Several soybean diseases in Arkansas

Overall, the soybean crop in Arkansas is a good one and disease problems have been erratic, to say the least.

Terry Kirkpatrick and Cliff Coker, Arkansas Extension plant pathologists for soybean diseases, noted that stem canker was more widespread than in the recent past. They say this is worrisome because the disease can build up in fields, and farmers should plant resistant varieties next year. Kirkpatrick noted that more new soybean varieties are tending to be susceptible to stem canker these days, not a good trend.

Kirkpatrick said that root-knot nematode was again a problem on mostly Group 4 soybeans in Arkansas. However, soybean cyst nematode complaints lessened this year compared to last.

Robert Riggs, internationally known expert on cyst nematodes, said that the timely and extra rainfall in the state this summer reduced visible cyst problems since good soil moisture reduces damage. However, both scientists agreed that cyst nematode was back, based on last year's problems and current races of the nematode cannot be controlled by our “resistant” varieties.

Cyst nematode can cause substantial yield loss to soybean even when above-ground symptoms are not very noticeable. In fields where yields have been declining, sample for cyst nematode.

Coker and Kirkpatrick said that Phytophthora root rot, a serious early-season disease, was more common due to the cool, wet May.

Coker also saw more aerial blight, especially in northeast Arkansas. Caused by the same fungus that causes sheath blight in rice, it has become more common recently. Most varieties are susceptible, he noted, and more research is needed. Coker and Roger Gipson, Clay County, Ark., Extension agent, have found Quadris fungicide to be very effective for aerial blight if applied at the right timing.

Coker said that frogeye leaf spot is also back this year with Group 4 soybean varieties hardest hit. Epidemics occurred in variety tests, so our disease ratings should be improved. With the loss of Benlate and trouble finding Topsin, Coker said, many growers tried Quadris to battle frogeye. However, Quadris has not been that great in his tests unless it was applied before the disease reached our threshold levels of 3 to 5 percent tissue damage at R-3 growth stage.

“As a preventative, Quadris is pretty good on frogeye, but weak once the fungus has infected the plant,” he said.

Cercospora leaf blight, which turns upper leaves bronze late in the season, has been common and will become more so near harvest. This disease can cause purple seed stain, and Quadris is not effective, he said.

With late drought conditions this year, charcoal rot has only been noticed recently, Coker said. He observed that it is now hurting late-planted Group 4 and Group 5 soybeans. As the roots shut down, soybean seed in infected plants will be small and shriveled.

Other seed quality problems were again troublesome with planting date, summer stress and difficult-to-control diseases all presenting routine challenges to seed growers in Arkansas, noted Pat Fenn, soybean seed quality expert with the University of Arkansas department of plant pathology.

Early-planted beans fill and mature during hot, humid July and August, resulting in lower quality seed, while later-planted varieties that fill after temperatures start cooling off have higher quality. The trade-off is lower yield on later varieties, he said.

Fenn observed that timely harvest is important since nothing good happens to mature soybeans sitting in the field. He is researching seed quality problems and cures.

Sudden Death Syndrome was sporadic this year in Arkansas, said John Rupe, a soybean disease researcher in the department of plant pathology. The disease is often worse under ideal growing conditions and moisture and can be more severe if cyst nematode is present, he noted.

Rupe and Coker noted that both green bean syndrome and stinkbugs, which have plagued Arkansas soybeans in recent years, were minor this time.

While it sounds like we had a lot of problems, Carl Hayden, Chicot County, Ark., Extension agent, noted that despite various challenges he is again seeing better fields cut 70 bushel soybeans or more.

Rick Cartwright is an Extension plant pathologist with the University of Arkansas Division of Agriculture.

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