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The Host-Pathogen Interaction of Alfalfa and Stemphylium botryosum. O. L. Borges, Former Graduate Student (Genetics), University of California, Davis 95616, Present address of senior author: Universidad Central de Venezuela, Facultad de Agronomia, Maracay, Estado Aragua, Venezuela; E. H. Stanford(2), and R. K. Webster(3). (2)(3)Professor of Agronomy, and Professor of Plant Pathology, respectively, University of California, Davis 95616. Phytopathology 66:749-753. Accepted for publication 22 December 1975. DOI: 10.1094/Phyto-66-749.

The nature of the host-pathogen interaction of alfalfa and Stemphylium botryosum was investigated by histological means. In a resistant plant of checkered alfalfa (Medicago cancellata), growth of the pathogen was inhibited and restricted to the cells surrounding the infection site. The host response was characterized as hypersensitive. On a susceptible host (M. sativa) the pathogen grew luxuriantly and produced abundant typical Stemphylium leafspot symptoms. Toxins produced by S. botryosum play an important role in disease development and their relationship to resistance and susceptibility was established. On susceptible plants, a crude filtrate from a culture medium inoculated with S. botryosum produced symptoms identical to those incited by the fungus. The resistant host was unaffected by the filtrate. A positive correlation between toxin test scores and those obtained by inoculation with the pathogen permitted the use of toxin tests as an improved rapid screening technique for resistance.

Additional keywords: toxins, hypersensitive reaction.