Diversity of Virulence in Phytophthora parasitica on Tobacco, as Reflected by Nuclear RFLPs

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Abstract
A worldwide collection of P. parasitica isolates was investigated for the ability to infect tobacco and tomato, as related to elicitin production. Elicitin was produced by all nontobacco isolates, and nonproducing strains all were isolated from tobacco. In addition, producing strains were isolated from tobacco and coexisted with nonproducing (TE¯) strains. Elicitin production generally was associated with low virulence on tobacco and frequent pathogenicity on tomato, whereas TE¯ isolates generally were highly virulent and specialized to tobacco. Analysis of both mitochondrial and nuclear DNA restriction fragment length polymorphisms indicated, for the first time, that black shank isolates can be distinguished from other P. parasitica isolates on the basis of genetic criteria. Our results suggest that severe black shank is caused by a limited number of TE¯ strains that have been disseminated by clonal evolution. Mutations in the TE¯ phenotype seem to have arisen independently in several genetic backgrounds and distinct geographic areas. The fortuitous absence of elicitin production has precluded population replacements in areas of intensive tobacco cultivation. Thus, monitoring the loss of elicitin production in developing tobacco areas should be considered in disease management.