 Woodland trees in North America are not quite as susceptible to the disease because they usually lack the root-grafting of the urban elms and are somewhat more isolated from each other.In France, inoculation with the fungus of over three hundred clones of the European species failed to find a single variety possessed of any significant resistance.The first, less aggressive strain of the disease fungus, "Ophiostoma ulmi", arrived in Europe from Asia in 1910, and was accidentally introduced to North America in 1928.