Histopathological and Ultrastructural Observations of Zanthoxylum armatum Infected with Leaf Rust Causal Agent Coleosporium zanthoxyli

The fungus Coleosporium zanthoxyli is the causal agent of leaf rust in Chinese prickly ash pepper (Zanthoxylum armatum ‘Hanyuan putaoqing’), seriously impacting its industrial development. However, little is currently known about the infection and pathogenesis of C. zanthoxyli on Z. armatum. In this study, the infection of Z. armatum by C. zanthoxyli was reported at histological and cytological levels by a fluorescence microscope and transmission electron microscopy (TEM) for the first time. Fluorescence microscopy with fluorophore Alexa 488 (WGA-FITC) stained samples revealed that the infection process comprised three distinct stages: penetration (0–1 days post inoculation, dpi), parasitic growth (3–5 dpi), and sporulation (≥7 dpi). The number of haustoria increased during the osmotic and parasitic periods and then decreased; the length of hyphae also increased rapidly and then decreased. TEM analysis during these stages demonstrated that as disease severity increased, chloroplasts and mitochondria enlarged significantly, accompanied by a marked accumulation of starch granules and osmiophilic granules. At later stages, the nuclei became irregular, the grana lamellae were blurred, and the lamellar structure was arranged disorderly, and leaf tissues were extensively colonized by fungal hyphae and haustoria, leading to cellular necrosis and distorted cell walls. Notably, the sporulation phase was characterized by dense rust spore clusters covering the leaf surface. These findings provide critical insights into the ultrastructural changes induced by C. zanthoxyli during infection, elucidating key mechanisms of rust-induced damage in Chinese prickly ash and identifying the parasitic phase as a critical window for control strategies. This study lays a foundation for further research on rust pathogenesis and the development of Chinese prickly ash targeted control strategies.