Resistance of Peronospora tabacina to Metalaxyl in Texas and Mexico. Martin D. Wiglesworth, Department of Plant Pathology, University of Kentucky, Lexington 40546. Moshe Reuveni, William C. Nesmith, Malcolm R. Siegel, Joseph Kuc*, José Juarez, and Tabamex. Plant Biotech Industries, Ltd., Mobile Post Ashrat, 25201, Israel; Department of Plant Pathology, University of Kentucky, Lexington 40546; and San Andres Tuxtla, Veracruz, Mexico. Plant Dis. 72:964-967. Accepted for publication 21 July 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0964.

Isolates of Peronospora tabacina were collected and tested for metalaxyl resistance in Kentucky during 1979–1987, southwestern Texas during 1983–1987, and Veracruz, Mexico during 1985–1987, using a detached leaf assay. The minimum inhibitory concentration (MIC) that prevented sporulation of all Kentucky isolates was 0.066 µg/ml. The Texas 1985 isolate was three times more sensitive to metalaxyl than Ky 1979. Reisolation of the Tx 85 isolate from leaves treated with the highest concentration of metalaxyl that allowed sporulation indicated an increase in the level of resistance. However, because of the approaching tobacco-growing season, it was necessary to freeze the isolate. When further experiments were conducted after the growing season, the isolate recovered from storage was as sensitive as the Ky 79 isolate. Isolates collected in all years from the gulf zone of Veracruz, Mexico were determined to be highly resistant. The MIC ranged from 12.5 to 27 µg/ml, and chlorotic lesions developed at concentrations as high as 66.6 µg/ml. With this level of sensitivity, metalaxyl is no longer an effective control. The proximity of metalaxyl-resistant P. tabacina isolates to the U.S. and Canadian tobacco production areas merits the continual monitoring of isolates in Texas and Mexico, as well as development of effective strategies to control such isolates should they arrive.