Evaluation location: Gelderland, Netherlands
The experiments were conducted in research nurseries at Alice Holt (England), Wageningen (the Netherlands) and Ames (USA). The chief inoculation technique was the same as that used in the breeding programme in the Netherlands and employed in the earlier experiments of Gibbs et al. (1972) and Gibbs and Brasier (1973). A spore suspension was prepared for each isolate in mid-June 1973 by shaking pieces of mycelium in 10 ml of Tchernoff medium for 2 days (Tchernoff 1965). An incision was made in the main stem of each tree with a knife to expose the wood vessels, and 3 drops of the spore suspension were allowed to enter the cut. In part of the work at Ames an equal number of trees were also inoculated by the 'pinprick' technique. This involved placing a drop of spore suspension on a 2-year-old branch and pricking into the wood through the drop with a needle until the spore suspension was drawn in. The inoculum averaged 5 X 10^5 spores per tree. In all the experiments, the isolates were applied in randomized blocks with single-tree plots, the number of replications varying with the number of trees available. Because some of the isolates were exotic to the countries in which the experiments were being conducted, all inoculated trees received regular sprays of insecticide (methoxychilor in Ames, lindane at Alice Holt and Wageningen). Disease was assessed as the percentage of the crown showing leaf symptoms at 8 weeks after inoculation. Each accession was inoculated with either four or five isolates (aggressive and non-aggressive strains) separately and the data reported here are the average of all isolates.