Ultrastructure of the early stages of Colletotrichum acutatum infection of strawberry tissues

Abstract

The early stages of the infection of attached leaves and petioles of strawberry (Fragaria ×ananassa Duch. 'Camarosa') by Colletotrichum acutatum Simmonds were studied using scanning and transmission electron microscopy. Pre-penetration events of these tissues were similar, but the production of secondary conidia (microcyclic conidiation) was detected only on leaves. At the ultrastructural level, different stages of maturation of appressoria were observed and described. In young appressoria, the cell wall was composed of two layers and the plasma membrane displayed a wavy appearance. In the following stage, the appressorium developed a third electron-transparent layer between the cell wall and the plasma membrane. This new electron-transparent material was especially visible in the region of the appressorium near the cuticle. The plasma membrane of this appressorium showed a smooth appearance. Afterwards, a penetration peg emerged through the pore penetrating the cuticle and reached the epidermal wall where it enlarged to form an intramural infection vesicle. Both structures of infection, the penetration peg and the intramural infection vesicle, produced during the early phases of infection of strawbery tissues by C. acutatum, have not been previously reported and confirm that its invasion strategy is that of a subcuticular intramural pathogen. Once the infection was well established, abundant subcuticular and intramural hyphae were produced on petioles, causing severe degradation of the host cell walls. Occasionally, the cuticle appeared disrupted in those regions where the host walls were very degraded and dilated. Differences between colonization of petioles and leaves were observed.Key words: Colletotrichum acutatum, Fragaria ×ananassa, infection vesicle, invasion strategy, penetration peg.