A Step Forward Toward Control of Disease Caused by Bean Pod Mottle Virus – First Identification of Field Tolerance

Populations of bean leaf beetles, the insect that efficiently transmits bean pod mottle virus (BPMV), have

been very high this year. The August 7 issue of the ICM newsletter lerted growers to a greater amount of BPMV in the soybean crop this year, and potential yield reductions and reduced seed quality this fall. But not all soybean varieties will respond the same. We have known for some time that soybean varieties can vary significantly in response to disease caused by BPMV and other viruses. But identifying resistant varieties has been tricky so far, because leaf symptoms do not correlate well with yield response, and seed staining does not correlate well with the amount of virus in the seed.

Therefore, in a cooperative program funded by the NCSRP, Iowa Soybean Association, and Wisconsin Soybean Marketing Board, researchers at Iowa State University and the University of Wisconsin decided to take a different approach by asking: can field tolerance be identified? With a certain level of field tolerance, a soybean plant is still able to produce a reasonable yield and decent seed quality in the presence of virus disease. Can field tolerance be quantified, so that we have a number that will indicate that variety A is more tolerant of BPMV than variety B, and by how much?

After searching through many parameters, we have found that a quantitative assay of the amount of virus in the seed and the amount of seed coat mottling provided the best measure of field tolerance. These parameters allowed calculation of a seed index which has proved useful in preliminary identification of tolerance. Thirty-three soybean accessions were evaluated for field tolerance at two Wisconsin locations over a three-year period.

Compounding identification of tolerance to bean pod mottle virus has been the presence of soybean mosaic virus (SMV), whose leaf mottle symptoms and stained seeds look like those caused by bean pod mottle. Soybean mosaic, efficiently transmitted by the soybean aphid and not the bean leaf beetle, has been increasingly prevalent since the introduction of the soybean aphid in the North Central region. Further, infection by both viruses can cause synergism, meaning that damage caused by both viruses in the same plant is more than each alone. Therefore, it has been very important for us to correctly identify the viruses prevalent at the test sites. The technology developed for this study required that the assay could quickly and accurately differentiate between both viruses.

The good news is that three accessions showed tolerance to bean pod mottle and soybean mosaic viruses, four were tolerant to bean pod mottle, and eight were tolerant to soybean mosaic. This study suggests that field tolerance can be used to reduce the damage caused by these virus diseases. The seed index method will allow the seed industry to easily mechanize evaluation of accessions in order to provide tolerant varieties to producers. Further, it gives us a tool to estimate virus incidence in soybean fields. Details will be published shortly in the journal Crop Science.

Submitted by John H. Hill, professor of Plant Pathology with research responsibility in virology, and Craig Grau, professor of Plant Pathology at the University of Wisconsin with research responsibility in diseases of soybean. August 16, 2006

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