Verticillium wilt of chrysanthemum: anatomical observations on colonization of roots, stem, and leaves

The morphology and distribution of Verticillium dahliae Kleb. within the ornamental chrysanthemum (Chrysanthemum morifolium Ramat) was determined at intervals after placement of rooted cuttings into infested potting mix. Most infection sites on the root system occurred on roots that developed after potting. Estimates of the number of infection courts per 100 cm of root ranged from 5.6 to 15.8. Root-cortex cells responded to invasion by producing lignitubers and a dark pigment. Microsclerotia had formed within root-cortex cells 8 days after potting. Hyphae, conidia, and conidiophores were observed throughout the vascular system. Hyphal widths ranged from 1.3 to 3.6 μm; conidial dimensions ranged from 1.8 × 3.6 to 2.6 × 5.8 μm. Colonization of vessels of stem and leaves was initially discontinuous and was therefore presumably initiated by conidia. Hyphae in root and stem vessels were generally narrow and there was rarely more than one strand per vessel. Occlusion of the lumen or pore plate of vessel elements in root and stem by hyphae, conidia, gums, gels, or tyloses was not observed. Conidia accumulated in petioles and basal portions of the midrib and lateral veins of leaves, often blocking pore plates. Initially, hyphae in leaf vessel elements were narrow but as wilt symptoms appeared, wide sinuous hyphae predominated, often coursing freely among adjacent vessel elements through pit-pairs and pore plates. Mycelium and conidia appeared to occur sufficiently abundantly in leaf vessels to significantly restrict water flow.