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The soybean crashers

Gil Gullickson 05/04/2011 @ 9:37am Crops Technology Editor for Successful Farming magazine/Agriculture.com

Ever had some pesky relatives or neighbors show up on your doorstep at the most inopportune time?

Most often, a polite brush-off gets them to leave. No such luck exists, though, with two unwelcome guests that have recently surfaced in soybeans. White mold and Sudden Death Syndrome (SDS) zapped yields in 2009 and 2010. Here are factors to consider to manage these pests.

How to fold white mold

Inoculum in your soils–sclerotia–sets up white mold infection. “Sclerotia can survive 8 years or more years in soil,” says Vince Davis, University of Illinois (U of I) Extension agronomist.

Sclerotia presence doesn't mean white mold will surface, though. Sclerotia morph into mushroom-like structures called apothecia. Each apothecium can produce millions of ascospores that can infect soybeans. Infections hinge, however, upon cool temperatures around 65°F., wet conditions, and good soil moisture.

“You normally see white mold symptoms surface in late September. But the disease actually starts in late June or early July,” says X.B. Yang, Iowa State University (ISU) Extension plant pathologist. “It attacks soybeans at the flowering stage.”

Yield losses from white mold quickly add up. A rule of thumb is a .3-bushel-per-acre yield loss for every percent incidence of disease, says Carl Bradley, U of I Extension plant pathologist. Sclerotia in grain also can spur quality discounts.

Hot weather helps curb white mold. “This fungus cannot actively grow at temperatures above 82°F.,” says Bradley. Steps to take to accompany the natural control include the following.

• Avoid narrow row spacings. Drilled or 15-inch rows spur an earlier dense canopy that festers white mold by curtailing air circulation.

• Plant tolerant varieties. This by itself, though, isn't a guarantee for curbing white mold. “Sometimes they work, sometimes they don't,” says Yang.

• No-till. When combined with rotating to corn or wheat, less white mold risk results. “Sclerotia on the surface or buried less than 1 inch can germinate,” says Yang. “Once spores germinate, they will not survive if they are in a nonhost crop.” Meanwhile, burying sclerotia with tillage guarantees survival for several years.

• Spray with a fungicide if environmental conditions deem a white mold outbreak likely. “Soybeans are an indeterminate plant, which means they are blossoming all the time,” says Randy Myers, Bayer CropScience fungicide product manager. “Over that period of time, soybeans will develop new infections. You want to apply by R1 (beginning bloom) to get protection, and you may need to reapply at R3 (beginning pod) if conditions remain favorable for white mold development.”

Contans, a biological product, effectively reduces sclerotia levels in high-value crops and is labeled for soybeans. “If you have high levels of sclerotia, you may have to apply it in multiple years to reduce the sclerotia population so it impacts disease levels,” says Bradley.

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