VIEW ARTICLE

Cytology and Histology

Anatomical Response of Resistant Alfalfa Infected with Verticillium albo-atrum. B. W. Pennypacker,Research associate, Department of Agronomy, The Pennsylvania State University, University Park, PA 16802; K. T. Leath, research plant pathologist USDA-ARS, The U.S. Regional Pasture Research Lab, University Park, PA 16802. Phytopathology 83:80-85. Accepted for publication 16 September 1992. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/Phyto-83-80.

Two resistant clones of alfalfa (1079 and WL-5) were stubble inoculated with Verticillium albo-atrum and grown for two 6-wk growth cycles before being used in the histological study. During the third 6-wk growth period following inoculation, regrowth stems were sampled for histological study at weekly intervals for 6 wk. The pathogen was confined to the crown of the plants by vascular occlusions for 2 wk, after which it was present in the stems of clone WL-5. Clone 1079 confined the pathogen to the crown for an additional 2 wk. Histologically the clones responded similarly to V. albo-atrum until week 5. Thereafter, vascular differentiation was disrupted in clone 1079, resulting in the absence of immature, developing vessel elements and the presence of atypically narrow metaxylem vessels in most vascular bundles in the stem. Dissolution of vascular bundles infected with V. albo-atrum was evident by the final week of the growth period. Confinement of V. albo-atrum to the crown until late in the growth period appeared to account for resistance in clone 1079. An additional resistance response was noted in stems of clone WL-5. The response consisted of hypertrophied xylem-parenchyma cells surrounding groups of infected vessel elements, eventually crushing and obliterating them. The hypertrophied cells frequently tested positive for suberin. Atypically narrow xylem-vessel elements were confined to infected vascular bundles in clone WL-5, and no vascular dissolution occurred. V. albo-atrum in the xylem vessels of both clones was frequently encased in a thick material that tested positive for suberin and lignin.