VARIABILITY IN SENSITIVITY TO THE FUNGICIDE PYROXYCHLOR WITHIN THE GENERA PHYTOPHTHORA AND PYTHIUM

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Abstract
A comparative study was made of the innate sensitivity of individual species within the genera Phytophthora and Pythium to the selective systemic fungicide pyroxychlor (2-chloro-6-methoxy-4-(Trichloromethyl)pyridine). In vitro sensitivity was evaluated on the basis of inhibition of mycelial growth on treated V8-juice agar. Little variation was found among the 10 Phytophtora spp. tested, the dosage response curves being linear and generally steep, with ED50 values ranging from 0.4 to 6.2 ppm. Similar steep, linear curves with low ED50 values were obtained with six Pythium spp., but five other species responded quite differently. P. sylvaticum was the most tolerant with no inhibition up to 25 ppm and only 20% at 100 ppm. The dosage response of the other species was irregular. Inhibition of P. irregulare and P. splendens reached a maximum of 50% near 1 ppm. The curve for P. ultimum also reached a maximum near 1 pp, but declined to a lower, constant level. The curve for P. polymastum was distinctly bimodal, ascending with dosage up to 0.5 ppm, descending sharply to 3.1 ppm, then ascending again. The effectiveness of pyroxychlor in controlling preemergence damping-off of pea seedlings caused by four of the Pythium spp. was compared in a growth-room test. P. aphanidermatum and P. ultimum were effectively controlled by postplant drench treatments, but P. irregulare and P. sylvaticum were not.

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