DIVERSITY OF VIRULENCE IN PHYTOPHTHORA-PARASITICA ON TOBACCO, AS REFLECTED BY NUCLEAR RFLPS

A worldwide collection of P. parasitica isolates was investigated for the ability to infect tobacco and tomato, as related to elicitin produ ction. Elicitin was produced by all nontobacco isolates, and nonproduc ing strains all were isolated from tobacco. In addition, producing str ains were isolated from tobacco and coexisted with nonproducing (TE-) strains. Elicitin production generally was associated with low virulen ce on tobacco and frequent pathogenicity on tomato, whereas TE- isolat es generally were highly virulent and specialized to tobacco. Analysis of both mitochondrial and nuclear DNA restriction fragment length pol ymorphisms indicated, for the first time, that black shank isolates ca n be distinguished from other P. parasitica isolates on the basis of g enetic criteria. Our results suggest that severe black shank is caused by a limited number of TE- strains that have been disseminated by clo nal evolution. Mutations in the TE- phenotype seem to have arisen inde pendently in several genetic backgrounds and distinct geographic areas . The fortuitous absence of elicitin production has precluded populati on replacements in areas of intensive tobacco cultivation. Thus, monit oring the loss of elicitin production in developing tobacco areas shou ld be considered in disease management.