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. It was steadily weakened by viruses in Europe and had all but disappeared by the 1940s. However, the disease had a much greater and long-lasting impact in North America, owing to the greater susceptibility of the American elm, "Ulmus americana", which masked the emergence of the second, far more virulent strain of the disease "Ophiostoma novo-ulmi". It appeared in the United States sometime in the 1940s, and was originally believed to be a mutation of "O. ulmi". Limited gene flow from "O. ulmi" to "O. novo-ulmi" was probably responsible for the creation of the North American subspecies "O. novo-ulmi" subsp. "americana". It was first recognized in Britain in the early 1970s, believed to have been introduced via a cargo of Canadian rock elm destined for the boatbuilding industry, and rapidly eradicated most of the mature elms from western Europe. A second subspecies, "O. novo-ulmi" subsp. "novo-ulmi", caused similar devastation in Eastern Europe and Central Asia. It is now believed that it was this subspecies which was introduced to North America and, like "O. ulmi", probably originated in Asia. The two subspecies have now hybridized in Europe where their ranges have overlapped. The hypothesis that "O. novo-ulmi" arose from a hybrid of the original "O. ulmi" and another strain endemic to the Himalaya, "Ophiostoma himal-ulmi" is now discredited.