things you need to know about Xylella fastidiosa Forest Research scientists Ana Perez-Sierra and Joan Webber, impacts range from wilting, growth stunting and leaf scorch, to dieback and with Animal and Plant Health Agency inspectors Edward the death of entire plants. In olive trees for Birchall and Lucy Carson-Taylor, provide an update on this example, the disease starts with withering 9emerging risk to plants and trees across Europe. and desiccation of the terminal shoots, but as the damage expands through the canopy, trees collapse and die. Typically on trees such as oak, elm What is Xylella? 2 Where has the disease plane, maple (Acer spp.) and Aesculus, 1 come from? visible symptoms caused by Xylella Xylella, or more properly Xylella fastidiosa, Before the findings in Europe,Xylella fastidiosa ssp. multiplex are mainly leaf is a disease causing bacterium that can was encountered mainly in the Americas. scorch so in the USA the disease is known infect many woody plants including Subspecies fastidiosa is found in Central as bacterial leaf scorch or BLS. Some broadleaved trees. There are at least four America, North America and Taiwan, effects, however, can be more severe accepted subspecies of Xylella which where it affects citrus, grapevines and and include twig and branch dieback or attack various plant types inciting different coffee and almond plants. Subspecies growth stunting of the entire tree. The types of symptoms. They are - multiplex has mainly been found in the characteristic leaf symptoms which are • Xylella fastidiosa ssp. fastidiosa USA and probably has the widest host most visible in later summer include • Xylella fastidiosa ssp. pauca range which includes trees such as oak, browning at the leaf margins (not along • Xylella fastidiosa ssp. multiplex elm and plane. Multiplex is considered to the main veins), often with a yellow edge • Xylella fastidiosa ssp. sandyi pose the greatest risk to trees in the UK. to the browned areas. However, other Until recently, Xylella was only known disorders can cause similar symptoms (see from the Americas and Taiwan, but it first What does Xylella do to Disorders box). became notorious in Europe following 3 infected plants? reports of the rapid decline and death of The bacterium invades the water 4 How does Xylella ancient olive trees in the Apulia region transporting xylem vessels in plants, spread? of southern Italy in 2013. X fastidiosa was blocking water movement and causing In the natural environment Xylella is confirmed as the cause and the subspecies symptoms that initially resemble water exclusively transmitted by insects such as identified asX fastidiosa ssp. pauca. Now stress. Depending on the susceptibility of leafhoppers and froghoppers which feed around a million trees in southern Apulia affected plants and the Xylella subspecies, on xylem fluid. There are various insect are affected by the disease, referred to as Olive Quick Decline Syndrome (OQDS). Subsequently other Xylella outbreaks in Europe have been found in France, Spain and Germany (see Timeline) with a wide range of plant species affected. In addition to X fastidiosa ssp. pauca causing OQDS in Italy, subspecies fastidiosa and multiplex have also been found in other European countries. To control this bacterial pathogen which is now treated as a quarantine pest, stringent measures are in place. All EU Member States are required to eradicate any outbreaks, or if that is impossible, to prevent further spread. Figure 1 Olive quick decline syndrome (left) Early Xylella fastidiosa symptoms (branch wilting) on an olive tree in September 2014; (right) symptom development on the same tree by May 2016. (Courtesy of Donato Boscia from CNR-IPSP, Italy) Xylella timeline 2013 2015 2016 (July) 2016 (October) 2017 (August) 2017 (June) X fastidiosa X fastidiosa ssp. X fastidiosa Infected almond Grapevines were X fastidiosa ssp. The most recent ssp. pauca multiplex identified ssp. fastidiosa trees were found also found to multiplex was figures from first confirmed affecting several confirmed in a on Mallorca and be affected on found on the Europe suggest in causing plant species German glasshouse then other affected Mallorca. The Spanish mainland around 150 shrub devastation to in France and on an oleander hosts on Ibiza and Balearics now have (south of Valencia) and tree species olive plantations in Corsica. plant and then Menorca. many outbreaks on Prunus dulcis have been infected. southern Italy. on Rosmarinus, involving three (almond). Erysimum and subspecies of X Streptocarpus. fastidiosa. 