Background
Forest and woodland ecosystems throughout the world are under serious threat from increasing numbers of decline and dieback syndromes. Dieback is the rapid mortality of forests, whereas decline describes reduced overall vigour, usually of the dominant tree species, in the ecosystem. These problems appear to be exacerbated under the present conditions of rapid climatic change. A range of abiotic and biotic factors may be involved in decline and dieback phenomena, but certain pathogens appear to be of great importance. Current models of climate change predict rises in average rainfall and temperatures for Europe, changes that will encourage increased disease problems particularly in poorly adapted ecosystems. Moreover, the latitudinal climatic ranges of pathogens will be altered: increased temperatures will enable survival and activity at higher latitudes than in the past; increased precipitation coupled with higher temperatures further exacerbates this problem. The overall outcome of these changes will be, at best, reductions in sustainable yields from, and amenity values of the affected forests, at worst large areas of forest could decline and dieback, as witnessed with Jarrah dieback in Western Australia.
Species in the Oomyctoa genus Phytophthora are consistently associated with the most significant of these declines.
Jarrah dieback is caused by P. cinnamomi, a particularly virulent and widespread pathogen with a recorded hostrange of over 950 plant species.
P. lateralis emerged in Oregon in the early-mid 20th Century and has totally altered forest productivity in that region, seriously reducing the amounts of Lawson cypress (Port Orford Cedar) timber available to local and international forest industries; this species has also been reported in France.
In 2007, it was confirmed that a newly recorded and devastating needle disease occurring on radiata pine in Chile was caused by a previously unrecognised species of Phytophthora, now named P. pinifolia.
Examples in Europe include:
Ink disease of sweet chestnut caused by P. cambivora and P. cinnamomi
The recently emerging problems of oak decline, where P. quercina is frequently a primary factor, sometimes in combinations with other Phytophthora species.
Riparian alders in northern Europe have been seriously impacted by a disease caused by a novel heteroploid hybrid Phytophthora, P. alni, which has throughout northern and central Europe within less than 10 years.
More recently, beech decline has been associated with a number of invasive Phytophthora species.
Newly recognised threats include, amongst others, the aerial pathogens P. ramorum and P. kernoviae. First characterised in 2000, P. ramorum causes widespread disease on a range of woody plant species, and is particularly noted for the 'Sudden Oak Death' phenomenon in California and Oregon. P. ramorum is commonly intercepted by plant quarantine authorities in European countries and is considered a great threat to European forest ecosystems. P. kernoviae, first recorded in 2003 in the south-west of the UK, causing serious stem cankering and dieback on beech and rhododendron, has since been discovered in native forests of New Zealand. P. kernoviae has been detected in several European locations since the initial recognition in England; the host range includes, apart from rhododendron and beech, oak, Liriodendron, Pieris, Magnolia and Vaccinium.