ABSTRACT

Brasier, C.M. and J.N. Gibbs. 1975. Highly fertile form of the aggressive strain of Ceratocystis ulmi. Nature 257(5522): 128-131.

The ability to recognize the aggressive and non-aggressive strains of Ceratocystis ulmi in culture 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.

The occurrence of the protoperithecial form of the aggressive strain, and the phenomenon of pseudoselfing, reveal previously unknown properties in C. ulmi with considerable ecological implications. Perithecia occur naturally in beetle breeding galleries within the bark of diseased elms and the ascospores, together with asexual spores, are thought to be carried by the newly emerging beetles in the spring. Until now it has been assumed that for perithecial formation the absence of an aggressive A type in Britain would mean that the aggressive strain would have to hybridise with the non-aggressive strain, leading to a decline in pathogenicity. It is now clear that this is not the case and that an aggressive B type could undergo pseudoselfing to produce two progeny genotypes, one the fluffy B form, close to the parent itself, and the other the highly fertile proto A form.