Oxalate and phosphates induce systemic resistance against diseases caused by fungi, bacteria and viruses in cucumber

Abstract

Spraying aqueous solutions (20 or 50 m m) of oxalate, potassium phosphate dibasic (K 2HPO 4) or tribasic (K 3PO 4) on the upper surface of leaf 1 of cucumber plants, or inoculating leaf 1 with a spore suspension of Colletotrichum lagenarium induced systemic resistance to C. lagenarium, Cladosporium cucumerinum, Dydimella bryoniae, Sphaerotheca fuliginea, Pseudomonas lachrymans, Erwinia tracheiphila, tobacco necrosis virus (TNV) and cucumber mosaic virus (CMV) when plants were challenged on leaf 2 with the test pathogens 7 days after treatment of leaf 1. Plants were rated for disease 5–15 days after challenge-inoculation, depending on the pathogen. The level of protection induced by the test solutions varied and generally was lower than that induced by C. lagenarium. Each chemical solution tested caused chlorotic stippling on the induced leaf within 48 h after application. The stippling later developed into restricted necrotic lesions. The nature of damage in the inducer leaf was related to the level of protection. Plants with distinct but restricted necrotic lesions on inducer leaves were better protected than those with extensive damage or few lesions. These experiments provide further evidence that induced resistance is due to a low level of persistent stress which can be induced chemically and not the result of a specific component of the inducing pathogen.