Toxin produced by Phytophthora cryptogea active on excised tobacco leaves

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Abstract
A sterilized aqueous extract of mycelium of Phytophthora cryptogea, grown on a defined medium, caused water-soaking of excised tobacco leaves within 12 h, laminar collapse within 20 h, and extensive dehydration within 48 h. This phenomenon was used as a bioassay to study the production and stability of the toxin. Many tobacco cultivars of diverse origin were all found to be susceptible, although degrees of susceptibility may exist. Bean, peas, pepper, tomato, and potato were not affected. Toxin was present in mycelium and culture filtrate throughout the growth cycle. The toxin was stable to autoclaving at acid pH but labile at alkaline pH. The activity of the toxin was augmented by dilution of mycelial extracts with 0.01 mol/kg NaH2PO4 rather than water. Treatment of leaves with light before or during incubation with the toxin reduced their susceptibility. Leaves collected for assay from the greenhouse in the afternoon were less sensitive to the toxin than those collected in the morning or at night. The toxin was equally active at incubation temperatures of 20 and 27 °C.

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