Abstract
Systemic acquired resistance (SAR) is a physiological defense response in plants conferring broad spectrum resistance to pathogens. SAR is inducible through infection by necrotizing pathogens or chemical inducers and involves the systemic activation of defense related genes. The objectives of this study were to evaluate resistance expression to Pythium soft rot in fruit of cucumber in response to foliar applications of 2,6-dichloro isonicotinic acid (INA), benzo(1,2,3)thiadiazole-7-carbothioc acid S-methyl ester (CGA 245704), or beta-amino-butyric acid (bABA). Excised leaves from three cucumber cultivars, Vlaspik, Sumpter and SMR 58, exhibited a resistance response to foliar applications of INA (50 ppm) and CGA 245704 (25 ppm) when challenge inoculated with Colletotrichum lagenarium. However, the reduction in lesion incidence by INA was 99% in Vlaspik and only 11% and 18% in Sumpter and SMR 58, respectively. bABA did not enhance resistance in vegetative tissues. Multiple foliar applications of INA and CGA 245704 prior to fruit set were found to be more effective than a single application. Harvested cucumber fruit, 3.0 to 5.0 cm in diameter, were wounded and challenge inoculated with Pythium sp.; no statistically significant differences in infection were observed between controls and fruit from chemically treated plots. In Vlaspik, however, INA and BTH did reduce the rot infection rate in fruit when foliar applications were made early in plant ontogeny, at the four-leaf stage. In addition, larger more mature fruit (4-5 cm diameter) exhibited slower lesion growth than less developed fruit (>3 cm). It is unclear if these differences were attributable to changes in physiological or anatomical factors. These findings indicate that the level of physiological resistance achievable through chemical induction in cucumber is a function of the specific pathogen and the plant organ being infected, i.e., leaves or fruit.
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