VIEW ARTICLE

Disease Control and Pest Management

Development of Laboratory Resistance to Metalaxyl in Phytophthora citricola. M. C. Joseph, Staff research associate, Department of Plant Pathology, University of California, Riverside 92521; M. D. Coffey, professor, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 74:1411-1414. Accepted for publication 25 June 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1411.

Isolates of Phytophthora citricola resistant to the systemic fungicide metalaxyl were obtained by treating zoospores with ultraviolet radiation or the chemical mutagen N-methyl-N' -nitro-N-nitrosoguanidine. Both methods produced similar numbers of resistant mutant isolates from a total of ~8.9 × 10⁷ zoospores. In vitro resistance was exhibited by 22 isolates that had growth rates indistinguishable from the parent isolate, and five of these isolates were also resistant in vivo. The stability of metalaxyl resistance was tested by using single-zoospore isolates of the resistant strains cultured for >20 generations in the absence of the fungicide. Three single-spore isolates proved to be extremely stable, both in vitro and in vivo. In vivo resistance to metalaxyl was associated with cross-resistance to four acylanilide fungicides: benalaxyl, RE 26745, cyprofuram, and oxadixyl. However, these resistant isolates remained sensitive to the chemically unrelated fungicide fosetyl-Al. The competitive fitness in vivo of one resistant mutant isolate was high as only that isolate was recovered from stem lesions that developed on seedlings of Persea indica previously inoculated with zoospores in the ratio of 1:1 of the parent and mutant isolates.