Evidence for the heritability of resistance to brown root rot of alfalfa, caused by Phoma sclerotioides

Abstract

Brown root rot (BRR) of alfalfa, caused by Phoma sclerotioides, a low-temperature pathogen, is associated with winterkill in the contiguous United States. Identified in Canada during the 1920s, BRR has been identified from legume roots in four western Canadian provinces. Some Canadian cultivars exhibit BRR resistance, although only one was consciously selected for the trait. Heritability of BRR resistance was evaluated in this study in which containergrown alfalfa plants were inoculated with P. sclerotioides. Plants that were evaluated are progeny (cycle 1) of one generation of selection and intermating of healthy plants drawn from six diverse North American alfalfa populations (five commercial cultivars, one experimental population) that survived exposure to P. sclerotioides soil infestation for several winters. These were evaluated together with plants grown from seed of the original unselected populations (cycle 0), and putative check cultivars ‘Multi-plier’ (BRR susceptible) and ‘Peace’ (BRR resistant). Plants were rated for disease severity for a total of 4 test years. Cycle 1 plants exhibited a reduced level of disease severity compared with cycle 0 plants (P < 0.05), indicating heritability of BRR resistance in alfalfa. Also, especially important for future alfalfa breeding efforts, this study demonstrates that ‘Multi-plier’ and ‘Peace’ are useful checks for screening BRR resistance.