Evaluation location: Utrecht, Netherlands
Two-year-old nursery grown callus cuttings of clone Belgica, which is susceptible to all strains of Ophiostoma ulmi, and of clone 390, which is resistant to the non-aggressive, but susceptible to the aggressive strains of O. ulmi (Elgersma, 1976), were inoculated with the aggressive strain H6 or with the non- aggressive strain E2 of O. ulmi with the techniques described earlier (Elgersma, 1969). The inoculum contained 10^6 conidia per ml. On the 3rd and 5th day after inoculation, 2 cm-stem pieces were cut 5 cm above the site of inoculation, using 10 plants in each combination at a time, which were fixed in formalin-ethanol (70%)-propionic acid (5:90:5) and sectioned transversely at 20 um thickness. The total number of vessels in the latest developed annual ring was counted and the proportion containing tyloses determined as described earlier (Elgersma, 1973).