42 FORESTRY & TIMBER NEWS • December 2017 CONFOR.ORG.UK species in Britain that could become Figure 2 Xylella bacterial leaf scorch symptoms on tree foliage (left) American sycamore vectors if Xylella was introduced, including (Platanus occidentalis); (centre) English oak (Quercus robur); (right) elm. (Courtesy of the common froghopper (meadow Theodore D. Leininger, USDA Forest Service, Bugwood.org, John Hartman, University of spittlebug). Although these insects Kentucky, Bugwood.org and Sandra Jensen, Cornell University, Bugwood)org) generally only fly short distances they can be moved much further by the wind. As they feed, they pick up the bacterium from Disorders with similar symtoms infected plants and pass it onto healthy plants. Movement in trade of infected Several disorders produce symptoms similar to those caused by Xylella fastidiosa. plants also aids disease spread. • Horse chestnut trees commonly suffer from a fungal disease which causes a brown leaf blotch with a yellow halo. The horse chestnut leaf miner also damages foliage but 5 How likely is it that Xylella might causes an inter-veinal browning of leaves rather than marginal browning. establish? • Elms suffer from Dutch elm disease which also causes wilting and browning of foliage. Xylella has not yet been found in the UK, • Plane trees suffer from a fungus called anthracnose which causes twig death and leaf but if it arrives at least one subspecies is blight. Powdery mildew can also cause yellowing and distortion of young plane leaves. likely to be able to survive and establish • Drought stress can cause cupping and browning at the leaf margins. most probably in the warmer parts of the UK, especially in regions with minimal frost episodes. Xylella fastidiosa ssp. multiplex 7 What happens if Xylella is 8 Good practice to combat which infects various forest tree genera found in UK? Xylella is active in temperate regions of North An outbreak in the UK could lead to • If importing plants, ensure that plant America. destruction of host plants within 100 m passports arriving with host plants are and a 5 km movement ban for ‘specified’ correct and keep the passports to aid 6 What are we doing plants for five years. Any findings will dealt trace back if necessary. to prepare? with to reflect risk and evidence of spread. • Source from known suppliers or The Forestry Commission undertakes Where Xylella is found on a plant and visit suppliers to view their procedures, targeted annual surveys of susceptible the disease has spread or there is risk biosecurity arrangements and the plants tree species as part of the surveillance that spread has occurred, then a 5 km they grow. See web resources. activities against X fastidiosa. The Animal demarcated area is required - the size and • Ensure imported plants originate from and Plant Health Agency (APHA) also how long it remains in place will depend and are sourced from disease free areas. conducts official surveys forX fastidiosa on a risk assessment. For regularly updated details on affected >> and works with businesses which import If infected plants are recently arrived susceptible plant species. Landscapers, but vectors carrying Xylella are not designers, retailers and anyone directly found, then a demarcated area is not importing plants are now subject to the needed. Plant Health Services will require same stringent requirements as growers destruction of the infected plants and any and suppliers. Prior to the movement of nearby potential hosts will probably have all potential Xylella ‘host plants’ across to be destroyed as well. the EU they must have been officially For isolated outbreaks resulting from inspected and be accompanied by a plant introductions of infected plants where passport to show they have been sourced there is robust evidence that spread has from disease free areas/sites. High risk not occurred and other actions are taken, hosts such as Coffea, Lavandula dentata, the buffer zone width may be reduced Nerium oleander, Olea europaea, Polygala to 1 km and restrictions revoked after a myrtifolia and Prunus dulcis have extra minimum of 12 months. requirements. Figure 3 Laboratory processing of tree foliage samples at Forest Research for the detection of Xylella fastidiosa (Forestry Commission). CONFOR.ORG.UK FORESTRY & TIMBER NEWS • December 2017 43 PLANT HEALTH VIEW FROM THE EMERALD ISLE SÉAN LENIHAN >> areas, known plant hosts and legislation, Ash dieback update Knowledge Transfer to the processing and see web resources. Since it was first discovered in County sale of first thinnings and firewood. There • Isolate or quarantine new plant batches Leitrim in October 2012, Ash dieback has are 2,000 owners now officially registered and monitor them during the growing been confirmed in 384 forest plantations, with Producer Groups which represents season for any symptoms – whilst not and is now present in every county in the about 20% of all private forest owners. a legal requirement it is good practice Republic.