Influence of xylem morphology in susceptibility towards mal secco disease in Citrus

In plants, mechanical support and conveyance of water and dissolved material are influenced by xylem morphology. Moreover, the level of susceptibility towards vascular disease was demonstrated to be determined by xylem vessel size and density as in the case of Petri and esca disease, Dutch elm disease and laurel wilt in grape, elm and avocado, respectively. In particular, xylem vessel morphology affects compartmentalization process efficiency and the plant's ability to obstacle pathogen spread within vascular tissues with the secretion of gum and gels. Several citrus species of economic importance (lemon, citron, lime, bergamot, chinotto, sour orange, rough and volkamer lemon) are severely affected by mal secco disease, a tracheomycosis caused by the mitosporic fungus Plenodomus tracheiphilus (Petri) Gruyter, Aveskamp and Verkley (syn. Phoma tracheiphila (Petri) Kantschaveli and Gikashvili). The pathogen penetrates through wounds and colonizes the xylem causing leaf vein chlorosis, leaf shedding and the progressive desiccation of shoots and branches. Being both chemical and agronomical measures not fully effective for the control of mal secco disease and taking into consideration the lack of information on the tolerance mechanisms, this study is aimed at investigating the xylem vessel morphology in 31 citrus genotypes characterized by a different degree of tolerance/susceptibility towards mal secco disease. Xylem vessel diameter (µm) and density (vessel number mm-2) were assessed on one-year-old stems cut into 50-µm sections, observed under optical microscope after staining in Safranine-O. The analysis highlighted a moderate correlation between xylem vessel density and susceptibility to mal secco disease.