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A Case for the Big Picture in Pest Effect Assessment

Pest management specialists are always on the lookout for new and/or damaging invasive pests in soybeans. And that is a good thing. Examples of such events follow.

•   In 2004, Asian soybean rust made its first incursion into the continental US, and that has kept soybean producers and production advisers on their toes to ensure this pathogen never gets a foothold into any year’s soybean crop. And so far, efforts to keep this pathogen from wreaking havoc to the US soybean crop have been successful.

•   In 2013-2015, the kudzu bug became a concern to Midsouth soybean producers, and entomologists are now vigilant to ensure this pest is managed when necessary (notice the word necessary).

•   The redbanded stinkbug became a significant soybean insect pest in Louisiana in the early 2000's. In 2017, this insect made its first major incursion into the Miss. and Ark. soybean crops. This caused entomologists to rapidly collect, assimilate, and disseminate all known information about this pest so producers could be on alert for possible future damaging infestations of this pest. That vigil became a full-force effort in the fall of 2017, and is now ongoing.

It is said that with humans, two things are certain–death and taxes. With soybeans, two things are certain–1) there will always be new and potentially damaging pathogen and insect pests, and 2) soybean scientists and industry personnel will always be investigating the potential impact of those pests and ways to manage/control them when they are at damaging levels. But of course, it must be proven that a new or recently identified pathogenic pest (such as taproot decline) is in fact damaging soybeans in producer fields before application of management strategies that cost money are justified.

Along this line, a recent (Mar. 2018) article titled “Incidence of soybean vein necrosis virus (SVNV) in Alabama soybean fields” by Sikora et al. (Plant Health Progress, Vol. 19, No. 1, p. 76) caught my eye. It reminded me that this viral pathogen was a somewhat hot topic a few years ago (first reported in the US in 2008), and this resulted in a flurry of activity to determine what the presence of this virus might mean to soybean producers.

In the above article, results are presented from a 4-year survey of Alabama soybean fields that was conducted to determine the prevalence of SVNV in the state. Ninety-nine fields were sampled over the 4-year (2013-2016) period. From this survey, SVNV was detected in 27 of the 28 counties tested, and the disease was detected most in the northern region of the state where most of the soybean acres are located. This study demonstrated that SVNV is prevalent in soybean fields throughout Alabama. According to the authors, “although SVNV has become widespread in the state, the potential impacts on yield are unknown.”

Interestingly, an article titled “Effect of soybean vein necrosis on yield and seed quality of soybean” by Anderson et al. (Canadian J. of Plant Pathology, Vol. 39, No. 3, p. 334) verifies that the SVNV pathogen does not necessarily reduce soybean yield when it is present. Results from field studies that were conducted in Wisconsin in 2013, Alabama, Indiana, Iowa, and Wisconsin in 2014, and Indiana in 2015 showed no case where plants with SVNV symptoms yielded significantly less than those without SVNV symptoms. In fact, in 2 of the 7 site-years, symptomatic plants produced significantly more seed yield, and in 6 of the 7 site-years (including Alabama), yields were numerically higher from symptomatic plants.

SVNV symptoms were associated with changes in oil and protein content in soybean seed in some site-years. The authors indicate that these seed component changes are a characteristic of viral infections in soybean. However, their impact on a producer’s bottom line will be negligible since yield is the economic component of concern for most commodity producers. The statement by the authors that “although SVNV symptoms were associated with changes in oil and protein content in soybean seed in certain years and locations, yield loss was not associated with SVNV in soybean in any of the six states in the study” is a very succinct summary of the lack of economic impact this viral pathogen presently has on soybean. This is supported by results from the annual survey to estimate “Soybean Yield Loss to Diseases in the Midsouthern US” that show viral diseases have little or no impact on soybean yield in the region.

The above summary of research involving SVNV presents an example of a pathogen that, although present in soybean fields, may not necessarily be detrimental to yield. Thus, it is incumbent on soybean research, extension, and industry persons to ensure that the presence of a pest of soybean does in fact represent an economic threat to producers before projects and management strategies that cost money are promoted/employed to manage or control the pest.

Composed by Larry G. Heatherly, May 2018, larryheatherly@bellsouth.net