Histopathology of Verticillium dahliae within mature roots of Russet Burbank potatoes

Roots of Russet Burbank potatoes (Solanum tuberosum L.) inoculated with Verticillium dahliae Kleb. were examined, primarily with the electron microscope. Penetration hyphae entered epidermal cells directly, apparently aided by hydrolytic enzymes. In most instances, penetration took place without eliciting any structural response. Most hyphae failed to penetrate deeper than the epidermal layer, owing primarily to the formation by exodermal cells of lignitubers that ensheathed the penetration hyphae. Apparently lignitubers were sometimes initiated as callose-like appositions opposite encounter sites. All cortical layers were capable of lignituber production. Few hyphae were encountered within cells deeper than the second cortical layer, and intercellular colonization was not extensive. Nonetheless, vascular infection of unwounded roots occurred. Wilt symptoms were first noted 2 days after hyphae were found in vessel elements of the root. Walls and pit membranes of colonized tracheary elements were coated with an electron-dense substance. As the disease progressed, hyphae invaded all tissues in the vascular cylinder and eventually grew back out into the cortex. The soft-walled tissues were destroyed. In wounded roots many severed vessels contained membranous remnants, conidia, and hyphae that tended to be accumulated at the perforation plates.