| dc.description | The allelic variation in four avirulence <em>(Avr) </em>and four extracellular protein <em>(Ecp)–</em>encoding genes of the tomato pathogen <em>Cladosporium fulvum </em>was analyzed for a worldwide collection of strains. The majority of polymorphisms observed in the <em>Avr </em>genes are deletions, point mutations, or insertions of transposon-like elements that are associated with transitions from avirulence to virulence, indicating adaptive evolution of the <em>Avr </em>genes to the cognate <em>C. fulvum </em>resistance genes that are deployed in commercial tomato lines. Large differences in types of polymorphisms between the <em>Avr </em>genes were observed, especially between <em>Avr2 </em>(indels) and <em>Avr4 </em>(amino-acid substitutions), indicating that selection pressure favors different types of adaptation (Stergiopoulos <em>et al</em> 2007)
Four <em>Avr </em>genes (<em>Avr2</em>, <em>Avr4</em>, <em>Avr4E</em>, and <em>Avr9</em>) have been sequenced from <em>C. fulvum</em>, and their encoded proteins trigger a hypersensitive response (HR) in tomato plants that carry the cognate <em>Cf-2</em>, <em>Cf-4</em>, <em>Hcr9-4E </em>(also known as <em>Cf-4E</em>), and <em>Cf-9 </em>genes, respectively (Joosten <em>et al</em>. 1994; Luderer <em>et al.</em> 2002; Van Kan <em>et al</em>. 1991; Westerink <em>et al</em>. 2004b). However, several strains of <em>C. fulvum </em>exist in natural populations of the fungus that are able to overcome as many as five different <em>C. fulvum </em>genes that are present in commercial tomato lines (Lindhout <em>et al.</em> 1989). Such strains with complex virulence spectra are thought to have emerged as a result of strong selection pressure imposed on the fungus after the introduction of the different <em>C. fulvum </em>genes into tomato cultivars since the 1930s (Lindhout <em>et al</em>. 1989; Rivas and Thomas 2005). The transition from avirulence to virulence is associated with DNA modifications in the <em>Avr </em>genes that include deletions, point mutations, or insertions of transposon-like elements, resulting either in complete loss of the Avr protein or in the production of altered forms that no longer trigger HR in tomato plants carrying the cognate <em>C. fulvum </em>genes (Westerink <em>et al.</em> 2004a). In most cases, loss or mutation of Avr proteins does not significantly affect the fitness of the fungus on cultivated tomatoes, suggesting that Avrs mostly represent virulence factors with minor effects (De Wit 2002; Westerink et al. 2004a).The purpose of this work was to isolate the organisms causing typical symptoms of leaf mold disease in tomato and identify the races of <em>P. fulva</em> from genomic DNA either of the organisms or leaf lesions. Five isolates were identified as <em>Passalora fulva</em> based on the ITS sequence. One isolate was identified as race “0”, which carried <em>avr2</em>, <em>avr4</em>, <em>avr4E</em> and <em>avr 9</em> while the rest of the isolates were identified as race 2 that lacks <em>avr2</em>. These isolates provoked disease on tomato cv Money Maker, race 0 (MMCf-0) and the remaining isolates on both cultivars MMCF-0 and MM Cf-2. | |
