| Project Title: | Studies on Soybean Viruses:
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| Project Leader: | G. L. Sciumbato, Plant Pathologist, DREC, Stoneville, MS. |
| Other Participant: | F. A. Harris |
We planted all of the entries in the Mississippi Varietal Trials in replicated plots and inoculated them with SMV. The summer of 1998 was very hot and no virus symptoms developed on any of the inoculated plots including the susceptible checks. In 1999 we plan to delay planting and attempt to avoid the hot temperatures which mask virus symptoms. We have a culture of Soybean Mosaic Virus which we are using to screen the entries in the Mississippi Variety Trials in replicated greenhouse trials.
We planted all of the entries in the Mississippi Varietal Trials in a late-planted-natural virus infection trials. Results for the Maturity Group IV Early trial are given in Table 1. In general virus infection was light. The entries TN4-86 and TN4-94 had the highest degree of viral symptom expression. Several of the entries did not have any symptoms (rating of 1.0). Results of the Maturity Group IV Late trial are given in Table 2. The entry H4998RR had the highest degree of viral symptom expression. Several of the entries had no virus symptoms. Results of the Maturity Group V Early trial are given in Table 3. The entry RVS 520 had a very high degree of virus symptom expression (4.0). Several of the entries had no virus symptoms. Results of the Maturity Group V Late trial are given in Table 4. The entry H5 181 had a very high degree of virus symptom expression (4.0). Several of the entries had no virus symptoms. In all of the Maturity groups there was a wide range in virus symptom expression. This indicates that there are varieties available which do not show virus symptoms and these varieties can be used in a breeding program.
2. Efficacy of trap crops and insecticides to control virus vectors.
Insect samples were collected utilizing sweep net and visual methods in soybeans during 1998 at Stoneville, MS. The samples were for the bean leaf beetle, 3-cornered alfalfa hopper, green stink bug, and 12-spotted cucumber beetle. These insects were chosen due to their frequency of occurrence in the field and possibility of their ability to transmit bean pod mottle virus and/or soybean Mosaic. Twenty-five sweeps were made at each sample site and observations by number and species of the above insects recorded. Sub samples of five or ten insects of each species were placed in separate containers and subsequently tested for the presence of virus. ALC insect blot samples for virus were sent to the Department of Plant Pathology at Virginia Tech University for testing.
Several samples of insects were assayed for the presence of bean pod mottle virus (BPMV) (Table 5) and soybean Mosaic (SMV) (Table 6) provided positive results for BPMV from the bean leaf beetle as expected. However, the 12-spotted cucumber beetle also showed positive for BPMV. This positive reaction for BPMV in the 12-spotted cucumber beetle was an unexpected result since most research sources do not include this insect as a possible vector of BPMV. Also, the 3-cornered alfalfa hopper and green stick bug proved to have positive but weak inconclusive reactions.
Samples of insects for SMV gave positive but weak reactions from the 3-cornered alfalfa hopper and bean leaf beetle. Only very weak or no reaction was noted from samples of the green stink bug. Some aphids were collected but not enough was available for samples to be tested for virus presence.
Additional insect samples were collected from Soybean Mosaic resistant and susceptible soybean plants (Tables 7,8). Insect vectors were present in both the SMV resistant and susceptible soybean plants.
These results of possible new vectors of soybean viruses show a need for continuation of sampling and possibly new research in this area of virus vectors. Early sampling and testing should be conducted from weed hosts as well as from seasonal soybeans to determine if the virus is present in over-wintering populations.
3. Identification of viruses present in Mississippi soybeans.
The growing season in 1998 was abnormally hot. Virus symptom expression in the early planted soybeans was masked by the hot weather. The state variety trial at Starkville was severely infected with viruses. Samples were collected and analyzed for soybean mosaic and bean pod mottle virus. The results of the analysis are given in Table 9. Almost 100 percent of the samples were infected with Bean Pod Mottle Virus. Six of the samples were also infected with soybean mosaic virus. We found a few entries which were not showing any virus symptoms in the field and are determining if they were escapes or had real resistance.
The results of the virus analysis in the late planted variety trial at Stoneville are given in Table 10. Almost all of the samples tested positive for Bean Pod Moddle Virus. About 20% tested positive for soybean mosaic. It appears from this and other trials that Bean Pod Moddle Virus is the main insect vectored virus which infects soybeans in Mississippi. We have also observed as is reported in the literature, that when both viruses are present in the same plant, disease symptoms are more severe than when a single virus is present in a plant. We have also observed that the virus can be present and the soybean plant may only show moderate viral symptoms.
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