Use of Race- and Cultivar-Specific Elicitors from Intercellular Fluids for Characterizing Races ofCladosporium fulvumand Resistant Tomato Cultivars

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Abstract
Intercellular fluids obtained by an in vacuo infiltration technique were used to compare and to characterize European and North American races of C. fulvum. In previous studies, it was found that such fluids contained specific elicitors of chlorosis and or necrosis. The races of North American origin tested included race 0 with cycloheximide tolerance (C+), a race 4(C+) mutant derived from the race 0(C+) isolate, race 2.3(C+) race 4 with benlate tolerance (B+) and 2 isolates of race 2.3.4 (#3 and #78). Races 4, 5 and 2.4.5.9 originating from the Netherlands were also tested. Intercellular fluids obtained from all compatible race-cultivar combinations showed race- and cultivar-specific elicitor activity when injected into tomato cultivars with genes conferring high phenotypic resistance (Cf2, Cf4, Cf5, and Cf9) and also when tested in a cultivar with gene Cf3 which allegedly confers only partial resistance. A cultivar with the gene Cf1, also giving partial resistance, behaved in a manner similar to cultivars with no known genes for resistance, i.e., none of the elicitor preparations induced chlorosis or necrosis. Fluids obtained from races with the same virulence spectrum but originating from different continents or by artificial means showed identical race and cultivar specificity. Fluids obtained by using the complex race 2.4.5.9, which is virulent on all European commercial tomato cultivars but not on 2 recently released Ontario hybrids, showed activity only in the hybrids. Polyacrylamide gel electrophoresis of intercellular fluids confirmed that preparations with necrosis activity on cultivar Sonatine contained a fast-moving basic peptide. The mannose/glucose ratios of fluids obtained from previously designed compatible and incompatible interactions had ranges of 1.45-3.10, and 0.50-0.71, respectively. In interactions in which partial resistance due to genes Cf1 or Cf3 was originally expected, the ratios ranged from 0.52-3.92, but for each interaction the ratio corresponded to the estimated degree of colonization based on elicitor activity and histological examination.

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