Dothistroma septosporum – native or exotic?

DOTHISTROMA septosporum (Dothistroma needle blight) is the fungal pathogen which put paid to Corsican pine (Pinus nigra subsp. laricio) as a commercial timber tree and now threatens the last vestiges of native Scots pine wildwood of the Caledonian Forest. This fungus has traditionally been regarded as an exotic pathogen, so a suggestion by the Forestry Commission (FC) that it could be native certainly deserves consideration.

D. septosporum is a parasite and as such requires a plant host. If truly native then the only realistic plant host is indigenous Pinus sylvestris (Scots pine). How long the fungus could have existed in Britain is anyone’s guess although we know that Pinus sylvestris crossed the land bridge and colonised post-glacial Britain circa 8500 BC (Rackham, 1976).

Novel views on nativity are apparently connected to research in the Caledonian Forest to determine whether D. septosporum co-evolved with Scots pine (Perry et al., 2016). The authors say there is evidence to suggest evolution of a low-disease-susceptible Scots pine, possibly in response to the presence of high historical levels of D. septosporum. This implies Dothistroma has been a part of the native Caledonian pinewoods ecosystem for a significant period of time.

Simply put, co-evolution is the process by which two species evolve together through exertion of reciprocal genetic selection pressures on each other. The result by definition is mutually beneficial, with Scots pine resilient to the pathogen and the fungus facilitated to persist in the parasitic mode. It is in a parasite’s best interest not to kill the host plant.

As the last authentic remnants of Britain’s pine wildwood, the Caledonian Forest in Scotland is a logical and likely place for a native pathogen to hole up. Perry et al. (2016) used ‘endemic’ to describe its status in the Caledonian Forest, but did not go as far as saying they believed the fungal pathogen to be native (to the British Isles). By implication, Dothistroma septosporum is exotic (non-indigenous) even though it may have been present in the British Isles for a long period of time.

Dothistroma needle blight can seek out and infect the highly susceptible Corsican pine wherever it is grown. Clumps of Corsican pine in the Chiltern beech woods of Oxfordshire are already riddled with Dothistroma needle blight.

DOTHISTROMA IN THE UK – ESTABLISHED FACTS

The pathogen was first identified on Corsican pine in a Dorset nursery in 1954. Subsequent findings throughout the 1950s and 1960s were confined to nursery stock with the exception of one in a single forest stand in Wales in 1958. There then appears to be a gap in documented history until the late 1990s, by which time the disease was firmly established in Thetford Forest (Suffolk and Norfolk) with the highest concentration of Corsican pine in the country. Corsican pine is an exotic conifer and now naturalised but without any worthwhile disease resistance and/or tolerance to Dothistroma, and is clearly the key to progress of this pathogen and disease in UK.

Corsican pine was introduced into the UK in the 18th century with planting already underway by the 19th century (Mabbett, 2016). By the 1950s there were 16,000 ha in total with extensive planting by the FC in places such as Thetford Chase in the East Anglian Breckland and the Morayshire Culbin Sands in eastern Scotland. By 1956, the FC was planting over 8 million Corsican pine trees every year (Edlin, 1956).

This begs the question as to why Dothistroma septosporum, if native, took so long (1850–1990) to infect Corsican pine. And if endemic in the Caledonian Forest, why did it not first appear on Corsican pine plantations in nearby Morayshire rather than in a nursery 500 miles away in Dorset? With first and subsequent outbreaks in plant nurseries, indications are of multiple introductions on imported planting material.

At the same time the disease was popping up all over the southern hemisphere, mainly in plantations of Pinus radiata (radiata pine), another highly susceptible Pinus species. Outbreaks were recorded in Tanzania (1957), Kenya (1964), South Africa (1960s), Chile (1965) and New Zealand (1967), suggesting the fungal pathogen could have arisen from a common source, perhaps in Asia (CABI, 2019).

