INDUCTION OF SYSTEMIC RESISTANCE TO PYTHIUM DAMPING-OFF IN CUCUMBER PLANTS BY BENZOTHIADIAZOLE - ULTRASTRUCTURE AND CYTOCHEMISTRY OF THE HOST RESPONSE

Benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH, CGA 245704), a non-toxic, synthetic chemical, was applied as a foliar spra y to cucumber plants and evaluated for its potential to induce defense mechanisms in root tissues infected by the soilborne pathogen, Pythiu m ultimum Trow. In non-treated cucumber plants, fungal colonization wa s intense and paralleled marked host tissue damage, whereas in BTH-tre ated plants, pathogen ingress towards the vascular stele was apparentl y halted by the massive deposition of a phenolic-enriched material whi ch occluded a large number of cortical and vascular parenchyma cells. This considerable increase in the accumulation of phenolics was accomp anied by cytological disorders of the invading pathogen at a time when the wall-bound cellulose component was preserved. In addition to phen olic compounds, the occluding material contained large amounts of beta -glucoside residues. These residues gradually decreased in the areas n eighboring fungal cells whereas phenolic deposition appeared to be mor e uniformly distributed throughout the occluded host cells. Pathogen p enetration in non-occluded cucumber root cells coincided with other ch anges, mainly characterized by both the deposition onto the inner surf ace of the cell walls of some heterogeneous wall appositions and the c oating of some intercellular spaces with an electron-opaque material. Evidence is provided in this study that BTH has the ability to induce SAR in cucumber. Exogenous, foliar applications of the chemical sensit ize susceptible cucumber plants to react more rapidly and more efficie ntly to P. ultimum attack, mainly through the massive accumulation of phenolic compounds at sites of attempted pathogen penetration.