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Chemotactic Response of Zoospores of Five Species of Phytophthora. K. L. Khew, Former Research Assistant, Department of Plant Pathology, University of California, Riverside 92502, Present address of senior author: School of Biological Sciences, Universiti Sains Malaysia, Penang, Malaysia; G. A. Zentmyer, Professor, Department of Plant Pathology, University of California, Riverside 92502. Phytopathology 63:1511-1517. Accepted for publication 15 June 1973. DOI: 10.1094/Phyto-63-1511.

Chemotaxis of zoospores of five species of Phytophthora (P. cactorum, P. capsici, P. cinnamomi, P. citrophthora, P. palmivora) was studied. Zoospores of the five species responded positively to a wide range of chemicals including vitamins, phenolic compounds, nitrogenous bases of nucleic acid, nucleotides, growth regulators, sugars, organic acids, and amino acids. A distinct, directionally oriented attraction to amino acids was observed. Several chemicals (e.g., vitamins, nucleotides, organic acids, and growth regulators) caused accumulation of zoospores by trapping and immobilization without evoking a directional movement of zoospores. A quantitative assay of chemotaxis was made with four amino acids (arginine, aspartic acid, glutamic acid, methionine). Both chemotactic index and threshold molarity for chemotaxis indicated that zoospores of P. cactorum, P. capsici, and P. palmivora were considerably more chemosensitive to the four amino acids than zoospores of P. cinnamomi and P. citrophthora. Aspartic and glutamic acids were better chemotactic agents than arginine and methionine. The chemosensitivity of both arg- and met- auxotrophs of P. capsici to arginine and methionine was less than that of the wild type. Since positively charged molecules were more attractive to zoospores than negatively charged molecules, ionic structure of the amino acid molecule is important in determining its chemotactic activity. Zoospores suspended in aspartic or glutamic acid solution responded positively to either amino acid, indicating that the chemoreceptors in zoospores are likely to be nonspecific. Various metabolic inhibitors and surface-active agents failed to prevent chemotaxis at a concn that did not affect motility of zoospores.