Genetics of resistance to alfalfa mosaic virus in red clover

Abstract

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 genotypes resistant to AMV within commercial red clover cultivars and to determine the inheritance of this resistance. Cultivars and progenies were screened for resistance by means of mechanical inoculations. Plants showing no symptoms after three successive inoculations were assayed for the presence of the virus using ELISA (enzyme-linked immunosorbent assay). 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 × R crosses produced a higher frequency of resistant plants (19.8%) in the progenies than R × S crosses (10.9%). S × S crosses produced no resistant progeny. For crosses of resistant plants from cycle 1 progeny, (R × R) × (R × R) produced 47.8%, (R × R) × (R × S) produced 31.4% and (R × S) × (R × S) produced 29.3% resistant cycle 2 progeny. Crosses of resistant and susceptible cycle 1 plants resulted in resistance frequencies in cycle 2 which were not significantly (P < 0.05) greater than cycle 1. When only resistant plants were selected for further crossing, the percentage of resistant plants increased from 1.3 to 47.8 over the two cycles of selection. It was concluded that resistance to AMV in red clover is under polygenic control and that it should be relatively simple to select a highly resistant population through recurrent selection. Key words: Red clover, Trifolium pratense L., virus disease, alfalfa mosaic virus, recurrent selection