STUDIES ON THE FUSARIA WHICH CAUSE WILT IN MELONS: I. THE OCCURRENCE AND DISTRIBUTION OF RACES OF THE MUSKMELON AND WATERMELON FUSARIA AND A HISTOLOGICAL STUDY OF THE COLONIZATION OF MUSKMELON PLANTS SUSCEPTIBLE OR RESISTANT TO FUSARIUM WILT

Abstract

It has been shown that more than one type of isolate of both the muskmelon Fusarium and the watermelon Fusarium occur naturally in infested soil. The isolates of both organisms could be divided into many cultural races, depending on the number of isolations made. Among these cultural races differences were demonstrated in their ability to establish successful host–parasite relationships with their respective host plants. The field reactions of various host varieties were shown to be a function of the races present in the soil at a given time. Fluctuations in the relative frequency of the races present in a field have been shown to occur, as well as changes in the races present.The muskmelon wilt organism penetrated a susceptible host variety between cells in the region of elongation. The cortex was then colonized intercellularly, later intracellularly. The fungus then penetrated the stele, establishing itself in the vessels. Eventually hyphae were present throughout the vessels and later passed out to infect stelar and cortical tissues at various loci.The fungus also invaded the primary meristem intercellularly, later intracellularly, and established itself in young protoxylem vessels or developing cortical cells. Penetration also occurred through tears in the cortex caused by developing secondary roots.Colonization of a resistant host occurred in the same manner, but there was always less fungus in the resistant host than in the susceptible host after comparable time intervals. This difference could not be correlated with the morphology of the resistant host.The effect of temperature on colonization appeared to be on the aggressiveness of the parasite, rather than on the susceptibility of the host.