Globalisation and free trade have given us a wealth of plants to enjoy from every corner of the world. However, where there are plants, there can also be pests. Hitchhiking pests like elm disease and ash dieback have caused serious harm to our environment. Other pests have arrived too, but not made such a noticeable nuisance of themselves.
The introduction and spread of fire blight has been a constant threat for over 40 years in Ireland, but fortunately there has been limited impact to date.
History
Fire blight, caused by the bacterium Erwinia amylovora, is one of the most destructive diseases affecting members of the Rosaceae family, particularly apple and pear trees. The disease causes wilting and dieback of susceptible plants, with no practical cure available.
Fire blight was first identified in the United States in the late 19th century. It has since spread to various parts of the world, including Europe. In Ireland, fire blight was first recorded in 1986 on a specimen of Cotoneaster in Dublin. Its presence in Ireland was of particular concern due to the country’s horticultural and fruit-growing industries.
Fire blight poses a significant threat to native and non-native species of Malus and Pyrus, which are important for both commercial production and ornamental purposes. Our native flora is susceptible, including the widespread hawthorn Crataegus monogyna and mountain ash Sorbus aucuparia.
Since first detected, the only control option for the disease has been to destroy infected and neighbouring susceptible plants and to maintain an observation area around the outbreak site. This has successfully limited the spread and/or establishment of the disease.
In an effort to improve disease control and traceability, plant passports have been used since 1 June 1993 for fire blight host plants. To date, the Republic of Ireland has a protected zone allowing import of plants only from areas known to be free of the disease.
Disease biology
Erwinia amylovora is a gram-negative bacterium that infects plants by entering through natural openings (such as stomata) or wounds, often caused by insects, pruning, or weather events such as hail. Once inside the plant, the bacterium multiplies and produces an exopolysaccharide called amylovoran, which clogs the plant’s vascular tissues, leading to the characteristic wilting and necrosis. This resembles fire damage, hence the name “fire blight.”
Source of disease
The primary source of the fire blight pathogen in Ireland is infected plant material, such as trees and shrubs, imported from regions where fire blight is endemic. Once introduced, the bacterium can persist in plant tissues and continue to spread within and between orchards, gardens and hedgerows.
Ireland is not the only European area that is largely pest free and holds a protected zone: Estonia, regions of Spain, Corsica in France, and some regions of Italy have this designation until April 2026, as do Latvia, Finland, and most regions of Lithuania. Northern Ireland and Great Britain do not have a protected zone.
There is an obvious risk of disease spreading by wind or insects to Ireland if outbreaks are not controlled in other territories. Pest-free areas can be established in countries that do not have a protected zone; however, these areas need to be strictly monitored for the disease.
Fire blight spreads via multiple vectors
Water droplets can carry the bacteria from one plant to another, especially during heavy rainfall. Wind can disperse the bacteria, particularly in areas with high tree-density. Pruning tools and infected plant material can also contribute to the spread of the disease. Regular disinfection of tools during pruning is known to be effective in limiting the spread of the disease.
Pollinators such as bees and flies are the most common carriers, spreading the bacterium as they move between flowers. As bees can carry the bacteria in their gut and spread it from flower to flower, the importation of beehives is restricted between 15 March to 30 June, when host plants are in flower. During this time, hives cannot be imported from non-protected zones without quarantine and testing.
Number of outbreaks
Since its first detection in Ireland, the number of fire blight outbreaks has fluctuated based on environmental conditions, primarily temperature and humidity, which are conducive to the growth and spread of the pathogen. Ireland’s cooler and more temperate climate and quick control measures have to some extent limited the large-scale outbreaks seen in warmer regions of Europe.
Detailed records from the Department of Agriculture, Food and the Marine (DAFM) indicate sporadic but significant outbreaks in specific regions. Records show this fluctuation: three cases in 1990, two in 1991, one in 1992, five in 1993 and 31 in 1994. Ten years later, surveillance has increased but detection remains low: in 2003 there were eight positives; in 2004, 13 positives; in 2005, 25 positives; in 2006, 19 positives; and in 2007, 25 positives, mostly from a large outbreak in the Bayside Sutton area of Dublin.
Recent records reported 17 cases in 2023 spread across eleven counties. 35 per cent of these were detected in garden centres, 24 per cent were roadside detection, 18 per cent were in nurseries, 12 per cent in public parks, and 6 per cent each for a single case in a Dublin industrial estate and a farm in Kerry.
In 2024, DAFM reported 108 outbreaks of fire blight to the EU Commission, across the 26 counties of Ireland. The majority of these outbreaks were in the wider environment and were found in the following hosts: Sorbus, Crataegus, Cotoneaster, Malus, Photinia, Pyracantha and Pyrus.
