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Disease Control and Pest Management

In Vitro Variability Among Isolates of Eight Phytophthora Species in Response to Phosphorous Acid. M. D. Coffey, Associate professor, Department of Plant Pathology, University of California, Riverside 92521; L. A. Bower, staff research associate, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 74:738-742. Accepted for publication 8 February 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-738.

The ED₅₀ values for inhibition of mycelial growth in vitro by phosphorous acid (H₃PO₃) ranged from 5.2 to 224.4 μg/ml for nine Phytophthora species. Among the most sensitive species were P. citricola, P. citrophthora, and P. cinnamomi. One of the most tolerant species was P. megasperma f. sp. medicaginis. At the extreme of the range, P. infestans from potato had an ED₅₀ value of 224.4 μg/ml. Even among a group of similarly sensitive isolates, it was possible to differentiate them at a species level in terms of their growth response to H₃PO₃. Isolates of P. citricola from avocado were inhibited by 48.3- 67.6%, and those of the A2 mating type of P. cinnamomi by 11.3- 38.5%, in the presence of 5 μg/ml H₃PO₃. Isolates of P. citrophthora from citrus were inhibited 80.3- 89.3%, compared with 27.9- 58.8% for isolates of P. parasitica in the presence of 10 μg/ml H₃PO₃.