VIEW ARTICLE

Resistance

Induction of Fungal Resistance to Metalaxyl by Ultraviolet Irradiation. G. C. A. Bruin, Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1; L. V. Edgington, Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1. Phytopathology 72:476-480. Accepted for publication 9 June 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-476.

Three minutes of ultraviolet (UV) irradiation of a suspension containing 3 × 10⁶ zoospores of Phytophthora capsici resulted in the development of 12 metalaxyl-resistant strains and 35 min of UV irradiation of mycelium of P. capsici and Pythium ultimum produced many resistant strains. The resistance was initially low, but a few transfers on metalaxyl-amended V-8 agar followed by 12 transfers on unamended V-8 agar resulted in a number of stable, highly resistant mutants that retained pathogenicity to their respective pea and pepper hosts. There was a high correlation between resistance expressed in vitro and in vivo. Resistance to metalaxyl was coupled with resistance to furalaxyl, Galben, milfuram, RE26745, and RE26940. UV irradiation of 1.05 × 10⁸ spores of Peronospora parasitica yielded no resistant strains. Large differences in levels of metalaxyl sensitivity among single zoospore isolates of P. capsici were demonstrated and may indicate the coexistence of metalaxyl-’sensitive’ and ‘resistant’ nuclei in the coenocytic mycelium.