Biological control of Fusarium wilt of cucumber by chitinolytic bacteria

Two chitinolytic bacterial strains, Paenibacillus sp. 300 and Streptomyces sp. 385, suppressed Fusarium wilt of cucumber (Cucumis sativus) caused by F usarium oxysporum f. sp. cucumerinum in nonsterile, soilless potting medium . A mixture of the two strains in a ratio of 1:1 or 4:1 gave significantly (P < 0.05) better control of the disease than each of the strains used indi vidually or than mixtures in other ratios. Several formulations were tested , and a zeolite-based, chitosan-amended formulation (ZAC) provided the best protection against the disease. Dose-response studies indicated that the t hreshold dose of 6 g of formulation per kilogram of potting medium was requ ired for significant (P < 0.001) suppression of the disease. This dose was optimum for maintaining high rhizosphere population densities of chitinolyt ic bacteria (log 8.1 to log 9.3 CFU/g dry weight of potting medium), which were required for the control of Fusarium wilt. The ZAC formulation was sup pressive when added to pathogen-infested medium 15 days before planting cuc umber seeds. The formulation also provided good control when stored for 6 m onths at room temperature or at 4 degrees C. Chitinase and beta-1,3-glucana se enzymes were produced when the strains were grown in the presence of col loidal chitin as the sole carbon source. Partial purification of the chitin ases, followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and activity staining, revealed the presence of five bands with molecular masses of 65, 62, 59, 55, and 52 kDa in the case of Paenibacillus sp. 300; and three bands with molecular masses of 52, 38, and 33 kDa in the case of Streptomyces sp. 385. Incubation of cell walls of F: oxysporum f. sp. cucum erinum with partially purified enzyme fractions led to the release of N-ace tyl-D-glucosamine (NAGA). NAGA content was considerably greater when pooled enzyme fractions (64 to 67) from Paenibacillus sp. were used, because they contained high beta-1,3-glucanase activity in addition to chitinase activi ty. Suppression of Fusarium wilt of cucumber by a combination of these two bacteria may involve the action of these hydrolytic enzymes.