Evaluation location: Maryland, United States
One year old trees were planted into a field plot at Glenn Dale, MD, in 1989 and 1990, in a randomized-block, split-plot design with 7 blocks, and, depending on the number of trees available, four trees per clone in each whole plot within a block. Half the trees in each whole plot were inoculated May 18, 1992; the other half were inoculated on May 27, 1992 (sub-plot treatment). Inoculations on each date were made into a 2.4 mm (0.1 in.) hole in the bottom one-third of the main trunk of each tree with 0.11 mL (0.003 fl oz) of an aqueous spore suspension containing 3 × 10^6 spores/mL (1 × 10^8 spores/fl oz) of a mixture of two isolates each of O. novo-ulmi and O. ulmi. This inoculation was intentionally severe in order to induce symptom expression on even the most disease-tolerant clones. The percentage of the crown's branches showing a lack of foliage, or dieback, was estimated one, two and seven years after inoculation. Percent survival was also assessed seven years after inoculation; trees that died back to below 1 m (1.1 yd) in height were considered dead, even if stump sprouts eventually arose from the base of the tree.