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Title: Population analyses of the vascular plant pathogen Verticillium dahliae detect recombination and transcontinental gene flow

item R. MICHAEL, DAVIS - University Of California
item Klosterman, Steven
item Hayes, Ryan

Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2010
Publication Date: 2/8/2010
Citation: Atallah, Z.K., Maruthachalam, K.K., Du Toit, L.J., Koike, S.T., R. Michael, D., Klosterman, S.J., Hayes, R.J., Subbarao, K.V. 2010. Population analyses of the vascular plant pathogen, Verticillium dahliae detect recombination and transcontinental gene flow. Fungal Genetics and Biology. 47:416-422.

Interpretive Summary: Verticillium dahliae is a soilborne fungal pathogen with a world-wide distribution. V. dahliae is cross-pathogenic on many crops and infests the seed of plants dedicated to seed production, such as spinach. Most spinach seed used for planting in California is produced in Europe and in the Pacific Northwest of the United States. Comparisons of DNA sequence markers between different isolates of V. dahliae from spinach, lettuce, peppers and other vegetable crops indicate that V. dahliae may have migrated from spinach seed production areas to the vegetable production area of the Salinas Valley, California. Potentially, the introduction of new strains of V. dahliae into this area may explain the relatively recent outbreaks of Verticillium wilt of lettuce. This has broad implications on the management of Verticillium wilt of lettuce and other crops in the Salinas Valley of California by limiting the pathogen introduced on seed.

Technical Abstract: The fungal pathogen Verticillium dahliae has resulted in significant losses in numerous crops in coastal California, but lettuce remained unaffected until the mid-1990s. Since then outbreaks have decimated entire fields, but the causes of this sudden susceptibility of lettuce remain elusive. The population structure of V. dahliae isolated from coastal California (n = 123) was investigated with 22 microsatellite markers, and compared with strains from tomato in central California (n = 60), spinach seed imported from Washington State and Northern Europe (n = 43), and ornamentals from Wisconsin (n = 17). No significant differentiation was measured among hosts in coastal California or with the spinach and Wisconsin ornamental sampling groups. In contrast, the tomato sampling group was significantly differentiated. Significant gene flow was measured among the various geographic and host sampling groups, with the exception of tomato. Evidence of recombination in V. dahliae was identified through gametic disequilibrium and an exceedingly high genotypic diversity. The high incidence of V. dahliae in spinach seed and high planting density of the crop are sources of recurrent gene flow into coastal California, and may be associated with the recent outbreaks in lettuce.