Genetics of reaction to peanut mottle virus in soybeans

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Virginia Polytechnic Institute and State University

Experiments were conducted at Blacksburg from 1975 to 1978 on soybean, Glycine max (L.) Merrill. The objectives were: (1) to study the inheritance of reaction to peanut mottle virus (PMV); (2) to determine the allelic relationships between genes for resistance from various germplasm sources; and (3) to screen a part of the soybean Plant Introduction germplasm collection and identify PMV-resistant strains.

Soybean cultivars, Plant Introduction (PI) strains, and advanced generation progenies derived from selected crosses were artificially inoculated with PMV-S/V74S (a Virginia isolate) and evaluated for their reaction to PMV in the field and greenhouse. Two cultivars, 'Virginia' and 'Pine Dell Perfection', that were previously reported as resistant to a mild PMV strain were found to be susceptible to PMV-S/V74S.

Crosses between resistant and susceptible lines and among resistant lines were made in the field in 1976 and 1977. The F₁, F₂, and F₃generation seedlings derived from selected crosses were tested for reaction to PMV in the greenhouse. It was shown that resistance in the cultivar 'Peking' is conditioned by a single recessive gene designated rpv₂. Evidence based on segregation in F₂ populations was also presented that indicates the presence of three other dominant genes for resistance to PMV-S/V74S. The three dominant genes are designated Rpv₁ (first reported by workers in Georgia), Rpv₃, and Rpv₄.

Preliminary F₁ and F₂ data were obtained from crosses between 15 resistant PI strains and two resistant "testers," 'York' and PI 89,784. The F₂ data obtained from five crosses indicate the possibility of still other genes for resistance to PMV. The two susceptible lines used in the study, Virginia and PI 229,315, differed markedly in their reactions to PMV. The presence of different alleles or modifying genes controlling the susceptible reactions in the two lines is suggested.

A total of 2161 FC and PI strains in Maturity Groups II, III, and IV were inoculated with PMV-S/V74S in the field during 1976 and 1977. Three hundred sixty-six strains that showed 10% or less virus infection were identified. These "resistant" strains provide a pool from which other genes for PMV resistance perhaps can be isolated. Differences in PMV disease reactions of plants from the same strain were noted when plants were tested in both the field and greenhouse. The differences were attributed to the following three factors: (1) differences in stage of plant growth at time of inoculation (field grown plants were generally larger at time of inoculation than plants inoculated in the greenhouse); (2) differences in environmental conditions between the field and greenhouse; and (3) the artist's airbrush inoculation technique was used in the field while the rub inoculation technique (mortar and pestle) was used in the greenhouse.