VIEW ARTICLE

Physiology and Biochemistry

Growth and Physiological Response of Resistant Alfalfa Clones Infected with Verticillium albo-atrum. B. W. Pennypacker, Doctoral candidate, Department of Plant Pathology, The Pennsylvania State University; K. T. Leath(2), and R. R. Hill, Jr.(3). (2)(3)Research plant pathologist, and research agronomist, USDA-ARS, The U.S. Regional Pasture Research Lab, University Park, PA 16802. Phytopathology 80:1247-1253. Accepted for publication 17 May 1990. 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, 1990. DOI: 10.1094/Phyto-80-1247.

Verticillium albo-atrum previously was reported to reduce the growth and flowering of resistant alfalfa cultivars. However, attempts to determine whether the pathogen directly affected the growth of individual resistant plants were thwarted by the fact that each alfalfa cultivar is a heterogeneous population of plants encompassing all levels of resistance to V. albo-atrum. Inoculation of an alfalfa cultivar and the subsequent death of susceptible plants resulted in the emergence of an inoculated-resistant subpopulation?a subpopulation that could not be duplicated in the absence of the pathogen. All growth measurements were taken on the resistant subpopulation, and the absence of an identical uninoculated subpopulation prevented researchers from unequivocally concluding that V. albo-atrum affected the growth of resistant plants. The present experiments were conducted on two resistant alfalfa clones to resolve this point. After a 6-wk establishment period, half the clones were inoculated with V. albo-atrum by placing a 20-?l drop of inoculum (3.65 ? 10⁶ spores/ml) on each freshly cut stem stub, and half were treated with sterile water. Growth and physiological parameters were measured 12 wk after inoculation and then weekly for an additional 6 wks. The experiment was repeated, and the data pooled. Verticillium albo-atrum caused significant reduction in height, percentage of plants flowering, and stem, leaf, and aerial biomass. Stomatal conductance of infected plants was significantly reduced during the afternoons of the last growth period. The significant reductions in these parameters are evidence of a host-pathogen interaction. Thus, the previously reported reduction in growth and flowering in alfalfa cultivars infected with V. albo-atrum was due, in part, to a host-pathogen interaction.