Is sweet chestnut next in the firing line for Phytophthora ramorum?

NO-ONE could have foreseen the future consequences of Phytophthora ramorum when the fungus-like pathogen first appeared in the UK shortly after the turn of the new millennium. In less than a decade, the scene was set for the loss of larch as a premium timber tree.

Documented reports are not crystal clear, but first identification in the UK was in February 2002 at a West Sussex garden centre on a container-grown Viburnum tinus ‘Eve price’, one of five plants sourced from a plant nursery in Cheshire. The disease was subsequently identified in May 2003 on container-grown English yew (Taxus baccata) at the same plant nursery in Cheshire. November 2003 saw first confirmation of P. ramorum on a full-blown tree, a 100-year-old southern red oak (Quercus falcata) in East Sussex.

Japanese larch planted in deciduous broadleaf woodland – foresters loved it, conservationists hated it but it extends good foliar colour into late autumn and early winter within otherwise drab and skeletonised woodland (picture courtesy Woodgate Sawmills).

Phytophthora ramorum had entered the UK multiple times on Rhododendron, Viburnum and other hardy nursery stock imported from Europe, most likely the Netherlands and Germany. A new condition affecting hardy nursery stock had already been reported in these countries, but was as yet unidentified and provisionally assigned to ‘an unknown species of Phytophthora’. But P. ramorum was already well known and indeed notorious in the Pacific North West region of the USA and Canada. Appropriately named Sudden Oak Disease (SOD), P. ramorum was sweeping through coastal forests in northern California and southern Oregon, killing at least one million native oak and tan oak trees along the way.

The new millennium saw UK plant pathologists and foresters preoccupied with Armageddon looming for native English oak trees, but two factors appear to have been on the side of these British native white oak species. Different genetic lineages of P. ramorum were operating in North America and in Europe. Initially NA1 (North America) and EU1 (Europe) were identified, joined later by NA2 and EU2. Whether it was a difference in genetic lineage or ultra-thick, resilient bark on Quercus petraea and Quercus robur which saved the day for English white oaks is unclear, but the net result was just a handful of cases of
P. ramorum on these iconic native trees.

However, P. ramorum’s catholic taste in trees and shrubs soon re-galvanised the industry as scores of different genera and species across dozens of plant families succumbed to the pathogen. And when the smoke had cleared, scientists had recorded a plant host list approaching 150 different trees species including horse chestnut, sycamore and silver birch, as well as Rhododendron ponticum and bilberry and scores of other small trees and woody shrubs in the horticultural and garden sectors, including Viburnum, Camellia, Kalmia, Pieris, Magnolia and Syringa (lilac).

The microscopic reproductive structures which sealed the fate of Japanese larch. The elliptical spore (sporangium) of P. ramorum is shown here squeezing out motile zoospores that will go on to infect other trees (picture courtesy of the Forestry Commission). (Image: FJ)

THE REAL THREAT TO SWEET CHESTNUT

Native English oaks may have escaped, but other trees in the plant family Fagaceae (beech family) including common beech (Fagus sylvatica) and sweet chestnut (Castanea sativa) had not. Disease manifestation by P. ramorum on sweet chestnut was especially interesting. Most tree species infected by P. ramorum display a frequently fatal bark necrosis, whereas manifestation on sweet chestnut is essentially foliar in nature and location. Epicormic growth positioned low on the stem is especially prone, with leaves showing water-soaked or discoloured margins and blackened petioles, with the discolouration extending up the mid-rib (main vein) of the leaf.

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However, collateral damage for sweet chestnut like Rhododendron ponticum is being a sporulation host with leaves acting as a template for production and liberation of asexual spores called sporangia. Thus sweet chestnut was not only sustaining infection and damage but also possessed the potential to spread disease through rain-splashed, air-borne spores.

Sweet chestnut grown and cut according to recommended coppicing cycles provides an ideal habitat for flora and fauna.

These 15-year-old findings should have set the alarm bells ringing for sweet chestnut. However, the appearance of P. ramorum on Japanese larch in 2009 in the west of England, with massive destruction of these conifer trees up the western flank of the British Isles, shoved any potential problem for sweet chestnut to the side. Coupled with later appearance of sweet chestnut blight (2011) and oriental chestnut gall wasp (2015), considered as the world’s worst disease and insect pest for the genus Castanea, this meant any potential threat to sweet chestnut from P. ramorum was pushed firmly onto the back burner. However, the latest reports and data on P. ramorum infection of Castanea sativa should set the alarm bells ringing once again, this time because the proverbial chestnuts may need to be pulled from the fire ignited by P. ramorum.

Sweet chestnut coppice in Kent, East Sussex and Surrey has so far been spared infection by P. ramorum.

