Dothistroma: Is it ever justified to burn thousands of trees?

DOTHISTROMA septosporum, the causal fungal pathogen of red band needle blight, has been present in Britain since the 1950s at least. The first identification was at a tree nursery in Dorset in 1954. The disease re-occurred sporadically in Dorset until 1966 and popped up on two occasions in Wales in 1958 and 1989. There were no more reports until the 1990s, when Dothistroma took off, with the majority of findings in East Anglia. 

It is now endemic throughout the wider environment of the British Isles. Of the three commercially grown pines, Corsican pine is the most susceptible, followed by lodgepole pine. These Pinus species, like the pathogen, are exotic in origin. Native Scots pine was spared for some time, but finally succumbed and now suffers widespread infection. Dothistroma has managed to get at the very roots of native Scots pine by infecting Scotland’s native Caledonian pine forests in the Cairngorms.

The disease is now present in over a quarter of Scotland’s 84 Caledonian pinewood inventory sites. 

There has been some suggestion that Dothistroma septosporum is actually a native fungus that was resident in Scotland’s native pine forests all along. However, those putting forward the proposition need to explain why the disease first popped up in Dorset and not in the large expanse of Corsican pine planted on the Culbin Sands on the shores of the Moray Firth starting in the 1920s/30s.

DIVERGENCE FOR THE WIDER ENVIRONMENT

Corsican pine being removed manually and clearly a bed of vigorously-growing trees. (Image: FJ)

Dothistroma septosporum is endemic across the country and for practical purposes clearly cannot be classed as a notifiable disease and quarantine pathogen. Foresters and landowners have learned to live with Dothistroma with no intervention from the relevant plant health authorities. But the situation could not be more different for forest tree nursery owners who at any one time will have hundreds of thousands of pine seedlings, mostly Scots pine, in the ground. They are subject to annual inspections for Dothistroma which are carried out in England by plant health officers on behalf of DEFRA. Just a whiff of Dothistroma means the owner is forced to lift, pile up and torch all pine trees in the affected beds and fields, with immediate losses running into the tens of thousands of pounds if not more. 

Why Dothistroma when found in tree nurseries is essentially classed as a ‘notifiable’ disease and treated as though it is a quarantine pathogen is not altogether clear. The Forestry Commission is a bit woolly when it comes to defining the status of Dothistroma septosporum in England, although Scottish Forestry is a lot more explicit and exacting in this respect and says: “Dothistroma septosporum is a ‘regulated non-quarantine pest’ for which statutory control actions are required if found in nurseries (but not in the wider environment).”

There may well be good reasons for such drastic measures, but they are not immediately apparent to me. I can’t understand why a pathogen and disease which is endemic in the wider environment can logically be treated as a quarantine organism when in the nursery. I could understand the caution if it was absent from significant parts of the country and/or not infecting one of more of the UK’s commercial timber-yielding pines. And perhaps even the dire consequences for nurseries found to have the disease, while remembering that absolutely no financial compensation is available.

I have been thinking about this for some time, ever since several Scottish forest nurseries were hit by Dothistroma in 2010 and 2011, and my interest was heightened when Dothistroma septosporum was identified in summer 2023 in beds of Corsican pine on one of the few remaining authentic forest nurseries in England. Plant health officers from the FC had inspected all the Corsican, lodgepole and Scots pine at the nursery, on behalf of DEFRA, amounting to hundreds of thousands of seedling trees.

This was the very first time the disease had been found at this forest nursery during the 40-plus-year tenure of the owner. Leaves (needles) showing symptoms were identified, samples taken and removed to a Forest Research laboratory for further scrutiny and testing, on which hinged the future of thousands of trees.

