Studies on the Fusarium of Muskmelon Wilt: I. Pathogenic and Cultural Studies With Particular Reference to the Cause and Nature of Variation in the Causal Organism

During the past decade a serious disease of muskmelon has appeared in Ontario. From diseased plants a Fusarium has been isolated, which proved to be highly pathogenic to muskmelons in infection experiments. Results indicate that the wilt organism occurs in nature as a form (or forms) that, when cultured on most artificial media, produces abundant aerial mycelium and this has been termed the 'wild type.' Mutation toward loss of aerial mycelium and darkening of the submerged mycelium is frequently observed in culture and, since many of the mutants are more aggressive than the parent, they tend to displace it. Evidence is presented that the wild type is genetically very stable in its natural environment. Since, moreover, certain of the commoner mutants caused less disease incidence under field conditions than the wild type, it is considered that variation of the sort observed in culture, if it does occur in the field, is unlikely to be of importance in the field pathology of the disease. The stability of the wild type in sterilized soil and soil agar indicates that these substrates may be of value for maintaining Fusaria in the pure state. Mutations were induced by irradiating spores with ultra-violet light. In general, these resembled those that appeared spontaneously and both groups were less pathogenic than the wild type. A phenomenon termed 'cultural interaction' was observed between certain of the mutant cultures and the wild type. Since this appeared to be of a specific nature, it may be of some taxonomic value in diagnosing species of Fusarium. The suggestion is offered that the concept of the 'wild type' may be of general application throughout the genus Fusarium.