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WOOSTER, Ohio - Environmental stresses, rather than disease pressure, may be the cause of below-average soybean yields throughout some parts of Ohio this season.
Ohio State University researchers found that the presence of diseases was not high enough to dramatically affect yields, and they speculate that extreme dry conditions in some parts of the state, such as the north and northeast, may be the contributing factor.
Anne Dorrance, an Ohio Agricultural Research and Development Center (OARDC) plant pathologist, said some soybean plants collected from fields showed evidence of viruses. Researchers are testing samples to determine which soybean viruses are actually present. Some plants have malformed pods, indicating the presence of bean pod mottle virus or another soybean virus, while other plants are showing signs of green-stem syndrome, a symptom of environmental stress or other plant viruses.
Dorrance said environmental stresses may be the cause of below-average yields in some parts of the state, rather than any particular soybean disease.
"Stunting from root rot and drought conditions in parts of the state, like the north and northeast, affected yields this year," said Dorrance. "The hardest message to get across to growers is that it's difficult to assess yield loss this year when we've had such dramatic environmental conditions. For example, areas of heavy drought were also the heaviest for the soybean aphid. It's hard to separate what really is the cause and effect relationship." Soybean yields in drought-stressed northern Ohio are 30 to 50 percent below average, while southern Ohio is seeing record yields because of ample rainfall, which may help maintain the state average of 43 bushels predicted by the U.S. Department of Agriculture.
"In drought areas we surveyed, plants were barely to my knees, but in good areas the plants were up to my waist," said OARDC entomologist Ron Hammond. "I'd say the vast majority of the areas in Ohio had fairly nice looking beans. We are expecting most of the state to have good yields." According to the USDA, 71 percent of soybeans have been harvested in Ohio and 58 percent of the crop is reported to be in good to excellent condition.
Reports of soybean diseases have been low to nonexistent so far. Hammond, who surveyed fields throughout the state, said the presence of diseases has been light. "Evidence of diseases has been found in areas throughout the state but has ranged from extremely light to very light," said Hammond. "Green-stem syndrome is much lower this year than last year and not much of a problem." Said Dorrance, "Last year, that's all growers talked about was green-stem syndrome. We might have found it in 1 percent of the fields. This year we are not hearing anything so far. It's very, very hard to find." The exact cause of green-stem syndrome is unknown. Parts of the mature soybean plant turn brown except for the stem, which remains bright green. Green-stem syndrome can cause major reduction in yields because the green stem makes it difficult to mechanically harvest the pods and pods are in the field during rainy periods where the seed then develops secondary fungal infections. Another type of "green stem" condition where the plants remain green and the pods are malformed is caused by plant viruses like bean pod mottle virus and soybean mosaic.
Hammond said populations of the bean leaf beetle, which spreads the bean pod mottle virus, were low this year compared to last year. "Populations were nowhere near what they were last spring," said Hammond. "There was very little pod feeding in the fields we looked at. Only two-three fields at the most had a noticeable level of pod feeding compared to what we found in previous years. At best, we can say that fields had moderate populations." Added Dorrance, "We had tremendous bean leaf beetle populations over the last two years and this year the population just crashed. That may be why bean pod mottle virus is very low this year, but we are not entirely sure why that is," she said. "In fields we found it in, it was less than .001 percent. Areas of the state, like western Ohio, had a lot of problems with bean pod mottle virus last year, and initial reports this year show seed quality to be outstanding." Bean pod mottle virus causes reduction in seed quality due to seed coat mottling and reductions in yield by producing malformed pods that contain beans so dry they are unsuitable for harvest.
Soybean aphid populations were high in northern Ohio counties this year, but researchers are unsure whether the insect or environmental stresses may have caused yield reductions in that part of the state. The soybean aphid can transmit soybean mosaic virus, which produces similar symptoms as bean pod mottle virus.
"The only significant populations of soybean aphids were found in the northern part of the state," said Hammond, adding that thousands of insects were found per plant in some fields. "The farther away from the northern counties we got, the lower the populations. But a lot of the areas with aphids corresponded with severe drought conditions, so it's going to be hard to determine what is causing some of the yield reductions in those areas." Ohio State researchers will continue to test soybean plants to determine exactly what might be causing the diseases. One theory is that the diseases were carried into the field via seed transmission. "The evidence of these viruses in the field was found very infrequently and at random," said Hammond.