The Mechanism of Host Penetration by Thanatephorus Cucumeris

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Abstract
Evidence is presented in support of the hypothesis that chemical stimuli control the development of infection structures on the host surface by T. cucumeris. On the stem of a young radish seedling there are discrete, susceptible areas which become fewer and eventually disappear as the seedling ages. Rubbing the stems of radish seedlings with or without an organic solvent increases the number of infection cushions formed by a crucifer-attack-ing isolate, and returns an older resistant seedling to the susceptible conditions. No infection cushions form on strips of cuticle and epidermis removed from the host. The susceptibility of several hosts to a number of isolates is closely correlated with the stimulation of these isolates by seedling roots enclosed in "cellophane". In vitro investigations on "cellophane" membranes show that root exudates from different hosts stimulate the growth of specialized and non-specialized isolates, and that under these conditions there is no correlation between specificity and stimulation. Crucifer-attacking isolates form infection cushions on artificial membranes covering intact radish seedlings, stem, or cotyledon pieces. Using a bioassay technique with collodion membranes mounted on van Tieghem rings, it is shown that exudates from radish stems and cotyledons stimulate infection cushion formation. This is the first direct evidence that the stimulant is a natural constituent of the host plant and is not produced by an interaction between host and pathogen.

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