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Rooted stem cuttings of 19 American elm (Ulmus americana L.) cultivars and selections, and rooted cuttings of two non-American elm selections, U. carpinifolia Gleditsch 51 and 970 (U. glabra Huds. x (U. wallichiana Planch. x U. carpinifolia)), along with a group of American elm seedlings, were planted in a randomized block design. When the trees were nine years old, they were inoculated with a mixed spore suspension of Ophiostoma ulmi (Buisman) C. Nannf. and Ophiostoma novo-ulmi Brasier, the causal fungi for Dutch elm disease (DED). Analyses of variance showed highly significant variation among clones in foliar symptoms 4 weeks after inoculation and in crown dieback one and two years after inoculation. After two years, 13 of the American clones showed significantly less dieback than the American elm seedlings, and 18 American clones showed significantly less injury than a randomly chosen, unselected American elm clone, 57845. The American clones with the most DED-tolerance were cultivars ‘Valley Forge,’ ‘Princeton,’ ‘Delaware,’ and ‘New Harmony,’ and selections N3487, R18-2, 290, 190, and GDH. The non-American selections 51 and 970 also exhibited high levels of disease tolerance. Most susceptible were American clones 57845, ‘Augustine,’ Crandall, W590, and the American elm seedlings. The most disease-tolerant American elm selections identified in this study are being evaluated further for possible naming and release to the nursery industry.

Controlled pollinations between five disease-tolerant elm (Ulmus L.) clones (Number 970, ‘Urban’, and clones that were later named ‘Homestead’, ‘Pioneer’, and ‘Prospector’) yielded 686 seedlings. Various crosses produced from zero to over 90 seedlings. Only one of four female parents produced any viable selfed seedlings. At age four, all seedlings were inoculated with Ophiostoma ulmi, (Buism.) C. Nannf., the causal fungus for Dutch elm disease. A factorial analysis showed male parent, female parent, and male x female interaction influenced disease symptoms 4 and 8 weeks after inoculation. After a few years of further evaluation of the seedlings, 10 clones were selected for a combination of disease- and insect-tolerance and horticultural desirability. These clones were propagated and established along with four disease-tolerant cultivars and American elm seedlings in a replicated field plot. Three-year-old clonal plants inoculated with O. ulmi varied significantly in their disease symptoms 4 weeks, 1 year, and 2 years after inoculation. Even clones from the same full-sib family showed significant differences in disease tolerance. Results indicate that both specific and general combining ability are important in determining tolerance to Dutch elm disease.

Ultrastructural effects of Ophiostoma ulmi (Buisman) Nannf..(= Ceratocystis ulmi Buisman C. Moreau) on susceptible American elm (Ulmus americana L.) callus tissue consisted of extensive vacuolation, accumulation of cytolytic substances, and plasmolysis when viewed with electron and light microscopy. Infected resistant American elm callus tissue revealed slight cell wall damage with some phenolic-like deposits but adjacent cells, membranes, mitochondria and nuclei were intact. Fungal hyphae that adhered to callus grown from susceptible American elm plants were regularly observed. Sparse hyphal growth was not suspended above resistant American elm callus cells. Callus from nonhost and resistant elms showed reduced hyphal growth between host cells compared to susceptible elm with increased accumulation of extracellular, phenolic-like deposits than in callus of American elm. The fungal structures on resistant tissue showed evidence of irregular growth and morphology. The latter suggest a phytoalexin-like response by the resistant American elm callus tissue.

We examined the effects of elm selection, explant source and media composition on growth of the Dutch elm disease (DED) fungus Ophiostoma ulmi on callus cultures. Calluses were generated from leaf and stem tissue of an American elm (Ulmus Americana L.) seedling (A), susceptible to the disease; an American elm selection 8630, resistant to the disease; and a Siberian elm (U. pumila L.) seedling, also resistant to DED. Calluses were generated on modified Murashige-Skoog (MMS) medium, either with (MMSC) or without coconut milk. Explant source did not affect the fungal growth rate on the callus. Rate of O. ulmi growth on American elm A callus was similar on both media; on Siberian and 8630, fungal growth rate was more rapid on callus cultured on MMS than on MMSC. However, in the absence of callus tissue, O. ulmi growth on MMSC medium was more than five times as rapid as it was on MMS. We observed significant interactions between explant source and selection, and between medium and selection. Fungal growth was always more rapid on American A, and American 8630 then on Siberian. Scanning electron microscopy revealed heavy fungal sporulation on American A, slight on Siberian and none on American 8630. High performance liquid chromatography analysis showed that the secondary metabolic profiles were distinguishable for callus tissue versus explant tissue, but were similar for calli generated from different explant sources.

We examined growth of the Dutch elm disease fungus, Ophiostoma ulmi, on callus derived from a susceptible American elm (Ulmus amencana, selection A), an American elm of intermediate resistance (U. americana, selection 8630), and a resistant Siberian elm (Ulmus pumlia) at 16, 22, and 28°C (61,72, and 83°F) and inoculation concentrations of 15 × 106, 2 × 106, or 0.3 × 106 conidia/ml. After 72 hours, the rates of fungal growth for all treatments were most rapid on calli from the American 8630 selection followed by the American A and Sibenan selections. While fungal growth was more rapid over American 8630, it was more dense on American A. Most rapid fungal growth occurred at 22°C (72°F) and was directly proportional to the inoculum concentration. A significant interaction was noted between callus source and temperature.

Ramets of nine American elm (Ulmus americana L.) clones or cultivars were planted with ramets of Ulmus ‘Frontier’, Ulmus ‘Prospector’, and American elm seedlings in a randomized block, split-plot design. When they were three years old, the trees were inoculated in the main trunk on either one of two selected dates in May with a spore suspension of Ophiostoma ulmi, the causal fungus for Dutch elm disease (DED). Analyses of variance showed significant variation among clones and between inoculation dates in disease symptoms four weeks and one year after inoculation. Inoculations made on May 18 generally created significantly more symptoms than inoculations made only nine days later. Four-week symptom expression was influenced also by a significant interaction between clonal or seedling group and inoculation date. When data from both inoculation dates were combined, six American elm clones (‘American Liberty’, ‘Princeton’, 680, R18–2, 180, and 3) showed significantly fewer foliar symptoms after four weeks than the American elm seedlings and three other American elm clones. Five of these same six more tolerant American clones averaged significantly less crown dieback after one year than the other American clones or seedlings tested. One of the American elm clones (clone 3) showed a level of disease tolerance equal statistically to ‘Frontier’ and ‘Prospector’, two cultivars which have shown a high degree of tolerance to DED in other studies.