Despite sweet chestnut turning out to be a sporulation host for P. ramorum, initial observations (pre-2010) showed only trees exposed to heavy inoculum pressures from other more powerful sporulation hosts such as Rhododendron ponticum were infected. That all changed in 2015, when a cluster of outbreaks was found in western England, mainly in Cornwall and Devon but as far east as the Somerset/Wiltshire border. This time, infected and diseased trees were isolated from the immediate presence of other sporulation hosts, therefore suggesting long-distance travel of P. ramorum spores and perhaps transmission between neighbouring sweet chestnut trees.

Figures released by Forest Research show 68 sites confirmed with P. ramorum between 2015 and 2020 (1 February, 2020) and SPHNs (Statutory Plant Health Notices) issued accordingly. Sweet chestnut trees showing dieback within these treatment areas are subject to felling or other control measures. Sweet chestnut trees retained within these treatment areas are subject to the removal of epicormic growth up to a height of three metres from the base. So far, 132 ha of sweet chestnut have been treated in accordance with issue of SPHNs between 2015 and 2020. The number of sweet chestnut trees affected within the treatment areas is not available.

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A greater number of sites (around 240) were investigated for P. ramorum infection over the same period and across a much wider area, with many in Kent, Sussex and East Surrey, the heart of sweet chestnut country in the UK, but apparently with no disease found. The map also shows a trickle of sites investigated along the border between England and Wales, with one confirmed finding and SPHN issued in Worcestershire.

These documented findings are almost certainly an under-representation of the spread and seriousness of P. ramorum on sweet chestnut, because the report only details cases where P. ramorum has been found affecting sweet chestnut (Castanea sativa) as a principal tree host.

“Cases where sweet chestnut is not a principal tree host are not reported here,” says the report. That said, it is not clear whether the report refers to infections (of sweet chestnut trees) found in areas where there were only a few sweet chestnut trees or whether to areas of the UK that do not traditionally support sweet chestnut and where, by default, sweet chestnut cannot be a principal plant host.

The situation in the counties of Essex, with Suffolk to the north, is especially interesting in this respect. Though not regarded as traditional sweet chestnut country, Essex – and particularly its southernmost region – contains a lot of sweet chestnut, but no investigations have been carried out, which presumably means no reports to the Forestry Commission of suspicious symptoms.

The reason for the apparent lack of disease in East Anglia and, likewise, Kent, Sussex and Surrey, is likely due to a Phytophthora requirement for moist and humid conditions to thrive. Indeed, early mycologists called the Phytophthoras the ‘water fungi’. Just as Phytophthora ramorum infection of Japanese larch is generally confined to the wetter western side of the British Isles, infections of sweet chestnut appear to be following a similar pathway and pattern.

After English yew and English oak, sweet chestnut provides some of our oldest trees – this specimen in Hertfordshire is nearing 400 years of age.

SWEET CHESTNUT VERSUS LARCH

Of particular interest is the Forestry Commission’s assessment of the relative importance of Phytophthora ramorum damage to larch and sweet chestnut. According to Forest Research, larch  – presumably all larch, including European larch (Larix decidua) and hybrid larch (Larix X marschlinsii) as well as Japanese larch (Larix kaempferi) – occupied about 154,000 ha or five per cent of Great Britain’s total woodland area before the first finding of P. ramorum on Japanese larch in 2009. As such, they say, the overall environmental, landscape and economic impact of significant larch losses would not be great in a Britain-wide context, although it would be (and has been) locally and regionally. In contrast, they claim sweet chestnut occupies about 29,000 ha of woodland and that the loss of veteran trees of this species would be a significant heritage loss, and relatively more so than larch forests, adding how most larch was planted after 1945, so there are few veteran larch trees on the landscape.

The focus of this article is on P. ramorum infection of Castanea sativa, but some defence of larch is required. The Forest Research view outlined above may give an impression that exotic Larix species, whether Japanese larch, European larch or hybrid larch, are universally negative in impact on the landscape and environment, which is simply untrue. Indeed, previous support for larch as a benefit to wildlife comes in part from research work carried out by the Forestry Commission.

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According to the Woodland Trust, European larch seed is eaten by red squirrels and a number of birds, including the siskin and lesser redpoll, while the buds and immature cones are eaten by black grouse. The caterpillars of many moths feed on the foliage, including the case-bearer moth and larch pug. Larch tortrix moth caterpillars eat the cone scales.

Phytophthora ramorum has essentially forced forest nurseries to give up on larch – will they be forced to do the same with sweet chestnut?

The findings from detailed research into the feeding habits of squirrels and birds on a variety of conifer cones and seed, including larch, are most illuminating. Factors which clearly contribute to how useful or effective a particular conifer is to wild animals and birds feeding on the cones and seed include: density of cones in the canopy, propensity to fall for ground-feeding flora and accessibility of seeds inside the cone.

Specific factors for which larches score highly include:

• Age of first large cone crop for common conifers in Britain.
• Cone density per square metre of canopy.
• Months of the year when seed is shed from cones by different conifer species.
• Fallen cone count.
• Amount of seed produced for a given area of woodland.
• Ease of seed access in cones by birds such as the common crossbill.

