SELECTION FOR RESISTANCE TO SCLEROTINIA CROWN AND STEM ROT IN ALFALFA

Efficient selection methods are needed for improving resistance in alf alfa (Medicago sativa L.) to Sclerotinia trifoliorum Erikss,, the caus al agent of Sclerotinia crown and stem rot (SCSR), The objective of th is research was to determine if SCSR resistance in alfalfa could be im proved by recurrent phenotypic selection, Selections and evaluations w ere conducted at Columbus, OH, between August 1991 and April 1991 in t hree alfalfa populations, 'Armor', ABI 9023, and ABI 9030. Seedlings w ere inoculated with the pathogen in the growth chamber and field, Surv iving plants were recovered and intermated for each cycle of selection . A standard seedling test was used to measure the progress of selecti on. Two cycle-3 populations and the base populations from which they w ere derived were also inoculated with five additional isolates of S. t rifoliorum to examine the durability of resistance in the selected pop ulations, Progeny of populations selected for two or three cycles in t he growth chamber a expressed significant (P = 0.05) reduction in dise ase severity index (DSI), Generally, progeny of one cycle of field sel ection were not significantly different than the unselected base popul ation. However, in Armor, one cycle of selection in the field (DSI 3.3 9) was comparable to two or three cycles in the growth chamber (DSI 3. 41), Realized heritability estimates for resistance to SCSR ranged fro m 0.07 to 0.27. Isolates of S. trifoliorum differed in virulence, but there were no isolate by population interactions. The growth chamber s election method developed, using a single isolate of S. trifoliorum, w as effective in increasing resistance in three alfalfa populations.