Highly fertile form of the aggressive strain of Ceratocystis ulmi

Abstract

THE ability to recognise the aggressive and non-aggressive strains of Ceratocystis ulmi in culture1 has enabled us to survey the C. ulmi population in the field and thereby monitor any variation that might indicate a change in the course of the present epidemic of Dutch elm disease in Britain. When sampling from diseased elm twigs in the outbreak areas, we have found that while most wild isolates can be assigned to the two strains, there also exist in low frequency (about 2%) isolates with some affinity to the ‘fluffy’ aggressive strain but which produce small black-brown sclerotium-like bodies throughout the culture and are dark centred because of the presence of brown pigment. We show here that these isolates are a highly fertile form of the aggressive strain, with a special function in the perithecial, or sexual stage of the fungus.