Biochemical and Histological Insights into the Interaction Between the Canker Pathogen Neofusicoccum parvum and Prunus dulcis

The number of reports associated with wood dieback caused by fungi in the Botryosphaeriaceae in numerous perennial crops worldwide has significantly increased in the past years. In this study, we investigated the interactions between the canker pathogen Neofusicoccum parvum and the almond tree host (Prunus dulcis), with an emphasis on varietal resistance and host response at the cell wall biochemical and histological levels. Plant bioassays in a shaded house showed that among the four commonly planted commercial almond cultivars ('Butte', 'Carmel', 'Monterey', and 'Nonpareil'), there was no significant varietal difference with respect to resistance to the pathogen. Gummosis was triggered only by fungal infection, not by wounding. A two-dimensional nuclear magnetic resonance and liquid chromatography determination of cell wall polymers showed that infected almond trees differed significantly in their glycosyl and lignin composition compared with healthy, noninfected trees. Response to fungal infection involved a significant increase in lignin, a decrease in glucans, and an overall enrichment in other carbohydrates with a profile similar to those observed in gums. Histological observations revealed the presence of guaiacyl-rich cell wall reinforcements. Confocal microscopy suggested that N. parvum colonized mainly the lumina of xylem vessels and parenchyma cells, and to a lesser extent the gum ducts. We discuss the relevance of these findings in the context of the compartmentalization of decay in trees model in almond and its potential involvement in the vulnerability of the host toward fungal wood canker diseases.