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Response of Elm Species and Clones to Inoculation with Verticillium albo-atrum. K. J. Rauscher, Research Assistant, University of Wisconsin, Madison 53706; D. T. Lester(2), and E. B. Smalley(3). (2)(3)Associate Professor of Forestry, and Professor of Plant Pathology, respectively, University of Wisconsin, Madison 53706. Phytopathology 64:702-705. Accepted for publication 3 December 1973. DOI: 10.1094/Phyto-64-702.

Seedlings of three elm species, and two clones from each of five different elm species or hybrid families, were inoculated with Verticillium albo-atrum. Response was measured as reduction in stem elongation. For seedlings, growth reduction was greatest in American elm and least in Siberian elm. Patterns of response to different concns of inoculum were curvilinear, with apparent threshold values at about 2 × 104 propagules/ml for American elm and 2 × 105 propagules/ml for Siberian elm. In most treatments, one or more seedlings grew as rapidly as control seedlings. The possibility that absence of growth reduction following inoculation has a genetic basis is being investigated. For clones, there was wide variation in the amount by which stem elongation was reduced. One pair of sibling hybrid clones showed little reduction but most variation was among clones irrespective of genetic similarity. Another experiment involving four clones inoculated and grown under three watering regimes substantiated clonal variation in response to the pathogen and illustrated additive effects of reduced moisture availability and the pathogen on stem elongation.

Additional keywords: host variation, wilt disease, disease resistance.