Tree diseases are affecting every temperate country in both hemispheres, causing massive economic and environmental damage. We have allowed and are continuing to allow non-native pathogens to spread unchecked, accepting the destruction of our trees on the basis that “nothing can be done”. We may not be able to eradicate tree diseases completely as early opportunities have been lost, but more vigorous direct action can be taken to control them, limiting their impact. Fundamental changes in government policy are a prerequisite for this.
Around the world, Phytophthora ramorumis the reason why the famous oaks of Monterey are dying in their thousands and the USA is losing millions of trees each year to a juggernaut of imported pests and diseases. In France, Ceratocystis plataniis rampant and the planes and poplars lining the historical Canal du Midi are being felled, causing damage which is difficult to comprehend. In Spain, vital food crops such as citrus, olives and nuts are being threatened by evolving Phytophthorastrains. Even isolated and supposedly bio-safe New Zealand is facing a disaster as the kauri trees die (Phytophthora taxon Agathis).
As an island, the UK is one of the last countries in Europe to feel the effects of non-native disease, although we well remember the second phase of Dutch Elm Disease (DED) in the 1960/70s. Over 45 million trees were lost and the elm has never recovered. New elm saplings still spring up but invariably contract DED and succumb at the age of 8-10 years. The lesson is that once a pathogen has established itself, it remains present and active even when the affected trees have been felled. Since DED, and especially in the last decade, new pathogens have arrived and have been allowed to establish themselves. The consequences are now becoming increasingly visible and our failure to learn lessons from DED will have serious consequences.
The impact of Chalara fraxinea(Ash dieback) has been massive and widely reported. Over 120million ash are under threat in the UK from a pathogen which attacks through the leaves. Evidence from Poland and Denmark shows that tree mortality is high – as high as 95% over a period of 10-15years. A few trees may be showing tolerance, raising false hopes of regeneration and encouraging and over-optimistic genomic researchthat has consistently failed to deliver disease resistant strains of elm and sweet chestnut.
Danish forest experts advised that there is nothing we could do to save our ash trees – a view based on the fact that 95% of their ash trees have been infected and will die. The real lesson here is that the Danes didn’t fight to save their trees. Neither did the Poles or any of the European countries where ash has been devastated. We don’t accept that laissez-faire attitude, but the underlying reason was that no effective treatment was available. The loss of these trees will have a major impact on UK woodlands in many parts of the country. In the Lake District, it will scar a uniquely beautiful landscape where over half a million individual ash trees dot the hills and valleys. Some of the UK’s finest and oldest ash specimens are to be found in Langdale, Borrowdale and Ullswater.
Waiting in the wings, but already causing massive damage to olive groves in Italy, Spain and Corsica, is Xylella fastidiosa – possibly the most deadly pathogen of them all. It is spreading northwards and can infect hundreds of carrier plants which show no symptoms. No level of biosecurity will keep Xylellaout. We have to be ready.
The future holds even larger and more costly problems because it is not government policy to counter these threats directly. A large part of the burden will fall on the private owners, large and small, who own over 75% of the UK’s threatened deciduous tree species. Local government will also have a heavy cost to bear and councils all over the country are already felling trees, partly due to disease and partly for budget reasons, at an alarming rate. The evidence emerging from Devon (see next section) is eloquent testimony.
One of the problems we face in the UK is that we have little experience of dealing with widespread tree disease. We also have an illogical aversion to attacking these pathogens head-on with fungicidal and bacterial treatments. The UK has extensive expertise in arboriculture and silviculture and the capacity and skills to adopt new methods and techniques – but anyone who attended one of the many meetings organised in 2012-13 in response to Ash dieback will know we had no credible answers then, and none now.
This is still not a comprehensive list and the environmental issues are significant. On the upside, we have now tested CuPC33 successfully, not only against Chalara fraxinea but also against Phytophthora cinnamomi, Massaria platani, Ophiostoma novo-ulmi, Cryphonectria parasitica, Armillaria mellea andVenturia inaequalis.