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Histopathology of Callus and Germinating Seeds of Arceuthobium tsugense subsp. tsugense Infected by Cylindrocarpon cylindroides and Colletotrichum gloeosporioides

Two fungi that parasitize western hemlock dwarf mistletoe (Arceuthobium tsugense subsp. tsugense), Cylindrocarpon cylindroides and Colletotrichum gloeosporioides, were evaluated for their virulence on germinating seeds and callus grown in vitro. Mistletoe seeds were germinating on Harvey’s tissue culture medium in one‐half of a petri plate, while the other half contained water agar on which fungal growth was initiated from a mycelial plug. Callus tissue was initiated on Harvey’s medium or modified White’s medium and challenged with fungi on Harvey’s medium (C. cylindroides) or modified White’s medium (C. gloeosporioides) because fungal growth rates were found to be moderate on these media. At 0.5, 1, 2, 3, and 7 d postcontact with fungi, mistletoe tissues were investigated by light microscopy. In seeds, both endosperm and radicle were colonized, and cushion development, cell wall degradation, and intercellular and intracellular colonization were evident with both fungi. Both fungi inhibited callus growth, degraded cell wall, and colonized inter‐ and intracellular spaces. Cells infected with C. cylindroides were disorganized and plasmolyzed. The in vitro screening method developed in this study was useful to elucidated host‐pathogen interactions and showed that C. cylindroides was more virulent in colonizing dwarf mistletoe tissues than C. gloeosporioides.