Histopathologic characterization of the process of Marssonina brunnea infection in poplar leaves

Abstract

Marssonina brunnea (Ellis & Everh.) Magnus, the causative pathogen of Marssonina leaf spot of poplars (MLSP), can lead to complete defoliation and tree death. Although MLSP has been studied for over 30 years, its precise process of infection is currently unclear. In this study, we present the process of M. brunnea infection in detail using several types of microscopy. When the conidia came into contact with the poplar leaves, they developed germ tubes to attach to the leaf surface. During the first 2 days post inoculation (dpi), infection vesicles (IV) and primary hyphae (PH) formed within host cells. The plasma membranes of the host cells penetrated by IV remained unbroken and intact organelles were visible, indicating that the IV did not kill the host cell. At 3 dpi, secondary hyphae (SH) began to appear within and outside the host cells. The ultrastructural evidence indicated that the SH could kill the host cells rapidly, producing black spots on the surfaces of the leaves. These results collectively suggested that M. brunnea is a typical hemibiotrophic fungus. In addition, M. brunnea could develop intercellular infective hyphae (IH) for expansion. Our findings also confirmed that the germ tubes, IV, and SH are crucial structures for the disease interactions and showed that the two formae speciales of M. brunnea share the same infection histopathological features. This provides important insight for further research into M. brunnea.