Xylella

The worrying disease Xylella fastidiosa (Xf) has been in Europe for more than five years. It has been causing problems in the Americas for many years and has been on the watch list for plant health officers for a long time. Coffee and olive plants coming to Europe have been closely inspected for Xf since becoming a notifiable disease in 2000. Without
decisive action, Ireland faces its most serious plant disease threat yet.

UNDERSTANDING XF

Xf is a bacterial infection and causes wilting and death of plants. It enters the water-conducting xylem vessels where it multiplies and blocks the transportation of water. As
spittlebugs feed on the fluid from the plant xylem they can become infected by the bacteria and pass it on to other plants. The three different subspecies of Xylella present in Europe are fastidiosa, pauca, and multiplex. The bacteria are very specific in their hosts and may not infect closely related plant species or even some varieties.

The bacteria is a major concern for a few reasons. It is likely to be fatal to its host. It has a very wide host range of 60+ plant species that show symptoms and more than 240 that can carry the disease without symptoms. Its vectors, spittlebugs, can be found all across Europe so it can easily be spread. Most worrying is the fact there is no cure.

The disease is currently in a number of confined areas in southern Italy, Spain, France and Corsica and on one site in Germany. The three different species of Xylella have different host ranges and favourable conditions. Xf has the potential to survive in a wide range of climates in Europe including Ireland. Xylella fastidiosa sp. multiplex has the widest range of hosts and survives in cooler climates. Some of the species attacked by multiplex include oak, elm, ash, and sycamore. This is significant because there is a very active trade in ornamental plants all across Europe. In Ireland, we are familiar with many plants sourced from Italy and France (e.g. lavender, rosemary, and the Celtic Tiger trophy plant, olive). Other countries like these plants too and there is a steady movement of them from market to market across Europe. Xylella is only a truck journey away from any country in Europe.

CURRENT ACTIONS

EU and private companies are taking action. Since the first detection of the disease, the EU has had a strict eradication policy which starts with identification and the establishment of an eradication zone. Once established, all host plants and vectors within 100m of the diseased plant are destroyed. A buffer zone of at least 5km in width surrounding the infected zone is set for five years, where movement of host plants is strictly controlled. Under certain conditions, which would guarantee that no further spread of Xf has occurred, the buffer zone may be reduced to a width of at least 1km around the infected site and be maintained for at least one year. Increased control measures were agreed by the EU at the end of October this year. DAFM played an important role in securing these stronger controls to protect our environment and our trade. Annual testing, likely to begin in March 2018, will now be required for the

most highly susceptible plant species; Coffea, Lavandula dentate, Nerium oleander, Olea europaea, Polygala myrtifolia and Prunus dulcis. These plants plus about 55 more susceptible hosts will require plant passports, most likely beginning before the end of 2017.

The disease has already had a big impact on trade in Europe. Some businesses have switched suppliers or stopped carrying stock that is sourced in identified countries. Growers and traders are looking for reassurances that everything possible is done to avoid the disease coming to their nurseries. The knock-on effect is that there is surplus stock in unaffected areas in Italy.

Close to home, the RHS has banned the 12 most disease susceptible plants from its garden shows and plant centres unless it has been grown in the UK for more than 12 months. The HTA has set up a voluntary scheme where growers have agreed not to import stock knowingly from affected areas. I hear there have been a good number of UK plant buyers in Ireland in the last few months finding alternative suppliers. Those buyers are also looking to know what we are doing to prepare against the disease. Speaking with numerous Dutch nurserymen in the past month, it’s apparent they are very anxious that it be kept out of the Netherlands. Many reputable nurseries have taken action and changed
suppliers or dropped lines altogether.

ERADICATION OR CONTAINMENT?

Thinking about possible control of the disease, it helps to consider it as part of the disease triangle: pathogen, vector, and host. By removing any one of the corners it is destabilized and chances of successful eradication increase.

The vector that spreads the disease is an insect like the native cuckoo spit. The insects can spread the disease as soon as they come in contact with it. Removing the vector is difficult but some continental growers are beginning to use netting/screens on tunnels ends. Control of the pathogen is effectively impossible. Control of hosts means the destruction of nearby susceptible species. Future spread could be reduced by making careful selections
of non-susceptible genus, species and even varieties. This has already happened with grapes and no doubt recommended ornamental varieties will develop in time.

Where eradication becomes impossible, containment is the next option. Corsica and the Balearic islands now join the Italian region of Apulia where the bacteria has established beyond the possibility of eradication; they are now working on containment. The EU has sanctioned control measures here that still include the destruction of all infected plants. Testing for the disease here is more focused to protect plants of cultural significance and plant trade.

THE FUTURE

Invasive and destructive species are nothing new to Ireland. In reality, we have been less impacted than many other countries, but change is constant and new threats will always be on the horizon. German nursery stock advisor Dr. Heinrich Loesing says, “There will always be new diseases, we have survived previous ones and we need to prepare for the next ones as they will arrive”. As an island with a natural water barrier, I think we could plan and hope for a better outcome. Dr. Richard O’Hanlon AFBI NI recently made a presentation to The Society of Irish Foresters highlighting threats to our forests. Of the known pests and disease that pose threats Phytophthora as a species appears to be growing and many forms are highly pathogenic. Dr. O’Hanlon’s list of incoming threats on the near horizon includes acute oak decline, Asian longhorn beetle, Castanea blight, the great spruce bark beetle, Phytophthora austrocedri (attacks juniper), Phytophthora siskiyouensis (attacks alder), and oak processionary moth. There are more threats out there, just a little further away e.g. emerald ash borer. Their threat is serious but not on our doorstep just yet.
Speaking at the recent Teagasc nursery stock seminar, Graham Spencer (Plants for Europe) strongly recommended growers talk to their neighbours to ask, “What are you
doing to stop the disease coming into your nursery and consequently infecting mine?”. We have a clean, green image on our island – we can do a lot ourselves to protect it. As the saying goes, you must be the change you want to see in the world.

More information and factsheets can be found here:
DAFM www.agriculture.gov.ie
Teagasc www.teagasc.ie
Richard O’hanlon www.rohanlon.org
EU PLANT HEALTH AND BIOSECURITY
https://ec.europa.eu/food/plant/plant_health_biosecurity/legislation/emergency_measures/xylellafastidiosa/latest-developments_en
EUROPEAN FOOD SAFETY AUTHORITY (EFSA)
http://www.efsa.europa.eu/en/topics/topic/xylellafastidiosa

Thanks to DAFM for contributions on updated controls. ✽

DóNALL FLANAGAN has been teaching with Teagasc for a number of years. Before this, he worked with Dun Laoghaire Rathdown Co Co Parks Department, landscaping and in protected crops. He will be based with the Teagasc Horticulture Development Department in Ashtown, serving the nursery stock and ornamental sector. Dónall Flanagan, Ashtown Food Research Centre Mobile: 087 703 5823; Tel: 076 111 4021 Email: donall.flanagan@teagasc.ie