Histopathology Of Leaves On Populus × Beijingensis Infected By Colletotrichum Gloeosporioides

Poplar anthracnose is a serious leaf disease of poplar in the world,which is mainly caused by Colletotrichum gloeosporioides.Poplar in Ningxia,Beijing,Henan,Hebei and Shaanxi of China have been harmed in varying degrees.However,the infection process and defense response between pathogen and poplar leaves are lacking in systematic research,which affect the effective control of the disease.In this study,the healthy leaves of Populus x beijingensis were inoculated with conidia suspension of C.gloeosporioides,observing infection process of pathogenic fungi and structure changes of the host cell by optical microscope technology,electron microscope technology to reveal the pathological mechanism and provide significant reference for breeding of poplar disease-resistant resources.The main results were as follows:1.The germination process of C.gloeosporioides on leaves of Populus × beijingensis was observed.The condium began to germinate,form septum and germ tube after 4h post-inoculation(hpi);The top of the germ tube expanded to form oblong appressorium after 8(hpi),the appressorium was pale yellow,not blackening and cytoplasm was sparse;The cell wall of appressorium gradually melanized,the cytoplasm was dense,and the infection peg formed in the central part after 12(hpi);A germ tube and appressorium formed at the other top of the germinal conidium,germ tubes of abundant branches differentiated into septate hyphae and produced appressorium after 24(hpi);The hyphae on the host leaf surface branched randomly and formed a network distribution,some hyphae produced secondary conidia on the top after 48(hpi).2.The invasion and expansion process of C.gloeosporioides was clarified.The C.gloeosporioides invaded the host in two ways.One was to penetrate the host cuticle directly through the form of appressorium and infection peg,the other way was that the germ tube or hyphae invaded from the stoma.The appressorium cone was continuous with the cell wall of infection peg through the penetrating pore after 3 days post-inoculation(dpi);Infection pegs invaded the host by penetrating the cuticle directly or from the intercellular space.The infection peg penetrated the host cell wall and expanded to form an infection vesicle at the top,which grew between the cell wall and the plasma membrane,then separated and differentiated into primary hyphae of 2.5?m to 4 ?m in diameter.When primary hyphae expanded to adjacent epidermal cells or mesophyll cells,they differentiated into secondary hyphae of 1 ?m to 2 ?m in diameter;The secondary hyphae continued to grow and reproduce in the host vascular bundle tissue,palisade tissue and spongy tissue after 4(dpi)to 5(dpi);The mycelium expanded in the whole tissue and formed acervulus under the epidermal cells after 6(dpi).Then the acervulus produced conidia after breaking through the host cuticle.3.The ultrastructural changes of the host leaves were illustrated.In the early stage of infection,there were accumulation of callose around the infection point and the cell wall of the leaf was obviously thickened,leading to the depression and dissolution of the host cell membrane during the hyphae invasion.After the hyphae invaded the cells,the protoplasm is disorganized and agglutinated,the organelles were basically degraded into fragments.The chloroplast layer was broken,the grana was disintegrated,and there were osmiphilic globules and many granules accumulating in the chloroplast.4.The symptoms and the lesion expansion process of Populus × beijingensis anthracnose were explained.The annual leaves of healthy Populus × beijingensis were inoculated with conidia suspension of C.gloeosporioides.The inoculation site began to turn less green after 3(dpi),resulting in visible brown lesion;The color of leaf lesion deepened after 4(dpi)and the lesion began to spread around the inoculation site leading to small visible white mycelium appeared after 5(dpi);The white mycelium surrounding the inoculation sites were distributed in a halo-like shape and continued to spread after 6(dpi)and the black granules were produced in the center of the lesions.