TAKING A CLOSER LOOK

Lifted trees loaded up to be taken away for burning. (Image: FJ)

I was intrigued by the whole procedure, including inspection, recognition of symptoms and testing as the determinant of any official action taken. In this instance it was the enforced destruction of a not-insubstantial number of trees. The nursery owner kindly supplied me with all the information he could, bearing in mind the UK plant health authorities are inherently unforthcoming on detail unless pressed. It all sounded a bit cut and dried, so I asked the nursery owner to push the authorities for as much detailed information as possible, while doing some research of my own into the procedures followed. 

The inspectors claimed to have found pine seedlings showing symptoms of Dothistroma. On this point I checked some websites in North America where Dothistroma is also a big problem. Despite the symptoms of Dothistroma considered distinct and specific, some sources in the US say symptoms on young pine plants are easily confused with those caused by other needle-cast diseases. It is worth pointing out that discussions on disease within North American forestry are more open, transparent and all-encompassing compared with what generally passes for information here.

Be that as it may, the nursery owner was duly informed that a fruiting body of Dothistroma was observed under magnification. A fruiting body is the reproductive structure produced by a fungus, which contains spores. Concerned that the destruction notice was issued on the basis of this single observation, the nursery owner pushed for clarification. The reply confirmed the fruiting body was produced by an infection in the leaf tissue which had tested positive. However, there was no mention of the type of test, the testing procedure carried out, its specificity or established level of accuracy. Presumably it was a PCR-based test, hopefully with a high degree of specificity and accuracy given that livelihoods are at stake. If I was being forced to torch thousands of trees with an accompanying financial loss of thousands of pounds I would certainly want to be given all this information and more.

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Corsican and lodgepole pine accounted for a relatively small proportion of pines on the nursery. The number of Corsican pine trees affected by the destruction order was not large in the greater scheme of things. Things could have been considerably worse if infection had been found on Scots pine. As such, the owner only pushed the matter so far, but told Forestry Journal in no uncertain terms how he would have asked a lot more questions if his Scots pine stock, amounting to over 100,000 trees, had been affected. This would have caused an immediate financial loss of around £30,000 and continuing losses thereafter, because it would take another four years to grow more stock for sale.

CONFUSING MESSAGES

Red band needle blight aptly describes the foliar disease of pines caused by Dothistroma septosporum. (Image: FJ)

No nursery owner wants to spread disease through sales of planting material, but Dothistroma is everywhere in the UK, which means healthy pine seedlings sold to customers will be exposed to and infected by airborne inoculum soon after planting. That makes such drastic measures for Dothistroma found in forest nurseries difficult to fathom. Indeed, the appearance of Dothistroma on this nursery is not surprising, given there is a pine plantation not too far away which is riddled with the disease. 

Also hard to fathom is the suggestion by Forest Research that forest nursery owners can spray an approved fungicide to manage Dothistroma, although management of the disease is all that such fungicide treatments can achieve. Copper fungicide was the traditional pesticide used to target Dothistroma, but is not curative in action and therefore will not eradicate the disease once plants are infected. Forest Research says that copper fungicides can suppress (mask or hide?) symptoms of disease, which, if true, is clearly not what nurseries want. Given that just one seedling tree found with disease means the destruction of whole beds containing tens of thousands of trees, then to put it politely any suggestion about applying fungicide feels like gaslighting and looks like a complete waste of time and money.

Forest Research says production nurseries must be located away from infected forest stands which can generate large amounts of inoculum and potentially result in the production of infected seedlings. But is this feasible in a small country like the UK with an endemic disease which counts one of the most common and widely spread native trees (Scots pine) among its hosts?

REASONS FOR CAUTION

The plant health authorities may reasonably say they are concerned about the generation of new and more aggressive forms of a fungal pathogen through mutation or hybridisation. Both of these are essentially a function of the volume of inoculum produced. Since the foliar canopy of a single mature pine tree in the wider environment is likely to generate as many spores as hundreds if not thousands of small seedling trees in the forest nursery, such arguments (in this case) don’t appear to add up.

