in various maturity dates so the rust can be monitored as it moves north.
The disease was found in early August in east-central Alabama, in Group III and Group IV soybeans, he says.
(For an earlier report on soybean rust in Alabama and recommended control procedures, click here).
At five locations in Alabama, researchers put in planting date tests, with soybeans being planted about every 10 days starting at the first part of June, says Delaney. The purpose of the study is to determine how yield decline corresponds with planting date.
“It was surprising last year that yield really didn’t drop off until after the fourth or fifth planting date on about the middle of July. That’s one year out of many, when we had a lot of late rain, which we don’t get every year.
Gulf Coast is exception
“The only exception was the Gulf Coast, where yield fell off after the first planting date. It’ll take several years to come up with good data from that study.”
As far as insects across the state, kudzu bugs continue to be a concern, says Delaney.
“They come in through northeast Georgia. They’re a stink bug, but they don’t affect pods like other stink bugs. They’re more like aphids — if there are enough of them, and a high-enough population, they’ll suck the juice out of the plant, but it takes a high population to do that.
“So if you see a few here and there, don’t get too excited. We haven’t had them in the U.S. long enough to really get a good handle on them. But in Georgia and North Carolina, they’re advising to just wait.
“The migration of adults won’t happen for about a month or so, so if you spray early, you’ll probably be spraying again and again, killing the beneficial insects. You probably should wait until you start seeing the nymphs and the egg masses. You might can get by with just spraying once.”
Corynespora leaf spot, a disease which researchers know little about, can be devastating in a cotton field, says Austin Hagan, Auburn University Extension plant pathologist.
“We’re used to seeing some leaf spot in a field, especially as we get towards the end of the season and potassium levels are low, but this is a little bit different.
Corynespora started out in Georgia about five years ago, he says. “They’ve had problems with it there, it moved into Alabama last year, and there’s a good deal of it this year.
“Just from observations, it appears the disease is rotation related. In other words, if you grow cotton after cotton, you’ll have it. If you’re in a reduced-tillage system when you’re in continuous cotton, you’re even more likely to have it,” says Hagan.
The other factor is irrigation, he adds. “It seems to be much more of an issue in those areas that get a lot of water. When you get down to Baldwin, Mobile and Escambia counties, where they get afternoon showers most every day, they don’t need irrigation to have issues with this disease, and they’re having a lot of problems with it right now.”
This disease “came out of nowhere,” says Hagan, and wasn’t an issue in the past.
“At this point in time, we really don’t have much information concerning management of the disease. We’re basing our information of the disease on experiences we’ve had with other foliar diseases in other crops. This is more of an infectious disease than those other leaf spots you see in cotton,” he says.
Found first on lower leaves
The lesions show up first on the lower leaves on the cotyledons and then spread upwards through the canopy, he explains.
“That’s not the pattern you see with what we consider the fertility-related leaf spot diseases where the spots themselves seem to show up in the top and middle canopy at almost any time of the year, particularly after the plant starts to set bolls.”
Corynespora spots are fairly large in comparison with the spotting that you see with fertility-related leafspot, he says. Spots are one-quarter to three-quarters inch in diameter, there’s a good amount of chorosis of the leaf, and the leaves fall off once they get four to six spots on them.
“There clearly is a defoliation issue,” says Hagan.
“Whether it affects yield clearly will depend on when the defoliation occurs. If it occurs late, it could help to control boll rot. What we don’t know is that if the leaf sheds while the bolls are forming, what impact will it have on the yield and quality characteristics of the cotton produced in that boll.”
There haven’t been any studies that specifically addressed the amount of lint loss you could have from this disease, says Hagan.
“We have studies where we’re looking at two varieties and different numbers of applications of Headline, which is one of the two fungicides registered for the control of this disease.
“We’re putting from one to six applications in different plots with the idea of generating different levels of leaf spot, and then we’ll relate that to yield. In those plots, the cotton is rank, and the leaf spot actually got ahead of us. There’s up to 50-percent leaf shed in the cotton.”
There are several varieties, he says, with between 10 and 25 percent defoliation, and other varieties are at 10 percent or less. Potassium-related issues will complicate this test, notes Hagan.
“One of the problems with fungicides is that when we spray over-the-top, we’re putting fungicides only on the top leaves. We may have to look at changing our sprayer systems to try and drive the spray down into the canopy.”
The only two products labeled for corynespora are TwinLine and Headline, he says.
“It looks like the more applications we put on, the better the leafspot control, and that’s not real good. Both of those have the same basic chemistry.
“We’re limited to two applications of either of those products, and that’s it. We might need options because the same fungus goes into tomatoes, and in a couple of years, they had control failures due to resistance using the same product we’re using right now on cotton.”