The white flowering horse chestnut (Aesculus

Transcription

1 BACTERIAL BLEEDING CANKER IN HORSE CHESTNUT Horse chestnuts are now under threat from an aggressive form of bleeding canker. Terry Mabbett outlines the current situation regarding this worrying plant pathogen. The white flowering horse chestnut (Aesculus hippocastanum) had five centuries of untroubled growth after its introduction to the UK from Turkey in the sixteenth century. Then at or around the new millennium the horse chestnut leaf miner arrived on road vehicles from Europe. What followed was bad enough for a prestige amenity tree relying on good looks for its continued planting and popularity, but something altogether more deadly had arrived around the same time. The most widely-planted amenity tree, endeared nationwide for its cultured canopy, fine flowers and seed (conkers) had collided with a microbial pathogen that destroys living bark and cambium. A fast moving and lethal bark necrosis and bleeding canker started to take down horse chestnut trees, young and old, red and white, the length and breadth of the country. In 2006/7 a joint Forest Research and Central Science Laboratory (CSL) team in the United Kingdom (UK), and the Aesculaap working group in the Netherlands, showed a new [to Europe] Fork infections and cracked bark right up the tree. bacterial pathogen was causing this more aggressive form of bleeding canker disease. The culprit turned out to be Pseudomonas syringae pv. aesculi of the Pseudomonas syringae group of gram negative bacteria. It was new for Europe but not to Asia. P. syringae pv. aesculi had been identified in India in the 1970s causing of a leaf spot disease on Aesculus indica (Indian horse chestnut). Most under threat are white flowering horse chestnut, popular amenity trees used especially for avenue planting. However, Dr Joan Webber of Forest Research says the red flowered horse chestnut (Aesculus x carnea) appears just as susceptible and recent evidence suggests it could be succumbing even more quickly due to its comparatively thinner bark. Pathologists isolated the bacterium from white flowered horse chestnut trees with bleeding cankers and then cultured and identified the pathogen. They subsequently reproduced the disease by artificial inoculation in healthy mature trees (Aesculaap) and healthy three-metre high saplings (Forest Research/CSL). They 277

2 QUARTERLY JOURNAL OF FORESTRY Classic cracked-bark symptom. had satisfied Koch s Postulates, a 100 year old standard verification procedure for microbial plant pathogens, and proved P. syringae pv. aesculi as the cause. Pseudomonas syringae is a rod-shaped, gram negative bacterium with polar flagellae. Pseudomonas syringae has over 50 different pathovars (strains) infecting a huge range of plants. Pathovars are morphologically the same but genetically distinct and differentiated by host range. Many were originally considered as separate species within the Pseudomonas genus, but molecular techniques such as DNA hybridization have shown all to be part of a single Pseudomonas syringae species. Pseudomonas syringae is named after the lilac shrub (Syringa vulgaris) from which it was first isolated. Different kettle of fish Until 2000 bleeding canker of horse chestnut in the UK was generally caused by two fungal 278 pathogens, Phytophthora cactorum and P. citricola, which are uncommon, slow moving and generally confined to southern England. Bacterial bleeding canker is a different kettle of fish, fast moving both between and on trees and spread throughout the country. The disease is terminal for white and red flowering horse chestnuts, no respecter of age and tree size, taking just several seasons to kill large mature trees. Thousands of A. hippocastanum baummanii (a double whiteflowered seedless clone) were planted in Dutch cities to avoid problems with falling fruit and seed, but most of these trees have already succumbed to bacterial bleeding canker. A 2006 Netherlands-wide survey by Aesculaap showed the disease had spread into local authorities across the country with up to one third of trees infected in the worst affected areas. Wood of the white-flowered horse chestnut is inherently soft, white, clean and moist which made it ideal for milkmaids buckets and shelving for top fruit (apple and pear) stores, but of little interest and use to the timber industry. By the same token infected wood is prone to fracture as it dries out causing serious potential issues for safety and damage, especially for large trees in sensitive situations. Source and dissemination Horse chestnut is relatively uncommon in natural or planted woodland but is the classic amenity tree with between 1 and 2 million specimens nationwide. They have been planted in avenues along roads and in stately homes and parks, and are common around playing fields, cricket pitches, municipal buildings and churches, with a not inconsiderable number in private gardens. Hence the burden is falling on local authorities, sports clubs, universities and large private landowners, including well-known organisations like English Heritage, the National Trust and the Church of England. Few British trees have been under such an acute and widespread threat (in recent history), which could mean end of horse chestnut as an amenity tree in the United Kingdom, an outcome predicted by some within ten years. Pathogen and disease are already here so