An almost completely defoliated stand of Corsican pine in East Anglia, pictured here as early as 2008 (picture: Forestry Commission). (Image: FJ)

UK PROVENANCE IS A RED HERRING

When and how Dothistroma septosporum arrived in the UK is irrelevant in relation to the damage caused to Corsican pine and increasingly now to Scots pine. More to the point is how it was allowed to progress in the first place, despite almost 50 years of disease management planning available from the first finding in 1954 until the disease got out of control.

Dothistroma is susceptible to sprays of protectant copper fungicide. As the description ‘protectant’ implies, the fungicide deposit and residue remain on the pine needles to protect the foliar canopies by killing the fungal spores as they germinate, and before the germ tube can penetrate into the pine needle (leaf) to establish an infection. Copper fungicide also prevents production and release of spores by the fungal fruiting bodies.

An unthinned 30-year-old stand of Corsican pine on Forestry Commission land in Suffolk with severe needle blight and needle drop caused by Dothistroma. The disease situation has been aggravated and heightened over the years by a lack of air circulation in the stand.

Pine plantations in southern hemisphere countries like New Zealand and Chile are aerially sprayed with copper fungicide on a routine basis. This has allowed commercial forestry to live with Dothistroma needle blight disease and successfully produce radiata pine timber, although like Corsican pine, radiata pine is highly susceptible to Dothistroma (Bulman et al. 2016).

In contrast, the UK continues to rely on what the FC calls good stand management, including stand thinning to improve air flow and reduce inoculum load, and falling back on other softwood species, having given up on Corsican pine. Good air flow to minimise disease development and spread is an essential cultural control whether or not fungicide spraying is used, though clearly insufficient when used as a stand-alone measure.

Dothistroma continued to spread and worsen throughout the 1990s and into the new millennium. By 2006, the FC had imposed a moratorium on planting Corsican pine on the public forest estate and removed grant aid from Corsican pine plantings on private estates.

Forestry Journal spoke to a leading forest nurseryman who saw his sales of Corsican pine start to collapse and then fall off the cliff in the 1990s from 150,000/year sales during the late1980s. He recalls how he and other nursery owners lobbied the FC for compensation to cover lost stock and sales although nothing was forthcoming. Instead of playing an important role in the production of timber and the sequestration of carbon dioxide, countless Corsican pine nursery seedlings were piled high onto bonfires to supplement atmospheric carbon levels.

Lodgepole pine growing in Scotland suffering from a severe dose of Dothistroma.

Since the late 1990s, Dothistroma has spread its wings to infect other Pinus species. First was lodgepole pine (Pinus contorta), followed by Scots pine, despite this native Pinus species being previously classed as resistant to D. septosporum. Scots pine and lodgepole pine remain as important commercial conifers in UK forestry with 218,000 hectares and 100,000 hectares respectively, according to FC Forestry Statistics 2019.

Causing most current concern is Dothistroma infection of the Caledonian Forest’s Scots pine trees, which are directly descended from the first pines to arrive in Scotland following the late glacial period. Caledonian Forest is the name given to the former (ancient old-growth) temperate rainforest of Scotland. Edlin (1956) said these few relic Scots pine had survived by virtue of their remoteness. Virtually all accessible pine timber had been logged out in the 18th and 19th centuries and floated across lochs and down rivers. The bulk of worthwhile stems were removed, leaving rugged veterans too crooked or otherwise defective to attract commercial foresters and sawmillers, he said.

Aerial spraying trials against Dothistroma needle blight carried out by FC Scotland in 2013 (picture: Forestry Commission).

Over a half-century later, FC Scotland was sufficiently concerned to carry out aerial spraying trials using copper fungicide for Dothistroma control. Helicopter spraying trials began in 2013 in Monaughty Forest near Elgin in Moray. The choice of copper oxychloride as the fungicide was strange since most commercial fungicide spraying in the southern hemisphere uses cuprous oxide and for very good reasons. The fungicidally active principle of a copper fungicide is the divalent copper ion (Cu2+). On a gram-for-gram basis, cuprous oxide (Cu2O) contains 88 per cent of active copper while copper oxychloride (Cu2(OH)3Cl) contains only 55 per cent. Therefore, on a weight-for-weight basis cuprous oxide has a significantly higher disease control potential, which translates into less fungicide formulation required and sprayed into the environment to achieve an equivalent level of control (Mabbett, 2013).

