In Vitro Variability Among Isolates of SixPhytophthoraSpecies in Response to Metalaxyl

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Abstract
A comparison of isolates of P. parasitica and P. citrophthora, mainly from citrus, revealed a range of inhibition of mycelial growth of 69.1-83.6% for the former, and only 0-14.6% for the latter, at a metalaxyl concentration of 0.1 .mu.g/ml. Similarly, P. palmivora was much more sensitive than either P. citrophthora or P. capsici (all from cacao [Theobroma cacao]) to the fungicide at 0.1 .mu.g/ml. Among isolates of P. megasperma, those of P. megasperma f. sp. medicaginis from alfalfa were extremely sensitive to metalaxyl, followed by P. megasperma f. sp. glycinea from soybean [Glycine max]. Isolates of P. megasperma from Douglas-fir [Pseudotsuga menziesii] were much less sensitive to metalaxyl. In contrast, isolates of P. infestans from potato did not display characteristic in vitro responses to metalaxyl, but variation in inhibition of mycelial growth ranged from a low of 11.8 to a high of 92.8% with metalaxyl at 1.0 .mu.g/ml. For various Phytophthora spp., ED50 values for inhibition of mycelial growth in vitro ranged 0.013 .mu.g/ml for an isolate of P. megasperma f. sp. medicaginis to 0.95 .mu.g/ml for an isolate of P. megasperma from Douglas-fir.