Selection for Root and Crown Rot Resistance in Alfalfa1

Abstract

Root and orown rot cause extensive damage to alfalfa (Medicago sativa). In this study, Fusorium solani followed by F. oxysporum, F. roseum, and F. tricinctum were most frequently isolated.Two techniques of inoculation, with one isolate of F. roseum var. acuminatum, were used for screening alfalfa for Fusarium root rot resistance. Both techniques were effective in producing a high percentage of infection. The application technique used on 3‐month‐old plants gave 5.9% resistant plants in the cultivar ‘Europa.’ Only 3.2% resistant plants were found when the bareroot‐soak technique was applied on 3‐week‐old seedlings. Resistance was evaluated by measuring the vertical discoloration in the split taproot. Plants with less than a 5mm spread of internal browning from the wound site were saved and intercrossed.The populations developed through each technique were established in the field at three locations in the spring of 1977 and rated for root rot in the fall of 1978. The two populations selected for resistance showed consistently less root rot than the unselected one although differences were not statistically significant.In a greenhouse study we were able to detect significant variation for root rot infection among as well as within polyerossed progenies of the selected genotypes suggesting that resistant genes for this disease are present in alfalfa. Narrow‐sense heritability was estimated at 22%. Some specific crosses were made between resistant (R) and susceptible (S) plants. The cross S ✕ S showed almost twice as much root rot infection as the cross R ✕ R. R ✕ S was better than expected based on mid value of R ✕ R and S ✕ S.Based on the results of this study, we believe progress is possible toward developing a crown and root rot resistant alfalfa.