Abstract
SUMMARYThe lethal dose (LC50) of metalaxyl for a wild isolate of Peronospora hyoscyami f. sp. tabacina was determined using a bioassay. Infection levels were recorded using 5‐wk‐old tobacco seedlings that had been treated with foliar sprays of metalaxyl (Ridomil®) 24 h prior to inoculation. Plants were maintained in a controlled environment cabinet. LC50 estimates determined by probit analysis of the data from three independent experiments ranged between 0–46 and 0–58 μg/ml. Pooling the results for all three experiments, probit analysis gave a slope of 8·733 (S.E. 0·805) with an LC50 of 0·51 μg/ml (95% fiducial limits 0·45, 0·57). This determination will be useful as a guide to future monitoring of P. hyoscyami for resistance to metalaxyl.
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