A Preliminary Study On The Cross Infection And Pathogenicity Differentiation Among The Pear Valsa Canker Pathogens,Which Isolated From China's Main Producing Areas

Pear is the important economic crop in our country.Pear Valsa canker is one of the three branches diseases in pear,it is mainly damage the pear's trunk and collateral.It has a high incidence,occured widely and difficult to control.In the serious orchard,the tree scarred and branches are incomplete.It can cause production cuts and wither of the tree and seriously threat the development of China's pear industry.At present,the research of the disease pathogens is less,and the study on pathogen species and pathogenicity is not perfect.In this study,we collected more than 600 samples from 13 provinces.By isolation and identification,we found 465 pear Valsa canker pathogens.Depending on the color of the strains' colonies on PDA plate,we divided them into five categories:type A(white),type B(gray),type C(yellow),type D(yellow-green)and type E(physalospora shape),each category of strains has diversity pycnidia,but different cultivate character of the strains are unrelated to origins or taxonomic status.Then we selected 122 strains by different origins and cultural characteristics from 465 isolates to measure their pathogenicity.Through the comparison of the lesion length at 6 days past inoculation,we found there're obvious pathogenic differentiations in these strains,but unrelated to origins or cultivate character.Becouse the pear Valsa canker in Korla fragrant pear and poplar canker in poplar around the pear orchard occurred seriously,and the symptoms are similar.We collected pear and poplar samples,separated 8 strains of Valsa mali var.pyri from pear Valsa canker samples,and 11 strains of Valsa sordida and 9 strains of Cytospora translucens from poplar canker samples.After validate the strains according to Koch's postulates,we did a cross infection experiment of them.The results showed the strains can infect each other,but the strain of Valsa mali var.pyri has so weak pathogenicity in poplar branches that it almost could't infestation poplar.The strain of Valsa sordida and Cytospora translucens were able to intect pear branches with moderate pathogenicity,but in pear,their pathogenicity were weaker than the strain of Valsa mali var.pyri,then we inoculated them to pear fruit of three generations and observed the pathogenicity,it showed that the pathogenicity of all the strains were increased at last,so we speculated that if these strains infect pear repeatedly,their pathogenicity will increase.Thus,it provides a new way of thinking for the choice of Korla Pear tree's shelterbelt and the prevention and treatment programs of the pear Valsa canker.