Effect of treatment with the phytoalexins medicarpin and maackiain on fungal growth in vitro and in vivo

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Abstract
The effect of medicarpin (3-hydroxy-9-methoxypterocarpan) and maackiain (3-hydroxy-8,9-methylenedioxypterocarpan). phytoalexins from red clover, on germ tube growth, oxygen uptake, and pathogenicity of Helminthosporium carbonum, Stemphylium botryosum, and S. sarcinaeforme was examined. Germ tube growth of all three fungi was inhibited by as little as 10 μg/ml of these compounds and oxygen uptake by H. carbonum and S. botryosum, but not by S. sarcinaeforme, was reduced by higher concentrations (about 60 μg/ml) of these phytoalexins. Medicaipin and maackiain added to inoculum drops prevented H. carbonum from infecting its host plant, corn. This protective effect was probably caused by the failure of H. carbonum to form appressoria in the presence of the phytoalexins. No protective action of the phytoalexins was noted when either S. botryosum or S. sarcinaeforme was used on their host plants, alfalfa and clover, respectively. Inoculation of red clover leaves with the nonpathogen H. carbonum before inoculation with the pathogen S. sarcinaeforme had no effect on symptom development, nor was there any evidence of increased susceptibility to H. carbonum if either S. sarcinaeforme or S. botryosum was added at the time of inoculation. The results are discussed in relation to the role of these phytoalexins in disease resistance of red clover.

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