DIVERSITY IN PATHOGENICITY TO TOBACCO AND IN ELICITIN PRODUCTION AMONG ISOLATES OF PHYTOPHTHORA-PARASITICA

The pathogenicity to tobacco of a large set of Phytophthora parasitica isolates has been assessed using several procedures: root inoculation of young plants, leaf inoculation on detached disks and stem inoculat ion of decapitated plants, with or without healing. Analysing various aspects of the plant-pathogen interaction with this array of tests led to the discrimination between three groups of isolates. In the isolat es from hosts other than tobacco, none was truly pathogenic to tobacco , and all but one produced parasiticein, a proteinaceous elicitor of t he elicitin family which induces a hypersensitive-like response in tob acco. Isolates producing parasiticein in vitro induced necrotic fleks on the leaves upon inoculation of roots or of freshly wounded stems. M ost tobacco isolates, including all the highly virulent ones, were cha racterized by a lack of elicitin production. However, those collected in Australia and Zimbabwe differed in that they exhibited reduced viru lence, induced leaf necrotic flecks and produced parasiticein. The inc idence of elicitin production on virulence and the significance of two types of tobacco-pathogenic strains for tobacco pathology are discuss ed.