In American and European breeding programs, numerous elm trees from many species (Ulmus spp.) and hybrids have been inoculated annually with the fungus Ophiostoma novo-ulmi (the Dutch elm disease pathogen) in screening tests for resistance. Because trees were inoculated at different ages, it appeared necessary to study the influence of host age on the symptoms shown. Four Ulmus minor trees and one U. minor × U. pumila tree were cloned annually from 1994 to 1999. The replicates obtained (usually n = 6) were inoculated on 17 May 2000 with an O. novoulmi strain. At the end of the season, 2-year-old U. minor trees showed average wilting of 8 ± 3% (ES), significantly less than that of 3-year-old trees (34 ± 5%) (P ≤ 0.01). Elms that were 4, 5, 6, and 7 years old showed wilting values of around 50%. A positive relationship between the symptoms shown and the diameter of the elm xylem vessels was observed, and the implications for elm resistance and breeding are discussed. Breeders and pathologists should use trees of the same age and physiological phase when determining the relative resistance among elm clones. It is concluded that under the growing conditions of this experiment, the optimal age for U. minor screening was 4-year-old plants.
Keywords: hydraulic conductance; juvenility; plant resistance.