Varieties with a high level of resistance provide an opportunity to manage root rot caused by Rhizoctonia solani in alfalfa

Rhizoctonia solani is one of the major fungal pathogens associated with root rot in alfalfa that severely limits commercial alfalfa production worldwide. Host resistance is considered the most cost-effective and sustainable way to manage diseases caused by this pathogen. This study aimed to determine whether any host resistances to R. solani exist among a wide-ranging alfalfa collection and whether host resistances were associated with a specific geographic origin. We found a large variation in disease responses among the 68 alfalfa varieties, with the range of disease indices of shoots from 23 to 94% and roots from 31 to 98% as well as reductions in dry weight of shoots from 35 to 96% and roots from 2 to 99%. Among these, three varieties (Gannong 9, Trifecta and Common), originating from three different countries, displayed a high level of resistance, with disease indices of shoots and roots as well as reductions in dry weight of shoots and roots being all ≤40%. Hierarchical clustering and principal component analysis revealed three main groups of disease responses existed among varieties, with five varieties (7%) showing resistance, 15 varieties (22%) being moderately resistant and the remaining ones exhibiting susceptibility. Besides, varieties with a similar disease response were associated with various geographic origins. This study provides valuable resistance sources for breeding programs to develop alfalfa varieties with improved resistance to R. solani, and for facilitating the identification of molecular mechanisms underlying the resistant varieties to this pathogen.