QUANTIFICATION OF YIELD BENEFITS FROM INCORPORATION OF VIRUS-RESISTANT WHITE CLOVER GERM PLASM INTO GRASS-LEGUME SYSTEMS

The performance of the virus-susceptible white clover (Trifolium repen s) cultivar Regal was compared in broadcast and spaced-plant plots wit h three experimental breeding populations (Southern Regional Virus Res istant synthetic [SRVR], Brown Loam Synthetic No 2 [BL Syn. 2] and Flo rida Experimental No, 4 [FL Exp. 4]) expressing varying levels of resi stance to viruses endemic to forage legumes in the southeastern United States. The experimental plots were established with and without gras ses in fields at Lexington, Ky., and Mississippi State, Miss., with th e objective of comparing the magnitude of yield losses from virus dise ases in spaced-plant nurseries with those in broadcast plots that more closely resemble farm conditions. Data from virus incidence in spaced plants in both locations indicated that SRVR had significantly (least significant difference, P = 0.05) lower incidence of peanut stunt vir us (PSV) and clover yellow vein virus (CYVV) than Regal. Although sign ificantly lower estimates of PSV and/or CYVV incidence were also obtai ned with BL Syn. 2 and FL Exp. 4 at some testing dates, there were lit tle or no differences at others. Furthermore, higher estimates of PSV and CYVV incidence were determined for spaced plants grown in plots wi thout grass than those with grass. Unlike PSV and CYVV, incidence of a lfalfa mosaic virus was similar among treatments. Data on yield, stand , and virus incidence in broadcast plots closely paralleled those of s paced plants. Percentage of virus incidence in broadcast plots in Kent ucky, however, was generally lower than that in the spaced plants. The overall data indicated that the performance of SRVR, based on yield ( 14 to 55% higher than Regal) and resistance to PSV and CYVV, was super ior to the other germ plasms and suggest that without virus resistance improved white clover yields and persistence cannot be obtained.