Author
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Larsen, Richard |
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Pederson, Gary |
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Submitted to: Trifolium Conference Abstract & Proceedings
Publication Type: Proceedings Publication Acceptance Date: 3/1/1996 Publication Date: N/A Citation: N/A Interpretive Summary: White clover is an important worldwide pasture forage crop, especially in the southeastern section of the U.S. where it is difficult to grow alfalfa. Alfalfa mosaic virus (AlMV) is responsible for significant yield losses and decreases in clover stand longevity. Southern Regional Virus Resistant (SRVR) white clover is a germplasm released with a high level of resistance to peanut stunt and clover yellow vein viruses but with low to moderate resistance to AlMV. The objectives of the current research are, therefore, to determine the level of resistance to several strains of AlMV, and to identify and select plants for use in breeding for increased resistance in SRVR. Test plant groups were inoculated with 1 of 6 different AlMV strains collected throughout North America. Infection rates for the strains from California, Georgia, Ontario, and Wyoming were 47, 53, 60, and 73% respectively. SRVR plants were not infected by AlMV strains from Montana or Washington. It was of interest in this study that the strain which was most infectious to alfalfa (Washington) in an earlier study, did not infect any of the SRVR test plants even after two successive inoculations. The plants which did not become infected are important in this study, as they are potential sources of additional virus resistance. These remaining noninfected plants were then inoculated with a strain different from the original. It is envisioned that plants which remain uninfected after inoculation with four to six different strains will be used in breeding programs to increase AlMV resistance in SRVR germplasm. This germplasm with resistance to a wide range of AlMV strains will have an important impact when incorporated into other breeding populations to further improve the virus resistance in white clover. Technical Abstract: Southern Regional Virus Resistant (SRVR) white clover germplasm was released in 1988 with resistance to peanut stunt cucumovirus (PSV), clover yellow vein potyvirus (CYYV), and alfalfa mosaic virus (AlMV). Evaluations of SRVR in the field indicated high resistance against PSV and CYVV and low to moderate resistance against AlMV. The objectives of the current research hare to determine the level of resistance to regional isolates of AlMV, and to identify and select plants for use in breeding for increased resistance in SRVR. Six North American isolates of AlMV were collected from California (CA-13), Georgia (GA-5), Montana (MT-4), Washington (WA-3), Wyoming (WY-1), and Ontario (ONT-2). Fifteen SRVR white clover plants for each of the six virus isolates were inoculated mechanically. Test plants were assayed using indirect ELISA. Infection rates for the Ca-13, GA-5, Ont-2, and WY-1 AlMV isolates were 47, 53, 60, and 73% respectively. SRVR plants were not infected by MT-4 or WA-3. Results from previous resistanc studies indicated that the most pathogenic isolate in alfalfa was WA-3 but the most virulent isolates both in alfalfa and in indicator plants were GA- 5 and MT-4. It was of interest in this study that the most pathogenic isolate (WA-3) in alfalfa resistance screenings did not infect any of the SRVR test plants. SRVR plants which did not become infected during the initial round of virus inoculations were then inoculated with an isolate different from the original. It is envisioned that plants which remain uninfected after inoculation with four to six isolates will be used in breeding programs to increase AlMV resistance in SRVR germplasm. The resistant germplasm will then be incorporated into other breeding populations to further improve the virus resistance in white clover. |