FC Scotland briefly reported the trials but with no mention of fungicide efficacy and disease control (Tubby, 2013). A follow-up trial planned for 2014 in Millbuie Forest on the Black Isle, Ross-shire was postponed until 2015 when Hugh Clayden, FC Scotland’s tree health policy adviser, said: “These trials will help us determine whether this aerial application technique has a place in future disease management strategies.” However, since no further findings appear to have been released during the last five years it is reasonable to assume FC Scotland sees no place for aerial spraying in the future management of Dothistroma.

Beds of pristine pine seedlings at a forest nursery in the West Midlands, inspected annually by Forestry Commission for Dothistroma. Just a trace of disease would mean all have to be destroyed even though the disease has been endemic in the UK environment for many years and is rife in a local Corsican pine plantation.

The FC started to enforce the testing of nursery pine seedlings for Dothistroma around 10 years ago, by which time it was well and truly established across the UK. I visited one nursery in the West Midlands where such inspections are carried out on an annual basis. Just a whiff of Dothistroma disease would result in whole beds if not entire fields of trees being dug up and burned. And just down the road is mature pine woodland riddled with Dothistroma, with disease control neither carried out nor demanded by the UK plant health authorities.

Dothistroma needle blight disease took half a century to reach full force on Corsican pine, in contrast to Phytophthora ramorum which disposed of Japanese larch in just a couple of years. The three commercial pines (Corsican, lodgepole and Scots) affected by this disease cover 364,000 ha and represent 28 per cent of UK conifer forest. All are moderately to highly susceptible to Dothistroma (Forestry Commission, 2019) with clearly much more scope for damage.

Recommendations for Corsican pine to be planted in mixed conifer stands in order to slow down or inhibit disease spread and development do not appear to hold water. I have seen pockets of Corsican pine planted inside beech woods in the Chiltern Hills where Dothistroma still seeks out, infects and defoliates the Corsican pine.

However, perhaps we should not write off Corsican pine just yet. 46,000 ha of Corsican pine were still standing in 2019 with much in southern England including Surrey and Sussex, where a wise old FC forester once told me how Corsican pine will still outperform and out-yield Scots pine, even when the former has a dose of Dothistroma.

REFERENCES

Brown A.V. and Webber J. (2008) ‘Red band needle blight of conifers in Britain’. Research Note. Forestry Commission. United Kingdom. ISBN: 973-0-85538-763-1.

Bulman L.S., Bradshaw R.E., Fraser S. et al. (2016) ‘A worldwide perspective on the management and control of Dothistroma needle blight’, Forest Pathology 46(5): 472–488.

CABI (2019) Invasive species compendium – Mycosphaerella pini (Dothistroma blight) https://www.cabi.org/isc/datasheet/49059.

Edlin, H.L. (1956) Trees, Woods and Man. Collins. 272 pages.

Forestry Commission (2019). Forestry Statistics 2019. https://www.forestresearch.gov.uk/tools-and-resources/statistics/forestry-statistics/

Mabbett, T.H. (2013) ‘Copper wires crossed – the aerial onslaught on Dothistroma’, Forestry Journal September, pages 26–27.

Mabbett, T.H. (2016) ‘Pining for the conifer from Corsica’, Forestry Journal January, pages 48–49.

Perry, A. Brown, A.V. Cavers, S. Cottrell. J.E., Ennos, R.A. (2016) ‘Has Scots pine (Pinus sylvestris) co-evolved with Dothistroma septosporum in Scotland? Evidence for spatial heterogeneity in the susceptibility of native provenances’. Evolutionary Applications Volume 9 Issue 8, pages 982–993.

Rackham, O. (1976) Trees and Woodland in the British Landscape, Phoenix Press. 234 pages.

Tubby, K. (2013) ‘Dothistroma aerial spraying trials’. https://www.forestresearch.gov.uk/tools-and-resources/pest-and-disease-resources/dothistroma-red-band-needle-blight/dothistroma-aerial-spraying-trials/

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