ObjectiveThe present experiment was conducted to study the infection process of poplar leaves by Colletotrichum gloeosporioides and the inoculated leaf reaction, providing scientific basis for further research on pathologic mechanism, as well as molecular breeding for disease resistance of poplar.

MethodThe healthy poplar leaves were inoculated with conidia suspension of C. gloeosporioides green-fluorescent-protein-marked strain BH12-2. The infection process and defense response between pathogen and poplar leaves were observed by optical and electron microscopies.

ResultThe results showed that condium began to germinate and form germ tube at 4 hour post-inoculation (hpi). The germ tube produced appressorium after 8 hpi. The mature appressorium formed infection peg after 12 hpi. A germ tube and appressorium formed at the other top of the germinal conidium after 24 hpi. Germ tubes of abundant branches differentiated into hyphae and produced secondary conidia after 48 hpi. The infection peg formed in the base of appressorium, swelled to form an infection vesicle after penetrating the host cuticle and epidermal cell wall after 3 days post-inoculation (dpi). The infection vesicle initially grew between the host cell wall and cell membrane, without penetrating the protoplast of the host, and subsequently produced primary hyphae and secondary hyphae. The secondary hyphae rapidly expanded in the host epidermal and mesophyll tissues within 4-5 dpi, gathered in the subcuticular to form stroma tissue in 6 dpi, from which conidiophore and conidia were then developed.With mycelial extension in host tissues, a series of alterations occurred in host tissues, including callose formation around the cell wall of mesophyll cells at the penetration point, downward deformation and dissolution of cell wall, degeneration of cytoplasm, disintegration of organelles such as chloroplasts and collapse of host cells, resulting in appearance of typical brown necrotic spots on the infected leaves.

ConclusionIn the infection process of C. gloeosporioides, conidium can germinate and form multiple germs and appressoria to increase the probability of successful infection. C. gloeosporioides adopts the intracellular hemibiotrophic infection strategy for colonization in poplar leaves.