STUDIES IN RACIAL TRENDS AND CONSTANCY IN CLADOSPORIUM FULVUM COOKE

Abstract

Intensive surveys of the physiological races of Cladosporium fulvum Cke. have been carried out during the past decade in southwestern Ontario, as a result of which seven such races have been identified. These are separated by differential reactions of the following hosts: Lycopersicon esculentum Mill. varieties Potentate, Stirling Castle, Vetomold, V-121, and V-473; L. pimpinellifolium (Jusl.) Mill., Vineland, No. 160 and No. 11-22-15 strains; L. hirsutum Humb. and Bonpl. and L. hirsutum var. glabratum Muller. Evidence is presented that only two of these races existed when the surveys began and that the other five have arisen, probably through mutation, during the period under study. There is further a strong suggestion that the stimulus responsible for these mutations is somehow related to the colonization of an incompatible host by a race which remains stable pathogenically while in association with a susceptible host. Comparable mutants have not been encountered under experimentally controlled conditions. A perfect stage has not been found or induced to form in C. fulvum. A limited number of types of cultural mutants are produced consistently and these have been studied with respect to variability and the factors initiating them. Such mutants frequently do not form spores and in most instances have a reduced sporing potential; they are unchanged in pathogenicity or are slightly less aggressive than the parent to all the resistant hosts. In mixed cultures of races 1 and 5 and 1 and 7, the components survived several transfer generations both in culture and on a susceptible host.