The role of medicarpin and maackiain in the response of red clover leaves to Helminthosporium carbonum, Stemphylium botryosum, and S. sarcinaeforme

Helminthosporium carbonum, a corn pathogen, and Stemphylium botryosum, an alfalfa pathogen, are both nonpathogenic on red clover (Trifolium pratense L.), while S. sarcinaeforme is a foliar pathogen on red clover. In clover leaves challenged with H. carbonum, medicarpin and maackiain were the only inhibitory compounds found in diffusates or in leaf tissue in a concentration sufficient to account for the inhibition of this fungus. Helminthosporium carbonum was inhibited by and could not degrade medicaipin and (or) maackiain in vitro. Both S. botryosum and S. sarcinaeforme were only slightly inhibited by these compounds in mycelial growth bioassays, and both fungi degraded medicarpin and (or) maackiain in vitro and some evidence was obtained that degradation occurred in vivo. In contrast with the relatively high amounts of medicarpin and maackiain that accumulated in leaves challenged with H. carbonum, relatively low amounts accumulated in leaves challenged with either S. botryosum or S. sarcinaeforme. The evidence suggests that the resistance of clover to H. carbonum can be accounted for by these phytoalexins; however, differences in relation to accumulation of, inhibition by, and breakdown of medicarpin and (or) maackiain were not enough to explain the difference in pathogenicity of S. botryosum and S. sarcinaeforme on red clover.