Biological Control Of Eucalypt Leaf Blight By Fluorescent Pseudomonas Spp

Based on the investigation of physiological characteristics of Cylindrocladium quinqueseptatum, the pathogen of Eucalyptus, bioassays were developed to study the biological control of eucalypt leaf blight by Pseudomonas putida WCS358r and its genetically modified stains WCS358::phl and WCS358::phz, P.fluorescens WCS374r> WCS417r, CHAOr and their siderophore-minus mutants. It is suitable for the growth of hyphae and germination of spores in a neutral or slightly alkali condition at the temperature of 25. When the temperature is below 10℃ or above 30℃, or pH value is lower than 5.0, the hyphae hardly grew, and the percentage of spore germination was very low. The periods of illumination has no noticeable influence on the growth of hypha and germination of spores. Comparing differently inoculation approaches, it was supposed that C. quinqueseptatum infects the plant mostly through stoma of the abaxial surface of leaf and wound of leaf, and hardly invades the upper side of leaf because of thick wax layer and compact cuticle structure. No inhibition was observed in in vitro antagonism tests against the growth of hyphae by all Pseudomonas strains, but the germination of spores stopped or decreased when all the wild types of bacterial strains were mixed with the spores of the pathogen. Among the strains tested, WCS358r, and CHAOr, producing several antibiotics are the strongest antagonists against spore germination. The siderophore-minus mutants had a lower capability to suppress germination of spores, demonstrating that siderophore is the main suppressive bacterial determinant. Salicylic acid (SA) has no inhibition in growth of hyphae, and no obvious influence on spore germination. SA can induce systemic resistance against eucalypt leaf blight by pouring SA to the rhizophere of Eucalyptus urophylla or dipping leaves into SA solution. However, no inducing effects of systemic resistance were observed by pouring fluorescent Pseudomonas spp. to the rhizophere of Eucalyptus or dipping leaves into the bacterial suspension. In bioassays with direct suppression, strain WCS417r, WCS358r and its genetically modified strain WCS358::phl(constitutively producing 2,4-diacetylphloucinol,DAPG) had the best control effects with a lower incidence of 30% than the control. The siderophore-minus mutant S680 and JM218 have no control effects at all, indicating that siderophore is a promising bacterial determinant to control eucalypt leaf blight. WCS358::phl had an enhanced effect against this fungal disease compared with its parental strain WCS358, inferring that DAPG had a suppressive effect against the disease.