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Late-season soybean diseases: Mississippi update

Tom Allen has served as Mississippi State University Extension plant pathologist since May, 2007. Previously, he’d worked with the Texas A&M system at the Amarillo, Texas, station on “a fairly rare wheat disease, Karnal bunt. It hasn’t been found in Texas since 2001-02, but it’s a problem in Arizona’s wheat-growing area. Originally, it’s from India and the disease is trade-limiting with many European nations.”

While wheat isn’t a current concern for Allen, the continuing spread of soybean rust in late-season Mississippi soybeans is. The plant pathologist spoke with Delta Farm Press on September 25. Among his comments:

A quick sketch of the Mississippi growing season…

“There were very low levels of disease in the state’s soybeans early on. Growers often put out a shot of fungicide at R-3/R-4. There was so little disease at R-3 this year that we suggested that application be held a bit later than the norm.

“As usual, we flailed around the soybean acres looking for rust. It’s often like looking for a needle in a stack of needles — forget hay. Late in July, those efforts paid off when rust was found in Jackson County.

“We suspected rust would be found in southwest Mississippi, close to where Louisiana had already confirmed some. That was also true of southeast Mississippi, since rust had also been found in south Alabama. Those are two areas where we focused scouting efforts.

“Now, though, post-hurricane and after so many weeks of rain, a large spore load came out of the Southeast — probably Florida — and is hitting our soybean crop. Apparently, the Southeast has been cooking a lot of spores in kudzu patches. Anyway, the effects of that began showing up in Mississippi in the last half of September.

“A lot of that spore load rode in on Hurricane Fay. Then, those spores were moved even more by Gustav. The rain between both of those events provided superb conditions for soybean rust development.

“At this point in the season, rust is common and easy to find. That said, it hasn’t caused any yield loss, to our knowledge. I feel safe making that claim.”

On the latest rust finds…

“Rust was found in Tunica County, Miss., on (Sept. 24) at very, very low levels. I think it was found on one leaf and that was it.

“Rust was also found in Amite County and Lincoln County and it wasn’t so sparse. There’s a good bit of rust in those fields which were approaching R-6. However, there aren’t a lot of soybeans grown there.

“We even went into Franklin County to look for rust. The only field that was close (to the other rust finds) was 25 acres that had been mowed down to about 4 inches in height by deer. The grower will hopefully have some good venison to eat, but he won’t have much of a soybean yield.”

How much of Mississippi’s crop was likely sprayed with a fungicide for soybean rust specifically?

“Most applications were for dual purposes, at least. Some fungicides were targeting rust alone — but that accounts for less than 5 percent, probably closer to 3 percent.”

On post-hurricane diseases other than soybean rust…

“There’s nothing overwhelming the soybeans. Right now, there’s more frogeye, but it isn’t causing much trouble. It’s showing up mostly in the late-planted crop, the double-cropped beans and beans that were hit hard with rains.

“Also, late-season cercospora-based foliar diseases are showing up. Depending on location, there can be a lot of that in a field.

“There’s still some downy mildew but the lower temperatures are helping keep that in check. That isn’t unusual.”


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