Defense expression in protected tissues of Medicago sativa is enhanced during compatible interactions with Colletotrichum trifolii

The defense response to anthracnose, induced in alfalfa by avirulent Colletotrichum trifolii race 1, was further enhanced following challenge by a virulent race of C. trifolii. Rapid, significant increases in the phytoalexins medicarpin, sativan, vestitol, and coumestrol were observed in cotyledons protected from anthracnose by inoculation with race 1 and challenge-inoculated 24 h later with race 2. Sativan and medicarpin levels were significantly greater than quantities accumulating in protected, nonchallenged tissues or in. tissues both protected and challenged by the incompatible race 1. Inoculation of nonprotected tissues with the compatible race did not induce protection, nor did the tissues accumulate significant quantifies of phytoalexins. The rapid increase in phytoalexin accumulation following challenge-inoculation of protected tissues with a compatible race was correlated with increased hypersensitive browning and the absence of disease lesions, acervuli, and secondary spores. The degree of disease protection from challenge-inoculation by race 2 was dependent upon the spore density of race 1 inducing-inoculum. Full protection from race 2 infection required inducing race 1 inducing-inoculum densities ranging from 1 x 10(4) to 1 x 10(6) spores per milliliter. These spore densities resulted in significant phytoalexin accumulation following race 2 challenge. The most virulent isolates of race 2 caused the greatest increase in concentrations of medicarpin, sativan, vestitol, and coumestrol. These results suggest that protection may not be solely a result of defense activation in response to inoculation by an avirulent race. The induction phase may serve to facilitate an enhanced defense expression upon challenge by a virulent race. The protected plant responds rapidly and specifically to challenge by the compatible fungus.