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LSU updates list of Louisiana soybean disease symptoms

With Asian soybean rust increasingly in the Louisiana soybean disease mix, Boyd Padgett believes it's time for an updated review of symptoms. That's especially true since so many ASR symptoms overlap with established diseases.

“When I began looking at Cercospora foliar blight, I didn't think it would be difficult to distinguish from Asian soybean rust,” said the LSU AgCenter plant pathologist at the recent Crop Demonstration meeting at the Dean Lee Research and Extension Center in Alexandria, La.

“In fact, there are symptoms that are very similar (between the two).”

The major Louisiana soybean diseases:

  • Cercospora blight.

  • Aerial blight in the central and southern parts of the state.

  • Frogeye leaf spot.

  • Pod and stem blight and anthracnose.

Purple seed stain, or Cercospora foliar blight, is “probably the number one disease we have to deal with in the state. We don't have any fungicides that are extremely effective.”

There are a few fungicides that suppress the disease — “and are certainly better than having nothing” — but producers have nothing that entirely cleans up the problem.

The blight, caused by a fungus that overwinters on plant debris, usually starts with a reddening, or rusting, on the upper leaves and brown lesions on the petioles. It also affects the pods and seeds.

The disease's ability to overwinter “might be a problem because I'm told we're supposed to have a mild winter. Of course, we were told this would be one of our worst hurricane seasons, too.”

Cercospora leaf blight is usually a late-season problem. Producers don't normally see symptoms until beans are headed towards maturity — R5/R6. It seems to be enhanced by stress and thrives at temperatures between 75 and 80 degrees.

“The first signs are usually on the leaves in the upper canopy. The reason is the fungus produced a toxin turned on by sunlight. That's where it differs from ASR, which typically develops initially in the mid- to lower canopy.”

At the St. Joseph, La., research station, where Padgett conducts research, “we've had some pretty (heavy) ASR and Cercospora blight,” said Padgett as a slide of two leaves — one infected with ASR, one with Cercospora blight — was projected. “These two leaves look very similar. The only difference is the ASR pustules appear almost three dimensional. You don't see that with Cercospora blight. But the coloration is very similar.”

Other notes for Cercospora blight:

  • Another difference — “and you'll see this before the foliar symptoms” — is petiole lesions.

  • There are also symptoms on the seed, the purple seed stain.

  • It can also be mistaken for salt injury.

“In our neck of the woods we have salt problems. Many times, especially in droughty years, salt injury could be confused with Cercospora blight.

“But that isn't the case — there are no lesions on the petioles. The foliar burn can be on leaf margins where the leaves are trying to get rid of the salt.”

Padgett thought downy mildew and bacterial pustule would be difficult to distinguish from ASR. Now, “I don't think it will be. This is usually favored by high humidity and produces light-colored, yellow specks in the upper canopy.”

Bacterial rust with raised pustule areas was the disease Padgett thought would be most confused with ASR. Last year, most of the samples brought in looked “very scary” until checked under a microscope.

“Something that distinguishes bacterial rust from ASR is quite a bit in the upper canopy with little or no disease in the lower canopy.”

Frogeye leaf spot is an occasional problem in the state. Most fungicides applied for Cercospora blight are effective at controlling frogeye as well.

This year, Padgett saw brown leafspot caused by septoria. “I never really noticed it previously — probably because we didn't (scrutinize) the beans as much in the past.”

Brown leafspot can appear similar to ASR and occurs in the lower canopy. It's fueled by cool weather — also favorable to ASR.

However, once you roll the leaves over, “you won't find the pustules. In most cases in Louisiana, this disease occurs early and can cause limited defoliation of the lower leaves. I haven't seen any raging brown leafspot epidemics.”

Padgett did see aerial web blight in tests he's conducting in Jefferson Davis Parish. The blight developed very late in the season.

“Often, the leaves are glued together by the fungus. That's one way to distinguish it from other problems. This disease, in my opinion, has as much potential to reduce yields as ASR.”

As for ASR, Padgett said the abundance of the disease this year has allowed everyone to “get a good look. Last year, if you'll recall, ASR wasn't reported in Louisiana until Oct. 28. This year, it's already been found in over 20 parishes.”

What's the potential for next year? “Well, after this year, I think the potential will be greater in 2007. That said, it will depend largely on what type of winter we have.”

After looking at ASR so much this growing season, several things impressed Padgett. Not long ago, “a group of us traveled to Brazil and we were told ASR would always be found in the lower canopy. Well, that may be true there but there are a lot of things that grow on our lower canopy, making it harder to distinguish ASR from everything else. It was much easier to find ASR in the mid-canopy this year.”

Researchers were also told there was no definitive yellow “halo” associated with ASR pustules. “But some general yellowing is associated with the spots. It's a dull yellowing — turn the leaf over and it appears dusty.”

Another thing of note: this year, Padgett didn't see ASR in beans until they reached R5/R6. However, other LSU AgCenter researchers did report ASR in early reproductive stages. Still, Padgett remains hopeful.

“We had sentinel plots throughout the state — Group 4s planted right next to Group 5s. The only thing separating them was the row. You could find ASR in the Group 4s easily. Take one 40-inch step to the next row and you wouldn't find the disease in the Group 5s until they reached R5/R6. That's good news and if it continues maybe we'll be dealing with a late-season disease. Time will tell.”

When scouting “you can roll (an ASR-infected) leaf and almost see a roughness. Even without a hand lens it's evident — sort of sandpapery.

Another thing that might help scout: holding a suspect leaf up to sunlight will exaggerate any spots. “You might not be able to see them otherwise. Utilize the sunlight when scouting.”

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