Objective The study aimed to determine the histopathological process of the pathogen and investigate the physiological and metabolic response mechanisms of the host during the specific infection of Gymnosporangium yamadae on apple (Malus domestica) leaves. Additionally, the study aimed to investigate the pathological mechanisms underlying the host-specific selection of the rust and lay the groundwork for further research on the pathogenetic molecular mechanisms of G. yamadae.

Method After artificially inoculating apple leaves with G. yamadae basidiospores, the infection structures of the pathogen and the cytology changes of the host were observed continuously using microtechnic; the contents of hydrogen peroxide, peroxidase (POD) and total phenol of infected apple leaves showing chlorotic flecks and at the pycnium and aecium stages were determined using TU-1810 UV-visible spectrophotometer.

Result G. yamadae basidiospores germinated and produced germ tubes after 6 hpi (hours past inoculation) and directly entered into apple leaves epidermal cells at 12 hpi, intercellular mycelia formed at 24 hpi and secondary mycelia with diaphragm were observed at 72 hpi. After 5 days, intercellular mycelia or haustorial mother cells entered into host cells, resulting in the formation of monokaryotic haustoria. After 10 dpi, the pycnium and pycniosporophores were observed on the surface of apple leaves and the aecium and aeciospores were formed on the undersides of apple leaves at 60 d. The morphological structure of host cells did not visually change before infected leaves showing chlorotic flecks. However, the cytoplasmic staining of the host cells lightened, and the morphology of organelles distorted when the host leaves showing chlorotic flecks (numbers haustorium were formed and developed). In the spermogonial stage, host cell membranes and organelles become ablation. During the formation of aeciospores, host cells began to necrotic. The content of hydrogen peroxide decreased first and then increased slightly, while the activity of POD and the content of total phenol showed an increasing trend of different amplitude in infected apple leaves.

Conclusion The basidiospores of G. yamadae germinated to produce germ tubes and appressorium directly entered the host epidermal cells (0-5 days post inoculation, dpi). The number of monokaryotic haustoria were formed by intercellular mycelia or haustorial mother cells to establish the biotrophic parasitic relationship with the host (5-10 dpi). Finally, the rust further developed to produce the spermatia and aeciospores (after 10 dpi). Susceptible reactions of the host cells began with the haustorium entering mesophyll cells, and necrosis of the host cells occurred in the aecial stage. Furthermore, the infection of G. yamadae resulted in the considerable accumulation of phenols in apple leaves, which might play a crucial role in maintaining low reactive oxygen species in the host.