Quantification of Yield Benefits from Incorporation of Virus-Resistant White Clover Germ Plasm into Grass-Legume Systems. N. L. Taylor, Department of Agronomy, University of Kentucky, Lexington 40546. S. A. Ghabrial, Department of PIant Pathology, University of Kentucky, Lexington 40546; and G. A. Pederson and M. R. McLaughlin, USDA-ARS, Forage Research Unit, P.O. Box 5367, Mississippi State, Miss. 39762. Plant Dis. 79:1057-1061. Accepted for publication 20 July 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-1057.

The performance of the virus-susceptible white clover (Trifolium repens) cultivar Regal was compared in broadcast and spaced-plant plots with three experimental breeding populations (Southern Regional Virus Resistant synthetic [SRVR], Brown Loam Synthetic No 2 [BL Syn. 2] and Florida Experimental No. 4 [FL Exp. 4]) expressing varying levels of resistance to viruses endemic to forage legumes in the southeastern United States. The experimental plots were established with and without grasses in fields at Lexington, Ky., and Mississippi State, Miss., with the objective of comparing the magnitude of yield losses from virus diseases 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 virus (PSV) and clover yellow vein virus (CYVV) than Regal. Although significantly lower estimates of PSV and/or CYVV incidence were also obtained with BL Syn. 2 and FL Exp. 4 at some testing dates, there were little or no differences at others. Furthermore, higher estimates of PSV and CYVV incidence were determined for spaced plants grown in plots without grass than those with grass. Unlike PSV and CYVV, incidence of alfalfa mosaic virus was similar among treatments. Data on yield, stand, and virus incidence in broadcast plots closely paralleled those of spaced plants. Percentage of virus incidence in broadcast plots in Kentucky, 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 superior to the other germ plasms and suggest that without virus resistance improved white clover yields and persistence cannot be obtained.