Development of Colletotrichum trifolii races 1 and 2 on alfalfa clones resistant and susceptible to anthracnose

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Abstract
Resistant and susceptible alfalfa clones derived from the cultivar Arc were spray inoculated with conidia from race 1 or race 2 isolates of Colletotrichum trifolii Bain in compatible and incompatible combinations. No significant differences were found in the frequencies of formation of immature or mature appressoria or in germ-pore formation by either race of C. trifolii on resistant or susceptible plants. These results indicate that incompatibility is not associated with the failure of conidia to germinate or to form appressoria with germ pores. In a small number of observations, penetration pegs were observed in tissue of both resistant and susceptible plants. Colletotrichum trifolii initiated infections in alfalfa by direct penetration of the epidermis via a penetration peg from the appressorium. Although the fungus spread rapidly throughout susceptible hosts, we observed fungus penetration only of epidermal cells of resistant hosts. Therefore, it appears that expression of alfalfa resistance to C. trifolii occurs near the time of epidermal cell wall penetration.