Analysis of genetic diversity among isolates of Verticillium dahliae from agricultural crops in the Central Valley of California

This study established differential virulence among eight race 2 Verticillium dahliae isolates independent of the genotype of tomato, cultivars tested. No tomato cultivars showed resistance to race 2 isolates comparable to that conferred by the Ve gene against race 1 isolates. There was a strong correlation between vascular colonization, visual symptoms and dry weight reduction among tomato isolates. Virulence of an isolates appeared lower on tomato than cotton, regardless of original host. All California race 1 isolates belonged to VCG2. VCG2 was also isolated from a wider host range in the field than VCG1 or VCG4. The appearance of race 2 isolates in diverse VCGs, supports the hypothesis of an independent origin for the race within each.; Distinct subpopulations of V. dahliae were found within agricultural region of the Salinas and Central Valleys. These subpopulations were identified by differential virulence on race 1 resistant tomato cultivars, differential degrees of virulence on tomato versus cotton, host origin and VCG. Correlating the first three markers with the latter, distinct patterns of virulence and host preference were revealed.; Vegetative compatibility groupings and PCR-RAPD haplotypes also supported the designation of V. dahliae as a strictly asexual-organism, and VCG2 and VCG4 as distinct, possibly clonal, populations with preferential host-pathogen relationships. PCR-RAPD haplotypes demonstrated that isolates within VCG1 fall into two separate subgroups. The appearance of race 2 isolates within multiple VCGs over a wide geographic range also argues for their selection in situ.; Greenhouse experiments proved fifteen leguminous intercrops grown in California's Central Valley had the potential to contribute to the incidence of Verticillium wilt in subsequent susceptible crops. Extensive vascular colonization, together with the formation of microsclerotia occurred with direct introduction of the pathogen into the host vacular system in all experimental legumes. However physical symptoms of such infection differed from those seen in Verticillium wilt disease in susceptible hosts. Evidence of infection resulting from microsclerotia infecting legume roots was not established. The usefulness of PCR-RAPD molecular techniques in distinguishing between V. dahliae and the physically similar vascular colonizer V. tricorpus was confirmed.