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The Generation and Quantification of Resistance to Dimethomorph in Phytophthora infestans

September 2004 , Volume 88 , Number  9
Pages  930 - 934

J. M. Stein , Texas Agricultural Experiment Station, Bushland 79012 , and W. W. Kirk , Department of Plant Pathology, Michigan State University, East Lansing 48824

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Accepted for publication 18 April 2004.

The generation of dimethomorph resistance in Phytophthora infestans was attempted using ethidium bromide/UV light mutagenesis and repeated culturing on dimethomorph-amended medium. Ethidium bromide/UV mutagenesis created two isolates of P. infestans with resistance factors for dimethomorph >20, i.e., the ratio of the 50% effective concentration (EC50) of the mutant to that of the wild-type. With repeated culturing on dimethomorph-amended medium, the rate of growth (mm diameter/day) increased until the tenth subculture for most P. infestans isolates. Resistance factors generated from repeated culturing were <8 for all isolates. For most isolates, the generation of dimethomorph resistance resulted in reduced growth rates on nonamended medium, regardless of the level of resistance or induction treatment. Additionally, the frequency of infection of leaf disks and whole tubers was significantly reduced in >20% of the isolates repeatedly subcultured on dimethomorph-amended medium. Regardless of the induction treatment, reduced fitness was common for all P. infestans isolates, indicating a potential biological cost associated with dimethomorph resistance. Based on these results, the development of field resistance to dimethomorph in P. infestans is unlikely with the currently employed resistance management strategies.

Additional keywords: fungicide, late blight, Oomycete, Solanum tuberosum

© 2004 The American Phytopathological Society