Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/1999
Publication Date: N/A
Interpretive Summary: Three new virus-resistant berseem clover germplasms, B1, B2, and B3, and one virus-susceptible berseem clover germplasm, B4, were released in 1998 by the US Department of Agriculture's Agricultural Research Service and the Mississippi Agricultural and Forestry Experiment Station. The B1, B2, and B3 germplasms were developed for resistance to bean yellow mosaic virus (BYMV). The B4 germplasm was developed concurrently and is useful as a susceptible control. BYMV causes the most widespread and serious virus disease of berseem and other annual clovers in the southeastern US. In the absence of resistance, BYMV, which is transmitted by aphids, spreads rapidly within clover plantings. The virus reduces forage and hay yields, and reduces the effectiveness of winter clover cover crops in controlling soil erosion. Researchers discovered the disease resistance within a broad range of exotic berseem clovers stored in the US germplasm collection. The new germplasms should be useful to plant breeders in producing BYMV-resistant cultivars. Small quantities of seed are available to qualified researchers.
Technical Abstract: Germplasms B1, B2, B3, and B4 of berseem clover (Trifolium alexandrinum L.), were released in 1998 by USDA-ARS and Mississippi Agricultural and Forestry Experiment Station. B1, B2, and B3 are resistant to bean yellow mosaic potyvirus (BYMV); B4 is BYMV-susceptible and was developed as an experimental control. The germplasms were developed from intercrosses of plant intoductions (PIs) screened for resistance to BYMV. Seedlings of 96 PIs and cultivars `Bigbee' and `Multicut' were inoculated with BYMV in greenhouse tests. Plants which remained healthy were selected and inter- crossed using honey bees, Apis mellifera L., to produce first generation (Cycle 1) seed, which was subsequently grown for a second cycle of selec- tion. Three BYMV-resistant Cycle 1 populations were produced: B1 from three PIs (468401, 517064, 517060) with 90% or more resistant plants; B2 from 11 PIs (220147, 291548, 291549, 517056, 468402, 517057, 445883, 420811, 163315, 445897, and 445882) and Multicut with about 80% resistant plants; and B3 from remnant healthy plants of 78 PI lines with <80% resis- tant plants, Bigbee, and Multicut. B4 derived from symptomatic plants of three PIs (517055, 201954, and 241475) with no resistance. Cycle 1 poly- cross seed were grown for a second cycle of selection in which symptomless plants of B1, B2, and B3 and noninoculated plants of B4 were respectively intercrossed to produce Cycle 2 B1, B2, B3, and B4 polycross germplasms. After one inoculation, <10% of B4 plants grown from Cycle 2 seed remained symptomless, compared to 92%, 80%, and 78% for B1, B2, and B3, respectively. Cycle 2 polycross seed were subsequently grown without further selection and polycrossed respectively, to produce seed for distribution.