3.7.1S
UNDERSTANDING AND CONTROLLING DISEASES OF WOODY PLANTS: AN HISTORICAL PERSPECTIVE

JN GIBBS

Forestry Commission Research Station, Alice Holt Lodge, Wrecclesham, Farnham, Surrey GU10 4LH, UK

The history of tree pathology provides many examples of ways in which an understanding of disease biology can be crucial to the establishment of successful programmes of tree care and to the production of valuable timber crops. The illustrations here are related to various points in the disease cycle.

Arrival of the pathogen on the site
One of the most dramatic events in the history of tree pathology was the discovery that Heterobasidion annosum, the cause of Fomes root and butt rot of conifers, enters new plantations when air-borne Basidiospores colonise freshly-cut stumps. This led to the development of systems of protective stump treatment using various chemicals - urea, boron etc., and, most famously, the benign wood-rotting basidiomycete Phlebiopsis gigantea.

The inoculum source
The importance of identifying the key source of inoculum is well illustrated by Coffee Berry Disease caused by Colletotrichum coffeanum. Primary inoculum comes from the 'maturing' bark on the branches but often this quickly becomes insignificant in relation to secondary inoculum coming from the first berries to develop disease. For this reason, control measures aimed at reducing primary inoculum are often ineffective.

The infection court
For infection, vascular wilt pathogens normally require freshly exposed xylem wounds. When Ceratocystis fagacearum, the cause of oak wilt, was described in North America in the 1940s, it was already known that in another vascular wilt, Dutch elm disease, the creation and inoculation of the infection court are performed simultaneously by vector bark beetles. However with oak wilt it was discovered that bark beetles are relatively unimportant and that much of the overground transmission occurs when sap-feeding Nitidulid beetles carry the pathogen from sporulating mats on diseased trees to preexisting xylem wounds on healthy trees. With good management the production of infection courts can be minimised, and some of those that are created can be eliminated.

Latent infection
Many tree pathogens can survive endophytically within host tissues for many years, only developing to cause visible symptoms when the host is under stress. This point is well-illustrated by Cryptostroma corticale, the cause of Sooty bark disease of sycamore (Acer pseudoplatanus) in north west Europe, which only causes damage following hot dry summers. Most of the sycamore in southern England already have endophytic infections in the xylem and sanitation felling of visibly diseased trees is unlikely to have a beneficial effect on disease losses.

Host Invasion
Although fungal pathogens are often more invasive on unthrifty hosts than on vigorous ones, there are exceptions. Thus with the pine stem rusts such as the Cronartium species, damage is reported to be exacerbated by activities that improve growth of the host. Is that because in vigorously growing trees, the amount of tissue available for infection is increased or is it because the tissues are more susceptible per se?

Saprophytic survival
Specialised plant pathogens are characterised by a declining phase of growth in the dead host. With knowledge of the characteristics of the pathogen, this process can often be accelerated. The root-rotting Basidiomycete Rigidoporus lignosus is a major cause of mortality in rubber plantations, whether developing from the stumps of old forest trees or from those of a previous rubber crop. It was noted that if a herbaceous leguminous cover crop was planted between the rubber trees, the fungus proliferated dramatically. At first sight this was alarming, but studies showed that a large source of inoculum, with a potential for long-term survival, was being converted into many small ephemeral sources. Thus an opportunity for disease management opened up.