Identification And Pathogenesis Of The Pathogen Causing Black-rot Disease On Zanhuang Jujube

This study was designed to identify the pathogens causing Zanhuang jujube black rot disease, which has done great harm to jujube fruit, and clarify their pathogenic mechanism from such perspectives as cell wall degrading enzymes, cell membrane permeability, reactive oxygen scavenging enzymes, defense enzymes and toxin. The results were as follows:1. The isolation and identification of the pathogen:Pathogens were isolated from diseased fruit segments and their pathogenicities were tested following Koch’s postulate. Identification of the pathogens was made through morphological study and rDNA molecular identification. The results showed that Alternaria tenuissima and Phoma glomerata were the dominant species which caused black rot on ZanHuang jujube.2. The activities of the cell wall degrading enzymes (CWDEs):The dynamic changes of CWDEs activities of Alternaria tenuissima and Phoma glomerata in vitro and in vivo were investigated. The results showed that the two pathogens could infect Zanhuang jujube by producing cutinase and CWDEs. The cutinase degrades the cutin of the plant cell wall in the early infection stage. A series of CWDEs including polygalacturonase (PG), polymethyl-galacturonase (PMG) and carboxymethyl cellulose (Cx) showed higher activities as compared with others. PG and PMG could make fruit soften by degrading the pectic substance of the cell wall at the early infection stage. Cx mainly degrade the cellulose structure to destroy the cell wall structure in the middle infection stage, but the emergence of Cx peak had slight differences after jujube fruit being infected by two pathogens. PGTE and PMTE showed no obvious in the process of infection.3. The changes of cell membrane permeability:The electrical conductivity of jujube pericarp, the content of MDA and LOX increased obviously after jujube fruit being infected by pathogens. At the same time, the activity of LOX in fruit infected by P.glomerata was higher than that in fruit infected by A.tenuissima. These results showed that cell membrane permeability and membrane peroxidation level increased and unsaturated fatty acid decomposition quickened. Thus we can see, the black rot pathogen could damage the cell membrane structure and function.4. The changes of reactive oxygen scavenging enzymes and defense enzymes:The reactive oxygen scavenging enzymes, including SOD and APX had higher activities in jujube fruits infected by A.tenuissima and P. glomerata in the early stage, but the activities dropped sharply later on. These results proved that two pathogens could activate the two reactive oxygen scavenging enzymes and avoid the accumulation of reactive oxygen in the fruit in the early infection stage, but later on, the system was gradually destroyed. The reason why CAT activity was lower in treatment than in control may be that the enzyme lost its activity owing to the infection of the pathogen or it has little role in the course of hydrogen peroxide elimination. For the defense-related enzymes, PAL which could enhance the lignification of cell wall had higher activities in jujube fruits infected by two pathogens than in control. But the POD activity in affected jujube fruits was mainly lower than that in control, except in the early infection stage, showing that it was not a major defense enzyme.5. The production of toxin:The toxin was one of the major pathogenic factors of A.tenuissima and P.glomerata. The optimum toxin-producing conditions of A.tenuissima and P. glomerata were defined respectively as follows:PDA medium, pH 7, at 25℃ shaking culture 7 days, and PSK. medium, pH6, at 30℃ shaking culture 14 days.