Studies on genetic and on chemically induced resistance of cucumber tissues to Cladosporium cucumerinum (Ell. & Arth.)

A detailed anatomical study was made of the host-parasite relations of cucumber seedlings inoculated with Cladosporium cucumerinum and of the effect of treatment with G33, the roots being immersed in a 100 p.p.m. solution of the compound for 48 h before inoculation. It was concluded that the resistance conferred by G33 was situated both in the cuticle and in the deeper tissues, and that the substance was effective only in cells appearing microscopically sound. The effect did not extend beyond the cuticle, and resistance did not seem to be due to plasmatic interaction.

Uptake by the roots of cucumber seedlings of 2,4-dinitrophenol, sodium diethyldithiocarbamate and sodium azide had no effect on either genetic resistance or that induced by G33. However narcosis of seedlings by exposure over solutions of 1. 5 % ether or 0.24 % chloroform caused breakdown of genetic resistance in the varieties Mabro and Proso, though these still remained resistant to Penicillium italicum and Botrytis cinerea (neither of which normally infects cucumber). But narcosis had no effect on the resistance to C. cucumerinum conferred by G33, which, unlike genetic resistance, seemed therefore unrelated to host metabolism. The sap expressed from resistant varieties and from the uninfected portions of older plants of a susceptible variety (Lange Gele Tros) did not inhibit growth of the pathogen in culture.