Origin of the Dutch elm disease epidemic in Britain.
Previous work by the authors indicated that two culturally distinguishable groups of Ceratocystis ulmi, the fungus causing Dutch elm disease, are present in Britain, one being fluffy and aggressive and the other waxy and non-aggressive; the former is responsible for the current outbreak. Examination of logs of Rock elm (Ulmus thomasii) recently imported from Toronto at Southampton Docks in January 1973 revealed the presence of live eggs, larvae and adults of Hylastes (Hylurgopinus) rufipes Eichh., a North American vector of the fungus, and a strain of C. ulmi that closely resembled the aggressive strain present in Britain. Comparison of the main disease areas in 1971 and the major ports of entry and centres of usage in the 1960's indicated that disease in two of the areas, that on the south coast and that round the Thames estuary, were associated directly with sites of importation and usage and that other disease centres, such as that in the Severn Vale, originated during the transport of imported logs by road or rail to sites for cutting.t<new para>ADDITIONAL ABSTRACT:<new para>Isolates of Ceratocystis ulmi obtained from logs of Rock Elm (Ulmus thomasii) imported into England from N.America were compared with fluffy aggressive and waxy non-aggressive isolates from England. On the basis of growth rate and morphology in culture the isolates of C. ulmi from the N. American logs fell into the fluffy group and closely resembled the aggressive strain present in England. The pattern of outbreaks of the aggressive strain is discussed in relation to ports of entry and centres of usage of Rock Elm, and it is concluded that the current epidemic of Dutch Elm disease in England has probably resulted from the importation of diseased Elm logs carrying aggressive N. American strains of C. ulmi.