Soybean growers this fall may have problems with green stem and mottled seed. Many of the problems may be caused by bean pod mottle virus (BPMV) or soybean mosaic virus (SMV). Both viruses cause the same leaf symptoms, green stem in the fall, and mottled seed coats. BPMV is transmitted primarily by bean leaf beetle; SMV is transmitted by at least 30 aphid species, including the newly introduced soybean aphid. Many soybean cultivars do not display obvious symptoms, and there is no positive correlation between severity of leaf symptoms and amount of seed coat mottling on fall-harvested seed.
One concern of growers is the risk associated with planting mottled seed. What is the chance that planting mottled seed will result in transmission of virus? Research sponsored by the Iowa Soybean Promotion Board and the North Central Soybean Research Program is addressing this question.
Growers must distinguish between seed-transmitted and seedborne virus. Both BPMV and SMV can be transmitted through seed (seed-transmitted virus) only when the virus is in the seed's embryo. Virus only in the seed coat (seedborne virus), but not in the embryo, cannot be transmitted through seed. Commonly, seed-testing laboratories prepare a seed extract for virus testing by grinding a seed sample; hence, seedborne virus is detected. The test does not distinguish between seedborne and seed-transmitted virus or predict whether there is a significant risk of seed transmission when the seed is planted. Seed-transmitted virus can only be detected by using grow-out tests where soybean seedlings are tested for virus.
Recent research with BPMV demonstrates this concept. Seedlings produced by seed from mottled seed lots of two commercial soybean cultivars were examined for BPMV in grow-out tests. Seed transmission was shown to be very low (0.04 and 0 percent in the two cultivars), even though the percentage of seed coat mottling was very high. When seeds were ground, both seed lots tested positive for BPMV. When individual seeds of one cultivar were ground, 31 of 35 nonmottled seeds tested positive for virus, whereas 47 of 65 mottled seeds tested positive. Similarly, with the second cultivar all nonmottled seeds tested positive for virus and 79 of 87 mottled seeds tested positive.
Two conclusions can be drawn from this research. First, testing for seedborne virus might suggest that the soybean seeds are unsafe to plant; however, they would be fine to plant. One cultivar contained no seed-transmissible virus, and the rate of seed transmission in the second cultivar was negligible. The seed transmission rate of BPMV seems to be so low that it probably does not contribute significantly to the amount of virus observed in Iowa soybean fields. Second, seedborne BPMV is found in both mottled and nonmottled seeds; furthermore, not all mottled seeds contained seedborne BPMV. Thus, seed-coat mottling is not a good indicator of seedborne BPMV and seed-testing results must be interpreted with caution.
Research at Iowa State University (ISU) suggests a similar outcome for SMV. Infected soybean plants may produce both mottled and nonmottled seed, seedborne virus can be detected in both nonmottled and mottled seed, and all mottled seed does not contain seedborne virus. Seed coat mottling is not a good predictor of virus presence in a seed. Nevertheless, the virus can be transmitted through seed at levels up to 50 percent; however, seed transmission rates in most currently grown Iowa soybean cultivars vary between 0 and 5 percent.
What, then, is the value of a seedborne virus test for these two viruses? First, it may reveal the cause of an evident seed quality problem. Because seed coat mottling caused by the two viruses is indistinguishable, only a laboratory test can identify the cause of the problem. Second, for SMV, a positive test may suggest the potential for seed transmission if seed is planted. This potential may be problematic in the future because the soybean aphid will efficiently transmit SMV. However, absence of the virus will prevent its transmission. Because SMV is introduced into the North Central states every year primarily through seed, planting SMV-free seed will prevent transmission by soybean aphid from becoming a problem. And third, potential for BPMV seed-transmission appears to be very low. Current research at ISU suggests that other sources of BPMV contribute more significantly to viral outbreak.
Thus, proper interpretation of results from a seed test is very important. An understanding of how the test is conducted and what it is measuring is necessary. Also, an understanding of the relative contribution that seed transmission may make to disease development is important.
This article originally appeared on pages 179-180 of the IC-486(22) -- September 17, 2001 issue.