Search

The cellular reactions of excised hypocotyl segments of 31 cultivars of Phaseolus vulgaris to infection by two races of Colletotrichum lindemuthianum were surveyed. Consistent differences in the extent of fungal development and host cell necrosis were used to establish six infection categories. Extreme susceptibility (category 5) involved initial formation of infection vesicles in living epidermal cells and a prolonged period of intracellular biotrophic development prior to necrotrophic growth and production of spreading lesions. Extreme resistance (category 0) involved rapid death and browning of penetrated epidermal cells, without formation of infection vesicles or a detectable biotrophic phase. Fungal growth and host necrosis were limited to single epidermal cells. Intermediate reactions (categories 1 to 4) initially resembled category 5, but after a biotrophic phase of varying duration, infected cells and some adjacent uninfected cells died and became brown, and fungal growth ceased. Symptoms ranged from flecks (small groups of dead cells) to large areas of necrosis (limited lesions). Similar intermediate reactions also occurred in normally susceptible hypocotyls which were transferred from 17 to 25°C, or which retained cotyledons. The finding that resistance is expressed at different stages of infection is discussed in relation to the regulation of cultivar specificity. [ABSTRACT FROM AUTHOR]