FREQUENCY OF PLANTS IN POPULATIONS OF ANNUAL AND PERENNIAL MEDICS (MEDICAGO SPP) THAT WERE RESISTANT TO DAMPING-OFF INCITED BY AG-4 ISOLATES OF RHIZOCTONIA-SOLANI

Twenty-seven annual and four perennial medics (Medicago spp.) were eva luated in a greenhouse or growth chamber for resistance to damping-off incited by five anastomosis group (AG) 4 isolates of Rhizoctonia sola ni that were known to be either mildly pathogenic, moderately pathogen ic, or highly pathogenic to alfalfa (lucerne; Medicago sativa). When t he medics were challenged with a mildly pathogenic isolate of R. solan i, in almost all cases the number of seedlings that survived was not s ignificantly lower than the controls. One of the two moderately pathog enic isolates was found to be more virulent than the other moderately pathogenic isolate. When the entries were challenged with rile former, 11 Medicago species and a hybrid Medicago exhibited seedling survival means ranging from 18.00 to 31.25. Of these, the means for six Medica go species and the hybrid were not significantly lower than the contro ls. Six entries had survival means ranging from 14.00 to 18.00 when ch allenged by the mole virulent isolate of R. solani. However, only M. p opovii had a survival mean that was nor significantly lower than the c ontrol. There was essentially no resistance exhibited by any of the 31 medics when they were challenged with either of two highly pathogenic isolates. Nevertheless, because some resistance albeit at a low level , was found to two moderately pathogenic isolates of R. solani, a prog ram of recurrent, phenotypic selection could eventually lead to the de velopment of Medicago cultivars that were resistant at least to modera tely pathogenic isolates of R solani. These results underscore the nec essity to use several isolates of this pathogen that exhibit various l evels of pathogenicity, when evaluating Medicago species for resistanc e to this fungus.