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Corn+Soybean Digest

Dealing With Disease

With high temperatures and sticky humidity over much of the year, southern growers can often see a deluge of diseases in soybeans. And that's not counting the threat of Asian soybean rust.

From Pythium to Phytophthora, purple seed stain to pod blight, Fusarium to frogeye leaf spot, soil- and airborne fungi can be as deadly for beans as the worst insect infestation. And now a new disease, soybean wilt, has joined the family as a threat to the South.

Infestations can start with poor-quality seed resulting from seed coat contamination by Phomopsis/Diaporthe and other fungi, says Tom Allen, Mississippi State University Extension plant pathologist. This can result in decreased germination and vigor.

Diseases can attack virtually any time during the growing season, he adds, while walking through a field infested with Phomopsis/Diaporthe complex symptoms last fall. Allen says there are “two main diseases in this fungal complex that include Phomopsis seed decay and pod and stem blight.” So growers must remain on their toes.

FOR SEVERAL YEARS, agronomists and plant pathologists have advised growers to plant earlier-maturing varieties in the early Group IV to late Group V range. This reduces vulnerability to late-season rainfall that often occurs even if tropical storms aren't in the forecast.

A shortage of soybean seed due to increased bean acres caused more growers to plant later-maturing varieties. Virgil King, an agronomist and crop consultant in Lexington, MS, says that created the potential for more late-season diseases. That potential transformed into reality when a series of hurricanes and tropical storms whipped up above-normal rainfall.

And then there's the new, unidentified disease.

“We started seeing a newer disease recently that we're calling soybean wilt,” says Allen. “We're seeing it; Georgia is seeing it. Louisiana is seeing it, and no one really knows what causes the disease.”

Allen has done extensive lab work on the mystery disease at his Stoneville, MS, location. “At the field level, the leaf symptoms can look a lot like sudden death syndrome (SDS),” he says. “Laboratory analyses have isolated an unidentified species of Fusarium and Macrophomina phaseolina, the agent of charcoal rot.”

King also points out that the new disease has symptoms that are similar to SDS, which begin as small, bright, pale green to yellow circular spots on the leaves. As the disease progresses, brown to tan areas surrounded by chlorotic tissue develop between the veins. The pith of affected plants is white.

This characteristic will separate SDS from brown stem rot, in which the pith is tan to brown in color. But it's hard to tell the new wilt from SDS. “Culturally, we are seeing some of the same fungi in this disease,” says Allen.

“We're working to determine what is causing it,” adds King. “It looks like it could be a big problem for us.”

The disease was initially observed as early as 2003. It was on mostly lighter soils that had traditionally been planted in cotton. “We have a lot more soybeans on those acres and a lot of double-crop wheat and soybeans,” says Allen. “However, in 2008 we started seeing the SDS-like symptoms on all types of soils and in irrigated and dryland fields.”

King, who works with growers over several western Mississippi counties, says that in the mid-season R3 growth range, the region saw many acres hit by Septoria brown spot.

In this disease, irregular dark brown spots, varying from minute specks to just less than 0.1 in. diameter, appear on both upper and lower surfaces of leaves. Leaves can quickly turn yellow and drop. Adjacent lesions frequently form irregular-shaped blotches. Numerous irregular, light brown lesions form on trifoliate leaves, says King.

Other typical diseases that emerge in southern soybeans at the R3 stage include frogeye leaf spot, which has the easily recognizable grayish spots on leaves. “Cercospora leaf blight or late-season Cercospora is usually picked up everywhere, and there seems to be no fungicidal control of the disease at all,” says Allen.

“We're not aware of any soybean variety that does not see late-season Cercospora.” Yield losses are likely low, but even a 2-3-bu./acre loss to the disease would mean close to $30 less return with beans bringing $10-11/bu.

“Also taking us by surprise were Phomopsis/Diaporthe seed and pod-rotting diseases,” says Allen. “It was due to the massive amount of (wet) weather we had.”

Other diseases that regularly aggravate southern growers include Pythium seedling diseases that impact emergence. The infected areas of the root are soft but rapidly turn brown, and tissues slough off to leave a “wire root” appearance.

Rhizoctonia on roots frequently kills young soybean plants. Look for sunken reddish-brown areas on the main and upper roots near the soil line, which resemble “sore-shin” on cotton. The infected area is firm, in contrast to Pythium.

Additionally, Fusarium occasionally causes a root rot problem on seedlings. Affected seedlings are stunted and weak. The lower part of the taproot and lateral root system may be destroyed.

SOME SOUTHERN GROWERS apply a strobilurin fungicide on every acre of soybeans, a practice that has its pro and con believers. Traditionally, this application has slightly boosted yield when beans are planted following the prior season of beans. King says a 6-oz. dose of Quadris or Headline is applied at the R3-R4 stage.

“We were able to extend this application because of environmental conditions and the absence of disease,” says King. “This allowed us to optimize and extend the protection further into the growing season.”

Within about 20 days, he and growers could determine if they needed another application of the same fungicide. “This application will last from 14 to 20 days,” says King. “At this time we can re-evaluate growth stages and disease pressure to see if we might need another fungicide treatment.”

Allen adds that, “Particularly this season (2008), we impressed upon growers to hold back applying the strobi until beans reached R4 to protect maximum nodes and since foliar diseases were not present in the crop at the time. But for the pod blight diseases, we suggested an option if bad weather was coming in.”

That option involved putting on a strobi or a pre-mix strobilurin/triazole.

King says the original strobi application can be tankmixed with a pyrethroid for combined insect/disease control.

Allen says that if soybeans follow rice, Quadris is often the fungicide of choice because it works well on Rhizoctonia aerial blight often seen after rice. He and King advise growers to seek information on their own soybean disease situations with their consultant or Extension pathologist.

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