Selection for resistance to root rot caused by Fusarium spp. in red clover (Trifolium pratense L.)

Abstract

Root and crown rots caused by Fusarium spp. are a major cause of stand decline of red clover (Trifolium pratense L.) in North America. The goal of this study was to evaluate the effectiveness of one cycle of selection for resistance to Fusarium root rot. Plants of the red clover cultivars Florex and Arlington were screened for their resistance to infection by two isolates of Fusarium acuminatum and one isolate of F. avenaceum. Plants of various infection categories were intercrossed, and progenies were screened to estimate the heritability of resistance to these pathogens. Synthetics produced by intercrossing either highly resistant (R) or highly susceptible plants were evaluated under sward conditions in the field for dry matter yield, root rot and adventitious root development. There were significant (P = 0.05) differences in aggressiveness among the three Fusarium isolates used, with F. avenaceum 814 being the most aggressive. Plants classed as R for all three isolates were found in both cultivars; however, disease reaction was not always correlated among the isolates. Parent-progeny regressions or correlations of disease reaction were negative in the cultivar Arlington, while for Florex, a relatively low regression coefficient of 0.26 was found. In the field evaluation in the fall of the third year after seeding, synthetics of plants selected for resistance were either not different from, or higher in infection score than the parental cultivars, indicating that selection using one or two isolates/species was ineffective in improving field root rot resistance. Plants with more severely rotted roots showed a tendency to produce more adventitious roots from the crown. It was concluded that progeny testing would be necessary for selection to be effective for resistance to Fusarium root rot in red clover and that more that one isolate or species of the fungus should be used in screening programs. In addition, not all populations of red clover contain sufficient genetic variation for resistance to justify a selection program. Key words: Red clover, disease resistance, selection, root rot, adventitious roots, Trifolium pratense L., Fusarium spp.