Rain Increases Risk For Sudden Death Syndrome In Soybeans

Rain Increases Risk For Sudden Death Syndrome In Soybeans

Wet weather so far during 2014 growing season has raised risk of this serious yield robbing bean disease.

Wet weather this growing season has increased the risk for sudden death syndrome in soybeans, says Darren Mueller, Extension plant pathologist at Iowa State University. "Unlike last year, most of the Iowa soybean crop this year was planted before the bulk of the rains started, which further increases the risk of SDS occurring. The early wet weather we have experienced so far in 2014 helps increase the root rot phase of the disease," he says.

SDS ON SOYBEAN LEAF: Sudden death syndrome is one of the most damaging diseases of soybeans in Iowa and much of the Midwest. There are two phases of this disease: a root rot phase and a foliar symptom phase.

One of the driving factors for late-season SDS development is significant rainfall during the late-vegetative and early reproductive stages. The rainfall totals for June 2014 are near the numbers that occurred in the SDS years throughout much of Iowa. For example, the precipitation total in Ames during June was 10.23 inches. The question is, will it keep raining? Rainfall in July and August will determine how bad this disease will infect fields this year.

Risk of Sudden Death Syndrome increases with rains
"One thing we've learned from outbreaks of sudden death syndrome in years past is that this disease likes it wet," says Mueller. "Last year we wrote about the risk of SDS increasing with the early season rain. But at the end of the article we threw in one caveat -- soybeans were planted very late in the spring of 2013, which reduced the risk of SDS developing. And after we published the article, the rains essentially stopped. Fast forward to the end of the 2013 growing season -- we still had some SDS in parts of Iowa in 2013, but it was not as nearly as bad as it could have been."

LEAF SYMPTOMS OF SDS: There are a few other diseases that may be confused with SDS such as brown stem rot and stem canker. All three of these diseases can have yellowing leaf symptoms. You need to look for lesions on the outside of the stems (stem canker) and browning in the pith inside the stem (brown stem rot) to distinguish from SDS.

This year's rain is triggering a similar increased risk in SDS developing. Unlike last year, most of the soybean crop in 2014 was planted before the bulk of the rains started which further increases the risk of SDS. And the early wet weather Iowa has experienced so far in 2014 helps increase the root rot phase of the disease.

Development favored by rain in early reproductive stages
As the table on rainfall data shows (see table accompanying this article), one of the driving factors for late-season SDS development is significant rainfall during the late-vegetative and early reproductive stages. As Mueller has noted, the totals for June 2014 are near the rainfall numbers that occurred in the past SDS years throughout much of Iowa. The precipitation total in Ames during June 2014 was 10.23 inches.

Table shows in years when SDS is prevalent, there's more rain in summer, especially June and July. Compare the average total precipitation in four years with high SDS prevalence (1993, 1998, 2008, 2010) and five years with low SDS prevalence (2001, 2004, 2005, 2007, 2011). These values are averages for two locations: Ames (central Iowa) and Mount Pleasant (southeast Iowa). The 30-year mean or average for period of 1981 to 2010 is also at Ames and Mt. Pleasant.

You need to know the symptoms of SDS when looking at soybeans and how to distinguish SDS from other diseases. "Remember, there are a few other diseases that may be confused with SDS such as brown stem rot and stem canker," says Mueller. Look for lesions on the outside of the stem (stem canker) and look for browning in the pith or inside of the stem (brown stem rot) to distinguish these diseases from SDS.

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