Sapstain and Vascular Pathogens

Joan Webber and Clive Brasier

A large array of species fall within this pathogen grouping, some on conifers, others on hardwoods. All are Ascomycetes-mainly within the genera Ophiostoma and Ceratocystis or related anamorph genera such as Leptographium and Chalara. Morphologically they are superficially alike, but often differ markedly in their pathogenic behavior.

The most aggressive of these pathogens are the vascular wilts. Spread by insect vectors, they invade the vascular systems of standing trees and disrupt water transport (Fig. 1). At the other end of the scale are the weakly pathogenic sapstain fungi. Instead of growing via the xylem vessels (or tracheids), they grow in the rays and axial parenchyma of the wood. Coloration imparted by their pigmented hyphae produces a disorder known as bluestain (Fig. 2). Sapstain fungi are mainly transmitted on a casual basis by a wide range of insects and by tiny arthropods such as mites. They are not, as is sometimes suggested, disseminated as windborne spores.

	Figure 1. Tylosis in elm xylem vessels.
	Figure 2. Typical wedge-shaped areas of discoloration caused by sapstain fungi.

Between these extremes, are a group of pathogens sometimes classified as vascular stains. Often these fungi are associated with bark beetle vectors, and when the beetles mass attack trees or logs to breed, the fungi produce lesions that expand around the entry wound in both the phloem and wood. Although only capable of limited movement in the vascular tissue (compared with vascular wilts), infected trees can show crown wilt symptoms, and the sapwood is often extensively discolored.

Special Characteristics
The microscopic fruiting structures of these fungi produce spores in sticky droplets of fluid (Fig. 3). This makes them suited to insect dispersal, as the spores adhere readily to the surfaces of insects or arthropods that come into contact with the droplets. Some of the vascular wilt and vascular stain pathogens also produce specialized toxins that are carried in the sap steam and disrupt the antifungal defenses of trees.

Figure 3. Stalked fruiting structures, each bearing a whitish droplet of sticky spores.

Vascular Wilts
By far the best known vascular wilt pathogens are the Dutch elm disease (DED) fungi, Ophiostoma ulmi and O. novo-ulmi. They are disseminated by various scolytid elm bark beetles, some of which are highly effective vectors. Spreading across the Northern Hemisphere, the moderately aggressive O. ulmi and the highly aggressive O. novo-ulmi, caused two disease pandemics during the 20th century (Fig. 4), destroying most of the mature elms across Europe, North America and southwest Asia. The precise geographic origin of the pathogens is uncertain but, following the rapid spread of O. ulmi across Europe, DED was introduced into North America, on veneer logs in the 1930s, killing many trees in the highly susceptible elm population. Against this background, the introduction into America of another even more pathogenic DED fungus, O. novo-ulmi, went unnoticed. O. novo-ulmi was then introduced into Europe on elm logs, complete with bark beetle vectors, imported by the timber trade. In Britain alone, it has since caused the death of more than 30 million trees.

Figure 4. Spread of (a) O. ulmi and (b) O. novo-ulmi during the first and second pandemics of DED. O. novo-ulmi exists as two distinct subspecies: the North America (NAN) and the Eurasian (EAN) subspecies.  Click image for an enlarged view.

Other well-known and highly damaging vascular wilt pathogens include oak wilt, caused by Ceratocystis fagacearum, and black stain root disease, caused by Leptographium wageneri. Both insect-vectored pathogens are endemic to North America. Unusual for a vascular wilt, L. wageneri affects conifers and may even be able to invade in the absence of any wound, spreading up the tree from infection points in dead and dying roots (see the paper by Everett Hansen). Both pathogens readily kill trees when they gain access to vascular tissue. In the case of oak wilt, entry is via man-made wounds, while black stain can be introduced by burrowing beetles. European Plant Health Regulations control the importation of plant material and timber that might allow the spread of these pathogens from North America. Until recently it was considered that European oaks were much less susceptible to oak wilt than the America red oaks. However, inoculation tests have shown this is not the case. Thus, the combination of a susceptible host population and a potentially much more effective vector (the oak bark beetle, Scolytus intricatus) could have disastrous consequences should the disease ever become established in Europe.

