Histological Changes in Mango Seedlings Following Infection with Ceratocystis manginecans, the Cause of Mango Decline

Ceratocystis manginecans-induced wilt and decline of mango has devastated the mango industry in Oman during the last decade. The histological changes in mango seedlings following inoculation with the fungus were investigated. Twelve-month-old mango seedlings were artificially inoculated with C. manginecans, and development of the disease was recorded weekly for up to six weeks. Inoculated mango seedlings developed typical wilt symptoms within one week and produced gummosis in the inoculated areas. Weekly assessment of upward and downward movement of C. manginecans in the wood showed that the pathogen moved at 6.3 and 6.1mm per day, respectively, with no significant differences in the rate of tissue colonization in opposite directions. Internal woody tissues of inoculated mango seedlings developed brown to dark brown discolouration. Discolouration of mango tissues was congruent or just behind the advancing hyphae of C. manginecans. Although there were no significant differences in the rate of internal discolouration in opposite directions from the point of inoculation, severity of wood discolouration was significantly higher above the area of inoculation compared to the area below inoculation. Tissues above and below the inoculation point were also examined microscopically. Tissues of inoculated mango seedlings were darkened, implying excessive production of phenolic compounds and gums as a defence mechanism following infection. In addition, tyloses and fungal mycelium were observed in the xylem of sections of the inoculated seedlings. This implies tyloses, mycelium movement in the vascular system and tissue discolouration as mechanisms responsible for wilt and death of infected mango trees.