3 BACTERIAL BLEEDING CANKER IN HORSE CHESTNUT origin and source is largely academic, but how it spreads between trees is of immediate concern. Pathovars of P. syringae are generally spread in water splashes and air-borne water droplets leaving little opportunity for prevention or control. Most enter trees via wounds but some are known to infect plants through intact and unbroken tissues whether tree bark or living epidermal cells. Large spreading mature horse chestnut canopies in avenue plantings virtually touch one another presenting ideal situation for rapid spread, especially with prevailing winds. Water splash, direct contact between branches, insects, frost damage, water-logging and drought, are all under consideration as agents of spread or factors affecting trees ability to withstand infection. Dutch scientists have suggested a relationship between the situation where a tree grows (e.g. constantly wet soil) and its susceptibility to this disease. Symptoms and mode of spread Recognition is complicated by type, extent and variation in symptoms between different trees. Brown fluid oozing from bark is a classic symptom but could also be a symptom of Phytophthora ramorum (sudden oak death) which has infected a few horse chestnut trees in the UK. Cankers caused by Pseudomonas syringae pv. aesculi vary widely between trees in extent, frequency and position on the trunk and/or branches. Most bleeding occurs during mild wet periods during spring and autumn. Ooze may dry up in summer on one side of the tree only, depending on aspect, air temperature and light and heat from the sun. Bleeding cankers may appear anywhere from root flares to high up on scaffold branches. Symptoms are rarely identical for any two trees, often minor and discrete or even absent, while under the bark the disease is spreading fast, girdling the stem or branch and progressively cutting off supply of water to the branches and foliage above. True extent only becomes apparent with severe cracking of the bark around the stem and often high up along the scaffold branches, followed by die-back, thinning of the crown with marked foliar Horse chestnut wood is soft and moist and fractures easily as it dries out during the course of an infection. Branch collapse was caused by the fork infection. yellowing and early fall of the leaves. Trees in this condition will probably survive into the following spring but not as fully functional and physically safe specimens. Infected trees can be identified by off-colour in the foliage accompanied by discrete die-back of the smallest shoots at the top of the canopy. But these are also symptoms of other diseases and physiological stress so should be taken as tentative indication. Leaf miner-damage makes recognition from the foliage more difficult. No means of management There is no available control measure and essentially no way of managing the disease. Under current UK pesticide legislation there is no bactericidal chemical with specific Label or Off label approval for use against this pathogen on horse chestnut trees. Advisory organisations 279

