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Corn+Soybean Digest

Fungus Among Us

Soybean growers may not have experienced significant yield losses from troublesome fungus problems in 2001. But being on the lookout may pay dividends in 2002.

Top-dieback, pod and stem blight and purple seed stain were noted by pathologists across the Midwest during last year's growing season.

They're a reminder that additional outbreaks can lead to inoculum build-ups in plant debris. And that may affect this year's crop yield.

During early spring rains, the fungus found on soybean residue, known as Phomopsis or Diaporthe, is splashed onto plants infecting them.

“The timing of the infection and condition of the soybean plant affects the outcome of the disease,” says X.B. Yang, an Iowa State University plant pathologist. If the fungus attacks during the early vegetative stages, the result is stem canker. If it occurs later in the growing season, it is top-dieback and pod and stem blight.

Hot, dry conditions late in the 2001 growing season triggered an increase in pod and stem blight by stressing the already infected soybeans. “If the majority of the infection occurs after pod formation, there's little effect on yield,” says Yang.

Soybean growers with pod and stem blight are urged to use tillage and rotate out of soybeans to reduce inoculum buildup in the residue. Soybean varieties aren't rated for pod and stem blight resistance. So Yang urges growers to plant the same varieties this year that had been grown in local fields showing fewer disease symptoms last year.

Many think purple seed stain, another common fungus problem in 2001, is only a problem for food-grade soybean producers. But its impact on germination and seed weight may affect row-crop profits as well.

The fungus is easily identified by light to dark purple discoloration of the soybean seed coat, reports Laura Sweets, plant pathologist at the University of Missouri. Before harvest, the fungus can be seen as reddish-brown bronzing in the upper part of the plant canopy.

Moist, warm conditions within the crop canopy favor development. Because the fungus overwinters in crop residue, Sweets urges crop rotation in infected fields.

Moderate infection levels reduce seed size and germination levels. “Producers should not plant seed with significant levels of purple seed stain,” says Sweets. A fungicide may be applied to infected seed but won't prevent infection during late-season infestations in the canopy.

“If growers notice purple seed stain at combining, it may be possible to reduce contamination throughout the seed lot by adjusting the combine,” says Sweets. Infected seeds are generally smaller and lighter, allowing them to pass through a properly set combine into the tailings.

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