Pathological Process Of Cankers On Popar Caused By Cytospora Chrysosperma

Poplar canker was a worldwide branches disease distributed in Northeast,Northwest,North China and other regions.The disease can cause the collapse stems rot,the formation of the ring spot,and even the whole plant death.The pathogenic fungi were Cytospora chysosperma(Pers.)Fr.However,the pathogenicity of pathogenic fungi was not clear,including the sprouting of spores,the mycelial growth process in the host tissue and the re-formation of new pycnidia and conidia.Therefore,this study used C.chrysosperma to inoculate the annual branches of Populus × euramericana(Dode)Guinier to observe the cytology and histopathological processes of poplar canker and provide the basis for the pathology and prevention and control strategies of the disease.The results were as follows:(1)After 2-3 days of inoculation,scald inoculated in vitro annual poplar branches and found that the lesion showed brown subsidence and continue to expand.After 7-9 days of inoculation,generation of ring spots,after 15-20 days of inoculation,a large number of conidia were developed on the surface of the branches.(2)The spore germination and invasion and expansion of the conidia were studied.The germination of conidia began at 24h,and reached the peak of germination at 48h.The conidia spread to the original 3-4 times in the long axis direction with a shape of oblong to spherical,and generating 1-4 germ tubes from both ends or middle,each germ tube continued to branch and invade from the wound of host surface.Pathogens expand in both lateral and longitudinal directions.Horizontal expansion occurs in the cortex,phloem and primary xylem,microscopic examination revealed that many brown glial sphere generated in plant cells,before the destruction of the cortex and phloem of mycelium by large-(5-10μm)and small-diameter(1-2μm)mycelia both between and within host cells.Large-diameter mycelia closed to the plant cell wall slightly expanded,and three to four fine filaments penetrate the cell wall under the enlarged part.This mycelia were found in the cortex,phloem,cambium,xylem and ray parenchyma cells of the host.The small-diameter mycelium aggregation specialized formed pycnidia in cortex.This mycelia appeared in the cortex destroyed by the large-diameter,vessel,and pits cells.At the same time,the cell wall of the host plant was significantly dissolved around the mycelia especially the hyphal tip.In the longitudinal direction,the mycelia were attached to the wall of the xylem vessel or through the pits and eventually gathered a large number of mycelia in the pith.(3)The development process of pycnidia and conidia in C.chrysosperma was elucidated.In the initial stage,mycelial septa and branches were produced frequently and the shape of cells became subglobose and shorter than normal ones.Mycelia interwove and expanded around the center point,then the pycnidial formed symphogenetically.Next,the primordial cells proliferated continuously,and a center loculus was formed through the assemblage of inner thick-walled mycelia.Several relatively independent loculi linked with the center loculus were formed and eventually formed a mature pycnidium.During this process,inner surface cells of the loculus developed into branched conidiophores and graduated in length.The sausage-shaped conidia were then produced from the conidiogenous cells enteroblastically.At last,with the maturation of central loculus,the pycnidia break out the surface of plant tissues and the opening ostiole formed.Mature conidia were then released from the opening ostiole under suitable environment.The mature pycnidia were usually located in the cortex of host plants.