Naturally occurring non cerato-ulmin producing mutants of Ophiostoma novo-ulmi are pathogenic but lack aerial mycelium

Cerato-ulmin is a protein implicated as a major toxin in the development of Dutch elm disease symptoms in elms infected with Ophiostoma novo-ulmi. O. novo-ulmi isolates typically produce fibrous-striate aerial mycelium on malt extract agar and secrete high levels of cerato-ulmin in liquid medium. However, two different genotypes of O. novo-ulmi (NAN race) from a population sample in Portugal, isolates MAFf8 and PG470, exhibited unusual flat-waxy 'non-aerial mycelial' colony types and were found to produce no detectable cerato-ulmin. The two isolates otherwise behaved as normal 0. novo-ulmi isolates, including being highly pathogenic to Ulmus procera. When MAFf8 and PG470 were crossed with wild-type NAN O. novo-ulmi isolates, non-aerial mycelial colony phenotype and non cerato-ulmin production was shown in each case to be controlled by a single pleiotropic mutation, termed cu−. A further cross showed the cu− locus was probably allelic in the two isolates. The influence of the cu− locus on colony phenotypes in O. novo-ulmi supports the view that cerato-ulmin is a fungal hydrophobin. The normal pathogenic ability of the two cu- isolates raises questions about the proposed role of cerato-ulmin as a wilt toxin