Vascular Stain Pathogens
In these organisms, the combined action of fungus and insect often leads to a suppression of the host’s resistance mechanisms, death of the phloem, and extensive invasion and staining of the wood (Fig. 5). Some of these fungi are highly pathogenic and associated with specific bark beetles. In North America, mountain pine beetle (Dendroctonus ponderosae) and its associated fungus, O. clavigerum, have a major impact on the productivity of conifer forests. In northern Europe, Ips typographus and the fungus C. polonica cause widespread damage to spruce, while the closely related beetle-pathogen system, I. cembrae and C. laricicola, attacks larch and spruce. However, many species of vascular stain fungi are not associated with a specific vector but with a number of bark beetle genera including O. ips, O. minus, and L. terebrantis.

Figure 5. Lesion produced by a vascular stain fungus in the phloem around the point of entry, a few days after introduction.

The impact of these pathogens is not confined to conifers. Chalara australis and C. fimbriata are examples of aggressive primary pathogens that attack hardwoods. They may be spread directly by insect vectors or when beetle activity liberates frass contaminated with fungal fragments. Chalara. australis causes myrtle wilt in the Nothofagus forests of Australia; it also poses a threat to Nothofagus in South Africa and New Zealand. C. fimbriata attacks a range of tropical species and acts as a vascular stain in some hosts, while causing cankers and stem or root rots in others. Its widespread distribution probably results from multiple introductions: into Malaysia in the early part of the 20th century and more recently into Australia. C. fimbriata var. platani, now the cause of a serious canker stain on Platanus trees in southern Europe, probably came from the United States.

Sapstain Fungi
Sapstain of lumber and processed timber is a major economic problem for the timber industry, even though the stain is largely cosmetic and has little impact on the strength properties of wood. Unseasoned pine is particularly susceptible. The disorder is caused by numerous Ceratocystis and Ophiostoma species, many of which are now ubiquitous. Commonly recorded species include O. piceae, O. piliferum, O.pluriannulatum, O. minus, and C. coerulescens. However, certain species such as O. piceae, which are cultured readily and frequently isolated from stained wood, may mask the existence of more damaging pathogenic species.

Sapstain fungi apparently thrive in wood-processing mills, where old and new logs and timber, sometimes from a variety of tree species, are mixed together. This provides many opportunities for contact and cross-infection. Large populations of insects and micro-arthropods aid this process by acting as casual vectors. Control is achieved through chemical treatments and kiln drying, but this is not always economical or environmentally acceptable. Global trade in timber probably contributes to the widespread distribution of sapstain fungi and has the potential to bring together almost infinite combinations of hosts and pathogens. The situation is further complicated by current confusion over the taxonomy of these fungi. Many of the accepted morphological species comprise several biological species with distinct breeding systems, host ranges, and vectors.

As with other tree pathogens, risk of serious disease often comes about because host trees are exposed to an imported pathogen for which they have evolved little or no disease resistance. In the case of these fungi, the situation is exacerbated where a new, more effective insect vector is available.

International spread of sapstain and vascular pathogens is strongly associated with movement of timber infested with an insect vector. Readily visible vectors such as bark beetles can be intercepted, but controlling casual micro-arthropod vectors poses greater difficulties.

The currently rather archaic taxonomy of these organisms presents an additional threat, because current quarantine protocols tend to rely solely on morphological criteria to identify and define a species.

Previously unknown, damaging tree pathogens exist in uncharted timber-growing regions of the world. One such example is the discovery of another highly aggressive DED fungus, O. himal-ulmi, found in the Himalayas in 1994.

Traditionally, the focus of concern about exotic pathogens is related to the immediate impact of the disease. It does not consider the opportunities for hybridization (horizontal gene-flow) between introduced pathogens that may result in accelerated pathogen evolution and emergence of entirely new diseases. This process is currently occurring between the two introduced DED pathogens (O. ulmi and O. novo-ulmi) leading to the rapid evolution of new forms of O. novo-ulmi.

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