Histopathology of the endophytic system and aerial shoots of Arceuthobium americanum infected by Colletotrichum gloeosporioides

Infection of the endophytic system of Arceuthobium americanum parasitizing Pinus contorta var. latifolia by the fungus Colletotrichum gloeosporioides was assessed on naturally infected and inoculated localized dwarf mistletoe plants. Two different techniques were utilized to attempt to inoculate C. gloeosporioides into the endophytic system: the fungus was applied as Stabileze formulated product onto cut A. americanum shoots and as a mycelial plug to wounded A. americanum infected host branches. The fungus was cultured from infected shoots and the basal cup region of naturally infected A. americanum, but not pine tissues that included the endophytic system. Following inoculation of cut shoots, the shoots were abscised from the dwarf mistletoe plant, and the fungus was not isolated 1 year after inoculation. The fungus was recovered from the wounded region of infections that were inoculated with a mycelial plug, but not outside of the wounded area. Hyphae with morphology that matched that of C. gloeosporioides were observed microscopically in the shoots and basal cup region of naturally infected A. americanum, but not in the samples from the inoculated treatments. Thus, we were unable to confirm that C. gloeosporioides infected the endophytic system of A. americanum parasitizing P. contorta var. latifolia, although xylem continuity between the aerial and endophytic systems was observed.