A cytoplasmically transmitted disease of Ceratocystis ulmi

Abstract

The massive epidemics of Dutch elm disease occurring across much of the Northern Hemisphere from North America to Europe and south-west Asia are due to the spread of two highly pathogenic forms of Ceratocystis ulmi: the Eurasian (EAN) and the North American (NAN) races of the aggressive strain of the fungus1–3. Most of the mature field elm population in these areas is likely to be destroyed as a result, and unless the pathogenicity of the EAN and NAN attenuates, successive generations of young elms can be expected to suffer a similar fate. Until now the factors that might contribute to any attenuation have remained a matter of speculation3, but I report here the discovery of a disease in the aggressive strain of C. ulmi, transmitted between mycelia by hyphal fusion, which severely reduces the growth and reproductive fitness of the fungus, and may be particularly severe when transmitted between NAN and EAN race isolates. It has features in common with transmissible hypovirulence in Endothia parasitica, the cause of the remission of chestnut blight in Europe. Its possible influence on the present Dutch elm disease epidemics and its potential as a disease control method are discussed.