Influences of cropping practices on Verticillium dahliae populations in commercial processing tomato fields in Ontario

The abundance of Verticillium dahliae in the soil and the incidence of V. d ahliae-infected plants were determined for 12 commercial processing tomato fields in Kent County, Ontario. Comparison of the data with those from a pr evious survey of fields in adjacent Essex County showed that soil inoculum levels and incidence of infection were generally lower in Kent County field s and that race 2 V. dahliae was not common in Kent County. From the two co unties, 128 isolates were characterized by restriction fragment length poly morphism (RFLP) analysis, using the subspecies-specific repetitive DNA sequ ence E18. A subset of these isolates was also characterized by vegetative c ompatibility and DNA hybridization analysis with a second subspecies-specif ic DNA sequence. Isolates with E18 RFLP profiles highly similar to those of isolates previously collected from potato fields in North America were pre valent in Essex County tomato fields but not common in Kent County fields. The data are consistent with the hypothesis that the group I isolates were introduced into southwestern Ontario with potato and that the different cul tural practices in Essex County and Kent County have contributed to the dif ferences in the accumulation of these isolates in the two regions.