GENETICS OF RESISTANCE TO ALFALFA MOSAIC-VIRUS IN RED-CLOVER

Virus diseases are known to reduce the yield and persistence of stands of red clover (Trifolium pratense L.). Alfalfa mosaic virus (AMV) is one of the causes of mosaic in red clover and may also cause mottling and leaf distortion. The goals of this study were to identify genotype s resistant to AMV within commercial red clover cultivars and to deter mine the inheritance of this resistance. Cultivars and progenies were screened for resistance by means of mechanical inoculations. Plants sh owing no symptoms after three successive inoculations were assayed for the presence of the virus using ELISA (enzyme-linked immunosorbent as say). Resistant plants were detected at a frequency of just over 1% in the five cultivars screened. Crosses among and between resistant (R) and susceptible (S) plants were carried out over two successive cycles of selection. In cycle 1, R x R crosses produced a higher frequency o f resistant plants (19.8%) in the progenies than R x S crosses (10.9%) . S x S crosses produced no resistant progeny. For crosses of resistan t plants from cycle 1 progeny, (R x R) x (R x R) produced 47.8%, (R x R) x (R x S) produced 31.4% and (R x S) x (R x S) produced 29.3% resis tant cycle 2 progeny. Crosses of resistant and susceptible cycle 1 pla nts resulted in resistance frequencies in cycle 2 which were not signi ficantly (P < 0.05) greater than cycle 1. When only resistant plants w ere selected for further crossing, the percentage of resistant plants increased from 1.3 to 47.5 over the two cycles of selection. It was co ncluded that resistance to AMV in red clover is under polygenic contro l and that it should be relatively simple to select a highly resistant population through recurrent selection.