In Britain, the first large cone crop for Japanese larch is produced in year 15, but it is a whole 10 years later for Scots pine. Japanese larch sheds seed for six months from October to April, while Scots pine sheds seed for only four months from April to July. Guide figures on seeds produced by stands in good seed years show that larches are the most prolific (up to 24 million seeds per hectare) followed by Sitka spruce (6 million seeds per hectare) and with the least seeds being produced by Norway spruce, lodgepole pine and Scots pine (1–2 million seeds per hectare). Although energy per individual seed in Scots pine is almost twice that of the larches, the seed crop produced by larches provides more energy per hectare (1–2 GJ ha-1) in a good seed year. Energy from seed production (at 0.1–0.5 GJ ha-1) is much lower in the pines as well as Sitka and Norway spruce.

Larix species are deciduous conifers and score heavily for aesthetic value. Their golden yellow autumn foliage reaches a climax in November and complements the landscape whether in pure stands or mixed with evergreen conifers and broadleaves.

Cones and seed of the exotic larches are eaten by native mammals and birds.

Apparently, negative comments relating to larch, or for that matter any other conifer, should be taken in the context of an organisation (Forestry Commission) having displayed decreasing interest in productive softwood timber forestry in England, if not Wales, over the last three decades. The turnaround in conifer planting across Scotland is recent and post-devolution.

Whether 150,000 ha of larch or 18,000 ha of sweet chestnut, such an order of tree, timber and ecological loss to plant disease is an unmitigated disaster.

AUTHOR’S NOTE

Compared with not-too-distant surveys, the current figure of 29,000 ha of sweet chestnut would appear to be on the generous side. Natural England Report 627 published in 2004, sourced from the Government Office for the South East (2001) and Braden and Russell (2001), calculated 18,758 ha of sweet chestnut in the UK – chestnut coppice, coppice with standards and high forest – with virtually all (18,120 ha) in England. No less than two thirds of this (12,270 ha) was concentrated in south-east England, partitioned between high forest (5,289 ha), coppice with standards (1,083 ha) and coppice (5,898 ha). The 5,898 ha of coppice was almost entirely in Kent (3,922 ha) and Sussex (1,779 ha) with 142 ha of the remainder in Surrey. In 2010, the British and Irish Hardwood Trust (BIHT) gave a ball-park figure of 18,000 ha for all sweet chestnut woodland in the UK, indicating virtually no change over the decade. Given the paucity of all tree planting over the last three decades, including the most recent, it is difficult to see how the area of sweet chestnut could have expanded by 61 per cent.

REFERENCES

Braden, N. and Russell, K. (2001) ‘Chestnut in the UK: forest area, management and utilisation of timber’, Forest Snow and Landscape Research 76: 505–510.

Broome, A. and Poulsom, E. (2006) ‘Validation of Forest Condition Monitoring Programme (ICP-Forests, level 1) cone index scores by measuring cone density using a field-of-view method’, Forestry 79, 159–66.

Broome, A. Summers, R. W. and Vanhala, T. (2016) ‘Understanding the provision of conifer seed for woodland species’. Forestry Commission Research Note, June 2016.

Buckley, P. and Howell, R. (2004) ‘Ecological impact of sweet chestnut coppice silviculture on former broadleaf woodland sites in South East England’, Natural England Research Report 627.

Bryce, J., Cartmel, S. and Quine, C. P. (2005) ‘Habitat use by red and grey squirrels: results of two recent studies and implications for management’, Forestry Commission Information Note, Forestry Commission, Edinburgh.

Forest Research 2020 Ramorum disease (Phytophthora ramorum) https://www.forestresearch.gov.uk/tools-and-resources/pest-and-disease-resources/ramorum-disease-phytophthora-ramorum/.

Gurnell, J., Lurz, P., McDonald, R. and Pepper, H. (2009) ‘Practical techniques for surveying and monitoring squirrels’. Forestry Commission Practice Note, Forestry Commission, Edinburgh.

McNab, E., Summers, R., Harrison, G. and Park, K. (2019) ‘How important are different non-native conifers in Britain to Common Crossbills (Loxia curvirostraz Bird Study Vol. 66 Issue 1, 2019.

Mabbett, T. H. (2011) ‘Sweet chestnut coppice’, Forestry Journal, August 2011.

Mabbett, T. H. (2014) ‘Japanese larch:  the Kamikaze conifer’, Forestry Journal, November 2014.

Nixon, C. J. and Worrell, R. (1999). ‘The potential for the natural regeneration of conifers in Britain’. Forestry Commission Bulletin 120. Forestry Commission, Edinburgh.

Sketchley, R. (2020) ‘Phytophthora ramorum in sweet chestnut’. England Situation Report, 1 February 2020. Forestry Commission February 2020.

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