Rôle of the phytoalexin medicarpin in three leaf spot diseases of alfalfa

Abstract

The possible rôle of medicarpin in limiting the size of lesions produced by foliar pathogens of alfalfa was investigated using Stemphylium botryosum, Phoma herbarum var. medicaginis and Leptosphaerulina briosiana. Medicarpin was extracted from alfalfa leaves infected with each of these pathogens after inoculation by spraying whole plants or by using the drop diffusate technique with excised leaves. Medicarpin was present in diffusate solutions only when P. herbarum var. medicaginis was used. In contrast to results obtained in drop diffusate experiments using the corn pathogen Helminthosporium turcicum, the amount of medicarpin accumulating in tissue infected by the alfalfa pathogens was relatively low. Mycelial growth of these three alfalfa pathogens and also of a fourth pathogen, Colletotrichum trifolii, was only slightly inhibited (0 to 12%) in V-8 agar containing medicarpin at a concentration of 75 μg/ml while H. turcicum was inhibited by 50% at between 25 and 50 μg/ml. P. herbarum var. medicaginis and L. briosiana caused a loss of medicarpin when it was added to cultures growing in Czapek medium. The first detectable degradation product formed by P. herbarum var. medicaginis was similar to the degradation product SB I formed by S. boryosum. This same compound was also present in diffusate solutions obtained using P. herbarum var. medicaginis. Preliminary evidence suggested that degradation of medicarpin by L. briosiana involved a constitutively produced enzyme or enzyme complex. The results do not eliminate the possibility that medicarpin plays a role in limiting lesion development in diseases caused by fungi pathogenic to alfalfa. A final conclusion must await data on the localization and concentration of medicarpin in the lesion area.