The soybean crashers

02/14/2011 @ 10:50am

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.

Cobra herbicide also can help slow white mold. A 2009 U of I trial evaluated several fungicides and Cobra. “By mid-September, the treatment that looked the best was Cobra,” says Bradley.

Slice SDS

The defoliated and dead leaf tissue that marked SDS-infested fields in August and September last year originated much earlier. The fungus that causes SDS to survive in crop residue attaches to the soybean seed as it germinates. The late-summer symptoms you see are caused by early-spring fungal infestation of the plant’s xylem system.

“As it (the fungus) colonizes, it produces toxins that act like a herbicide,” says Yang.

The presence of SDS fungus in spring doesn’t translate into disease. Prolific water is needed to fuel fungal survival.

Ditto for temperatures below 70°F. “The longer soybean seed stays in the cooler soil, the more infection will occur,” says Yang.

Planting during warm and dry conditions help soybeans dodge SDS. It’s important to balance this with planting date. ISU research shows planting soybeans in northern Iowa by the first week of May boosts yields 83% of the time compared to mid-May plantings. Dates in other areas vary, but the early-planting edge applies.

Yang advises targeting high-risk fields with preventive measures. Here are three examples of high-risk scenarios.

• Fields infested with SDS the last two or three soybean crops.

• Flat river-bottom soils saturated with water.

• Seed corn fields. Leftover corn kernels provide a haven for SDS fungus.

What You Can Do

Yang recommends four practices on fields with high risk for SDS.

• Avoid planting into cool soil.

• Plant SDS-resistant varieties. “Not every variety marketed as SDS-resistant will deliver satisfactory SDS resistance,” reminds Yang. Resistant varieties in early maturity groups are less reliable than those in later maturity groups. “Even with good resistance, a 20% yield loss can still occur under severe SDS,” says Yang.

• Have well-drained soils. Tiling would not have helped flooded areas escape SDS in 2010, says Yang. Yet, tile that whisks away standing water in normal years can help check SDS.

• Manage soybean cyst nematode (SCN). Left unchecked, SCN aids SDS in triggering toxin production.

Unfortunately, fungicides don’t dent SDS. Another complicating factor is that white mold and SDS can infect fields in the same year.

That’s why taking steps to manage one disease puts you at odds with another. “Tillage reduces SDS risk, but increases white mold risk,” says Yang.

In these cases, Yang recommends planting a reliable SDS-resistant variety and using a fungicide to control white mold on fields plagued by both diseases.

“In June and July, we normally have a better view of conditions conducive to white mold,” Yang says. “You can then determine if you need a fungicide.”