 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".