DEVELOPMENTAL AND ULTRASTRUCTURAL FEATURES OF PHAEOMONIELLA CHLAMYDOSPORA AND PHAEOACREMONIUM ALEOPHILUM IN RELATION TO XYLEM DEGRADATION IN ESCA DISEASE OF THE GRAPEVINE
Résumé
Esca is a devastating disease of grapevines involving xylem-inhabiting fungi. Among these, Phaeomoniella chlamydospora and Phaeoacremonium aleophilum are considered as the causal agents. We show that they present different structural features and act on plant cells by very different mechanisms. Their sensitivity to temperature differed, with optimum growth rates for P. chlamydospora at 25°C, and for P. aleophilum at 30°C. Low temperatures (5°C) applied over a long period (40 days) did not affect the viability of either fungus, whereas high temperatures (35°C for P. chlamydospora and 40°C for P. aleophilum) rapidly suppressed their growth. Alkaline pH, which reduced mycelial growth, increased the formation of conidia in P. chlamydospora, whereas the same effect was observed for acidic pH in P. aleophilum. Unlike P. chlamydospora, P. aleophilum penetrated the lignified walls in host xylem cells degrading the hemicellulose, cellulose and lignin components. However, the deleterious effect of these two fungi cannot explain all the observed wood-degrading processes occurring during esca disease. These agents may facilitate the access of opportunistic saprophytes whose mode of action should now be investigated.