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Induction of Auxotrophic Mutants in Phytophthora by Ultraviolet Light. Jesus Castro F., Former Research Assistant, Department of Plant Pathology, University of California, Riverside 92502, Present address of senior author: Centro de Investigaciones Agricolas de el Bajio, Apartado Postal No. 112, Celaya, Gto., Mexico; George A. Zentmyer(2), and William L. Belser, Jr.(3). (2)Professor, Department of Plant Pathology, University of California, Riverside 92502; (3)Associate Professor of Microbiology, Department of Life Sciences, University of California, Riverside 92502. Phytopathology 61:283-289. Accepted for publication 23 September 1970. DOI: 10.1094/Phyto-61-283.

Zoospores of Phytophthora capsici and P. drechsleri were irradiated with ultraviolet (UV) while motile and after encystment, at a dosage that killed 99% of them. Mutants were screened by the “rescue method” and by the “starvation method,” which are discussed in relation to Phytophthora. Sensitivity of zoospores to UV light was different for motile and encysted zoospores, as shown by different survival curves and yield of mutants. No auxotrophic mutants were found among 685 survivors when zoospores were irradiated while motile. Four of 359 survivors of zoospores irradiated after encystment were auxotrophic mutants. Two (P. capsici) were methionine-dependent; two (P. drechsleri) required tryptophan. These results support the concept of haploidy in the vegetative stage of Phytophthora.