| Pear Valsa canker is one of the most important stem diseases of the pear. It occurres in all pear producing areas of China and causes more serious damage in the northeast, northwest and north areas of our country. The disease mainly endangers the main stem and branch of pear trees and causes bark rot and branch die. When serious, it can cause the tree die or even the orchard destroy. But only little systematic research about pear Valsa canker has been done in China. The preliminary identification for the pathogens causing pear Valsa canker in China and pathogenic analysis of the strains showing different cultural characters and rDNA-ITS sequences were carried out in this study, for providing reference to research the epidemic rules and effective prevention and control measures of this disease.The results were as follows:1. Some strains of different provinces from the pathogen isolates of pear Valsa canker which were saved in the laboratory were purified in the study and79strains were obtained by isolating single spore. Colony morphology and pycnidia of these strains were observed on PDA medium at25℃in darkness. The pathogenicity of these strains was tested by wound inoculation with mycelia plugs on one-year ’cuiguan’ in vitro branches. The rDNA-ITS sequences of partial strains were amplified and analysed. The results showed that there were two colony types, designated as type Ⅰ and Ⅱ, in the pathogen strains of pear Valsa canker. Different pycnidia of these strains were formed on PDA medium. The pathogenicity of these strains was different on ’cuiguan’ in vitro branches of pear. The nucleotide sequences of rDNA-ITS from different strains showed high identities, ranging from99.98%to100%. Thus, the pathogen of pear Valsa canker belonged to Ⅴ. mali var. pyri which were clustered into two branches of the same subgroup with the pathogen of apple Valsa canker based on analysis of nucleotide sequences of rDNA-ITS genes, though the pathogen of pear Valsa canker in China have different colony types.2. The biological characteristics of three strains (F-SD-8, F-BJ-2c-2and F-HN-2a-1) of Valsa mali var. pyri causing pear canker and one strain (F-SX-A6) of Ⅴ. mali var. mali causing apple canker in China were comparatively tested and the pathogenicity were comparatively by wound inoculation with mycelia plugs on in vitro twigs of pear, apple and some other woody plants, and in vivo twigs of pear. The results showed that there was no significant difference in the incubation temperature and kind of carbon and nitrogen sources of the four strains. The pathogens could grow at10～35℃but growed poorly at4℃and below or and35℃and above. They could grow in Czapek medium and the tested medium with carbon and nitrogen sources other than urea and growed well with the sources of mannitol, maltose, starch and yeast extract. There were significant differences in the requirement of light and adaptation to pH among the four strains. The illumination significantly promoted the growth of the F-SD-8and F-BJ-2c-2, but had no obvious effect on F-HN-2a-1and F-SX-A6. The optimum pH of strains F-SD-8, F-BJ-2c-2and F-HN-2a-1were4～6, but F-SX-A6was4. The significant pathogenicity differentiation was detected in V. mali var. pyri. Generally strains F-SD-8and F-BJ-2c-2were highly pathogenic on pear although their culturing characteristics differed greatly. The strain F-SX-A6was more aggressive on apple than on pear, and the strain F-HN-2a-1showed significantly lower pathogenicity on ten pear cultivars and other seven species of woody plants. Our results confirmed that two varieties of V. mali had host preference and were also aggressive to crabapple, apricot, and peach besides apple and pear. Meanwhile, strains F-SD-8and F-BJ-2c-2could induce the formation of pycnidia on in vivo twigs of pear, which was not observed on in vivo twigs inoculated with F-HN-2a-1and F-SX-A6.3. The type of enzyme, the enzyme activity and the change of pH were tested in the lesions of in vitro pear twigs which produced by inoculating the four tested strains of pear and apple Valsa canker for4d and the pear skin liquid medium that had cultivated the four strains for10d. The results showed that in the internal and external of pears, all the four strains could secrete PG, PMG, Cx and xylanase. The xylanase activity of pear Valsa canker strains F-SD-8and F-BJ-2c-2were the highest both in in vivo and in vitro, but the activity of PG was the highest of apple Valsa canker strain F-SX-A6. Meanwhile, all the three strains could make the pH of the lesions inoculated for4d and the the pear skin liquid medium become lower. The enzyme activity and the change of pH of the weak virulence strain F-HN-2a-1was different from the other three, the pH value as well as the enzyme activity were also different in the in vivo and in vitro pear twigs. |
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