INDUCTION OF SYSTEMIC RESISTANCE IN CUCUMBER BY PLANT GROWTH-PROMOTING RHIZOBACTERIA - DURATION OF PROTECTION AND EFFECT OF HOST-RESISTANCE ON PROTECTION AND ROOT COLONIZATION

Two plant growth-promoting rhizobacteria, Pseudomonas putida strain 89B-27 and Serratia marcescens strain 90-166, which previously induced systemic resistance (ISR) in cucumber 'Straight 8' against anthracnose caused by Colletotrichum orbiculare, were used to determine the duration of ISR in cucumber. ISR activity induced by strain 89B-27 occurred at the first-leaf stage, increased over time, and was maintained at least to the fifth-leaf stage. With strain 90-166, ISR was less stable, occurring at the second-, fourth-, and fifth-leaf stages. Cultivar specificity of ISR and colonization capacity of the two strains were studied in one resistant and three susceptible cucumber cultivars under greenhouse conditions. Strain 89B-27 induced systemic resistance in all three, whereas strain 90-166 induced systemic resistance in two of the three susceptible cultivars. Both strains failed to induce resistance in the resistant cultivar. Root colonization patterns of strains 89B-27 and 90-166 also differed. Populations of strain 89B-27 declined at a consistent rate over time, dropping from log(10) 8.2 CFU/g fresh weight of roots 1 week after planting to log(10) 3.2 CFU/g fresh weight of roots by 4 weeks after planting. In contrast, populations of strain 90-166 declined rapidly from log(10) 7.4 CFU/g fresh weight of roots 2 weeks after inoculation and then decreased more slowly to log(10) 3.2 CFU/g fresh weight of roots by 4 weeks after planting. There was no relationship between ISR activity and populations of the two strains on roots. ISR increased over time, while populations of the bacteria decreased.