Forest Research Agency, Alice Holt Lodge, Farnham, Surrey, GU10 4LH, UK

More than 100 species are found within the genera Ophiostoma and Ceratocystis, and many are economically important fungi. Large sums are spent each year on chemical treatments to prevent the discoloration caused by species of these fungi when they colonize freshly processed lumber. Yet others are capable of killing standing trees, causing serious losses to food crops or timber production, and often changing the appearance of the landscape. Some species, particularly those of Ophiostoma, have life cycles intimately linked with insect vectors, often bark beetles, which introduce the fungi into susceptible hosts when they attack trees to feed and breed. Well known examples of the latter include Dutch elm disease, oak wilt, and various root diseases of pine and spruce.

Despite the significance of many of the fungi within these genera, however, there is still considerable confusion about their identity and origins. Only relatively recently has it been revealed that, although morphologically similar, Ceratocystis and Ophiostoma are polyphyletic and an example of convergent evolution. And when some of the more common and widely recognized species have been examined, they have frequently been found to comprise more than one biological species with differing pathogenicity and host specificity. This has resulted in the misidentification of at least one serious pathogen, allowing introduction into areas where it had previously been absent. In addition, modern silvicultural practices and the worldwide trade in timber and forest products often inadvertently bring together novel combinations of tree species, fungi and insect vector populations. With their multitude of spore forms, varied dispersal mechanisms and potential for hybridization, these fungi have the capacity rapidly to exploit new hosts and to evolve in a changing environment.

These issues are illustrated by various examples of tree pathogens: C. fagacearum, the cause of oak wilt, and C. polonica, which causes bark killing and sap stain in Norway spruce; Ophiostoma anamorphs Leptographium wageneri (black stain root disease of pine and Douglas fir), and L. calophylli, a vascular wilt disease of takamaka; also C. coerulescens which causes sap stain of softwood but has been identified as the cause of sap streak of maple. Some of these problems are of long standing, but each demonstrates how the threat these fungi pose is often misjudged, sometimes because of taxonomic confusion about the identity of the fungus, or because the host range of the pathogen and/or its potential to be vectored by other insect species not present in its native environment are not fully recognized.