Selection of a new set of homogeneously reacting bean (Phaseolus vulgaris) differentials to differentiate races of Colletotrichum lindemuthianum

Twenty-two cultivars of Phaseolus vulgaris, mentioned in the literature as differential cultivars or sources of resistance, were screened with isolates representing 10 races of Colletotrichum lindemuthianum, to test their suitability as differentials. Single-plant selections from eight cultivars were chosen, with genetic uniformity for resistance and with clear positive or negative reactions following inoculation with all 10 individual races. These differentials are proposed as a standard set for international use, to make the identification of C. lindemuthianum races internationally more comparable. The criteria for a set of differentials, and the influence of cultivar, fungus race and environment on the host-pathogen reaction are discussed.