With between 1,500 and 2,000 tests carried out per year on suspicious plants, the rate of detection continues to be reassuringly low.
Impacts
Fire blight has the potential to have a significant economic impact on Ireland’s horticultural sector. The disease can kill entire trees, reduce fruit yield, and affect the aesthetic value of ornamental plants, leading to significant losses in both fruit production and the ornamental plant trade.
Moreover, the disease necessitates costly control measures, involving the complete removal of infected and neighbouring plants. This cost is borne by the nurseries or garden centres affected, or the relevant local authorities.
The fire blight case in Bayside and Sutton in north county Dublin in 2007 resulted in the felling of over 25 mature street trees of Sorbus aucuparia and Sorbus aria. This had a significant impact on the character and amenity value of the streets affected, not to mention the financial cost of the destruction of the trees and replanting with new resistant varieties.
Commercial nurseries and garden centres with sporadic positive findings also suffer the same control actions, leading to loss of valuable stock. Fire blight outbreaks can also result in trade restrictions, especially for nursery stock, which can impact Ireland’s ability to export host plant varieties. Indeed, garden centre owners have remarked that they suffer negative impacts just from the appearance of official hazard tape around suspicious plants, even though they almost always return a negative result for the bacteria.
Treatment and control
As with all bacteria, there are no practical plant protection options available for fire blight prevention. In countries like the USA, where the bacteria is endemic and can’t be eliminated, control is through good hygiene, variety selection and non-antibiotic sprays during flowering. Monitoring of the disease through AI image analysis and flower and fruitlet sampling helps inform disease levels and forecasting tools. As the disease is not established in Ireland, a strict control programme is required.
EU plant health regulations require yearly checks in areas with a protected zone for the disease. When a disease is found, an infected zone and a surrounding buffer zone must be set up in a demarcated area based on the pest’s behaviour. For fire blight, this marked area is a 5 km circle around where the disease was detected.
The DAFM then surveys the area to check if more infected plants are present. If new cases are found, the marked area is adjusted. Control measures of plant destruction and safe disposal are taken to keep the protected zone intact.
Risk mitigation can reduce spread should an outbreak happen. Pruning of susceptible species should be carried out with the regular disinfection of equipment, especially between zones of planting or work sites. Planting fire blight-resistant varieties can be an effective strategy to reduce hosts.
Strict control measures on imported plant materials are enforced by the DAFM to limit the introduction of infected plants into Ireland. Use of quarantine bays could help in limiting collateral damage should a symptomatic plant test positive.
Nurseries and garden centre owners are reluctant to speak on the record, but have told me that they are frustrated with the cost of destruction being borne entirely by the grower/centre owner, while they see other sectors getting compensation for similar destructive actions.
Impact of climate change on fire blight
The fire blight bacterium thrives in warmer conditions. Climate change is likely to result in milder winters and warmer springs, extending the periods during which the disease can proliferate. More frequent heavy rain events, as projected by climate models for Ireland, could facilitate the spread of the bacteria via rain splash.
Changes in insect populations and behaviour, particularly pollinators, due to climate change may increase the transmission of fire blight between plants. An extended growing season could provide more opportunities for infection, particularly during flowering periods.
Resistant varieties
Resistance can vary based on environmental conditions and pathogen strains, but underlying genetics are the key ingredient. While the US is leading the way in developing resistant varieties of trees and shrubs, there is a slow transfer of new varieties to Europe.
Crab apple Malus ‘Adirondack’ (‘Admiration’), Malus sargentii (‘Tina’) and Malus (‘Prairifire’) are US-bred selections that overall have very good disease resistance to fire blight, scab and mildew. Some European introductions of ornamental plants include the French-bred Pyracantha ‘Saphyr’ range (Orange-Cadange, Yellow Jaune-Cadaune and Red-Rouge Cadrou) which are also resistant to scab.
Teagasc top fruit researcher Alberto Ramos Luz says, “Some apple breeding programmes – for example Agroscope in Switzerland and some universities in the USA – are including as one of their aims the selection of new fire blight resistance genotypes.”
Conclusion
Preventing fire blight presents an opportunity for Ireland’s horticultural sectors to help maintain our protected zone status. Climate change can potentially exacerbate conditions favourable to fire blight’s spread. Being prepared for the potential for increased outbreak is important.
We have seen over the past 40 years that containment is possible, but it comes at a cost. The disease’s economic impact can be severe, affecting commercial fruit production, ornamental plant industries and the wider environment. It is worth the effort to prevent the disease becoming a more familiar challenge, given that we currently have limited tools available to tackle it. Continued vigilance remains our best defence.
