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Cross Protection in Mints by Verticillium nigrescens Against V. dahliae. H. A. Melouk, Research Associate, Department of Botany and Plant Pathology, Oregon State University, Oregon 97331; C. E. Horner, Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Corvallis, Oregon 97331. Phytopathology 65:767-769. Accepted for publication 5 February 1975. DOI: 10.1094/Phyto-65-767.

Peppermint and spearmint were cross protected against disease induced by a virulent isolate of Verticillium dahliae when inoculated first with the weak pathogen, V. nigrescens. Cross protection effects were observed in peppermint when inoculation with V. nigrescens preceded inoculation with V. dahliae by 2 days. When inoculations with V. nigrescens preceded V. dahliae by 7 and 9 days, wilt severity was greatly reduced and many plants were symptomless 4 weeks after inoculation. No cross protection was observed when inoculation with V. dahliae preceded inoculation with V. nigrescens by 2, 5, 7, and 9 days. In cross-protected peppermint, propagules of V. dahliae decreased substantially 22 and 29 days after inoculation. Four weeks after inoculation the number of propagules of V. dahliae in stems of cross-protected peppermint were one-fifteenth to one-twentieth of those of nonprotected controls. Numbers of propagules of V. nigrescens remained stable, but at a low level, in peppermint plants when V. dahliae was the challenger.

Additional keywords: Mentha spp., biological control, premunity.