Dothistroma septosporum is not the only fungal pathogen causing red band needle blight disease. It is also caused by the closely related Dothistroma pini fungal pathogen which is widespread in continental Europe, including France, as well as further afield in the US and elsewhere in the world. According to Forest Research, Dothistroma pini is not present in the UK, but I would not like to put money on that given the free trade in pests and diseases which occurred while the UK belonged to the EU and has apparently been ongoing since Brexit. If this is the reason for all the caution, then the solution is simple: place a ban on pine trees coming in from Europe, just as pine imports from North America and most other countries outside of Europe are prohibited. 

This nursery owner has not had cause to import any pine tree planting material from Europe for many years. However, after enquiring on my behalf he was initially told by the Animal and Plant Health Agency that there is nothing to stop him importing Pinus from Europe, but they would strongly advise against it. However, this advice did not fit with recent information put out by the UK plant health authorities following interception of pine processionary moth from the EU in 2022.

Further enquiries solicited a more detailed response from APHA, with the conclusion on our part that Dothistroma does not appear to be a consideration when importing pines from the EU. The reply in totality said:

“I would like to provide some more information regarding the importing of Corsican pine from the EU. I was under the impression that due to interceptions of pine processionary moth on imported material from the EU last year that emergency legislation was brought in to prohibit imports of pine.

New restrictions have been put in place to protect GB biosecurity but importing of pines from the EU is permitted from areas where these specific requirements are met:
• Countries officially confirmed by the National Plant Protection Organisation as free of pine processionary moth; 
• Officially designated pest-free areas; 
• Nurseries where the trees have been 
grown under complete physical protection for their lifetime. 

All EU imports of high-risk material must be pre-notified on the PEACH system and will be subject to an import inspection upon arrival.” 

We took from this statement that Dothistroma is not part of considerations relating to the importation of Corsican pine (and presumably other Pinus species) from the EU. As regards the PEACH system, the nursery owner said the import of oak from Europe comes under the same system, but he was keen to point out that a number of his recent oak consignments were not inspected. 

STILL IN DEMAND

Corsican pine trees undergoing lifting using a tractor-drawn lifting machine. (Image: FJ)

Shortly after lifting, piling and burning the Corsican pine as ordered by DEFRA, the nursery owner received an order for 2,000 Corsican pine trees for planting in a mixed stand on an estate in coastal north Wales. The client was clearly prepared to bear the full cost of planting, because grant money has not been available for planting Corsican pine since the early years of the new millennium,  due to perceived problems created by red band needle blight disease.

However, the estate owner was apparently quite confident about planting Corsican pine given that it was in a mixed stand at generous plant spacing and in a location near the sea to provide excellent all-round aeration within the stand. I recall some two decades ago writing about the work of a postgraduate MSc student at Imperial College London who showed the supreme importance of planting density and stand thinning to maximise aeration and minimise the effects of red band needle bight disease.

Historically, Corsican pine was considered to be a premium timber tree species for UK forestry, outperforming native Scots pine and more recently earmarked as a species that could withstand climate warming, although Dothistroma and its effect on the availability of grant money for planting Corsican pine shot down that proposition as soon as it was floated. 

I would argue that withdrawal of grant money, which essentially sounds the death knell for any tree species thus affected in commercial UK forestry, has done more to damage the prospects of Corsican pine than any direct damage caused by Dothistroma. I have been told by experienced on-the-ground foresters how Corsican pine, even with a dose of Dothistroma, can still out-yield Scots pine. This was first relayed to me by an FC man working in the Surrey Hills who asked me not to quote him lest he get bitten by predators further up the FC food chain. The same message was subsequently relayed by the manager of a big private estate in West Sussex.

Looking at the whole picture I sense something historical in these rules and regulations. Having been done this way since the last century, perhaps no-one can be bothered to look at and reassess the situation. Given that forest nurseries cannot keep taking financial hits like these, perhaps the situation should at least be looked at again.