4 QUARTERLY JOURNAL OF FORESTRY This 70-year old horse chestnut is dying top down. The upper smaller- circumference branches are girdled more rapidly by the disease. like the RHS (Royal Horticultural Society) confirm there is no chemical control available in the UK for control of bacterial bleeding canker of horse chestnut. Traditional copper fungicides (Bordeaux), containing cupric (copper) sulphate as the active ingredient, are approved for use against two related pathovars (Psedomonas syringae pv morsprunorum and P. syringae pv syringe) which cause bacterial canker mostly on cherries and plums, but also peach, apricot and ornamental Prunus including gean (Prunus avium) and cherry laurel (Prunus laurocerasus). Traditional copper fungicides are approved for amateur use and therefore may be used if so desired and practical in amenity and forestry (see note at the end of this article). They are clearly considered safe for use on food crops and should, therefore, have no problems with approval for use on ornamentals like horse chestnut. Copper compounds like cupric sulphate are primarily used as fungicides but show additional high efficacy against many plant pathogenic bacteria. 280 Phosphite, chemically related to active ingredients in proprietary fungicides such as fosetyl-al (or their breakdown products), and which function according to the same principles, have provoked some interest. The actives are not classic (conventional) fungicides because they stimulate the plant s defence mechanism in addition to any direct fungicidal control of the pathogen. In the past they have proved to be significantly effective only against Oomycete fungi (e.g. Phytophthora, Pythium etc). Dutch scientists are exploring use of antioxidant chemicals to counteract and inhibit bacterialinduced bark necrosis caused by oxidative chemical reactions Dutch authorities recommend strictly handsoff management of infected horse chestnut trees, advising owners to monitor but not excise cankers and adopt minimum pruning programmes. They should also avoid unnecessary contact management (including disturbance of soil around the trees) that may aggravate bacterial disease spread. Plant pathologists at the RHS endorse this strategy claiming that older specimens with large trunk girths may recover if left alone. The disease they say is less likely to completely girdle large trees, and that bark renewal may eventually compensate for tissue previously lost through necrosis. That said infected trees may have to be felled even before their last throes because naturally soft and moist wood of horse chestnut trees fractures easily when drying out. This may cause potentially dangerous collapse of large scaffold branches and cleavage of stems at points of infection and wood weakness. Horse chestnuts are large trees often in highly sensitive situations, along roads and near houses with high risk of collateral damage. Rigid instructions for pruning-tool disinfection and disposal of infected wood must be followed to prevent spread of what is a highly contagious disease. Some local authorities are felling and grubbing out diseased trees from tree avenues and clusters and then replanting immediately with young horse chestnuts to preserve uniformity on the landscape. Dutch experts advise the longest possible delay when

5 BACTERIAL BLEEDING CANKER IN HORSE CHESTNUT replanting with horse chestnuts. Once one or two trees in a line or cluster have the disease most of the remainder are likely succumb over in the following years. Newly planted trees succumb just as often as mature specimens. However, not all trees in a cluster or line are coming down with bacterial bleeding canker disease. A minority appear to be stubbornly resisting infection, even when many trees nearby are showing advanced infection and disease. Dr Joan Webber at Forest Research believes there is need to investigate this phenomenon because there may resistant germplasm in the wider white-flowering horse chestnut population of the United Kingdom. In addition, Forest Research are investigating whether A. indica, which P. syringae pv. aesculi invades but only as a foliar disease, has bark resistance to infection and the bleeding canker disease dimension of this pathogen. On-site diagnosis of trees requires a pocket diagnostic instrument called Lateral Flow Meter available from CSL. Utilising a rapid serological testing technique it will provide a positive or negative result for Phytophthora. If negative cause of the bleeding canker is almost certainly down to Pseudomonas syringae pv. aesculi. Grim outlook for horse chestnuts Damage caused by leaf miners looks bad but does not threaten tree survival. Bacterial bleeding canker is mostly fatal and rarely offers recourse to recovery. Many experts doubt the wisdom of trying to prop up a tree species that is seriously destabilised in its environment, so the authorities may have look at other species like walnut to fill gaps left by horse chestnuts. Bacterial bleeding canker could finish horse chestnut as an amenity tree in the United Kingdom. The only other tree to recently suffer such fate was English elm. Infection mechanisms are not comparable because one was caused by a fungus carried and transmitted by an insect vector, while the other is bacterium spread in water-droplets by wind. Ironically neither tree species is native but the effect on longevity of horse chestnuts as mature trees in the United Kingdom could turn out to be the same. Important note: use of chemicals and pesticides Mention of any chemical in this article is for information only and not recommendation or endorsement for use. Pesticide legislation covering the chemicals and use situations mentioned here is particularly complex and not altogether clear. Use of chemicals and application techniques not approved for a particular plant host-pathogen relationship and in a defined use situation is illegal. Anyone wishing information about potential for use of any chemicals mentioned and discussed in this article, even for experimental purposes, should seek advice and clarification from the supplier, the manufacturer and the Pesticide Safety Directorate (PSD). Bacterial bleeding canker is no respecter of age and size as canopy thinning on this 200-year old horse chestnut shows. 281