STUDIES IN SEPTORIA LYCOPERSICI SPEG.

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Abstract
Investigations into the biology of Septoria lycopersici Speg., a leaf-spotting fungus parasite of the tomato, reveal that this fungus species is composed of at least two physiologic races which show both qualitative and quantitative differences in their pathogenicity. Other factors, such as humidity, temperature, light, and host nutrition, which influence host–parasite interaction, have been shown to cause further variability in the symptomatological picture. Studies of spore germination and the prepenetration phase of host–parasite relationships suggest an interaction before actual penetration which, in some cases, is sufficient to condition the germination of the spores themselves. A minimum period of 48 hr. at saturation is required to promote germination and stomatal penetration, but this experience at high humidity need not be continuous. The fungus inhabits an intercellular locus, but is very intimately associated with the cells of the tissues parasitized. There is no evidence of action in advance; rather a certain measure of host–parasite equilibrium is maintained, and the pathogen is enabled to complete its life cycle and reinitiate infection. Although this evidence suggests a rather high degree of specialization of parasitism, the nature of the host–parasite association is yet considerably removed from that of an obligate relationship. It is pointed out that while S. lycopersici might be considered, in the sense of de Bary, either as a facultative saprophyte or an obligate parasite, its peculiar behavior makes it exceedingly difficult, if not impossible, to place it into any one category which might describe its biological activity.

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