The Mechanism of Host Penetration by Thanatephorus Cucumeris

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 a�reas 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 crucifor-attacking isolate, and returns an older resistant seedling to the susceptible condition. No infection cushions form on strips of cuticle and epidermis removed from the host.