Is the cure worse than the disease?
The UK is bracing itself for a serious plant disease, but we should be wary of applying the ‘slash and burn’ policy being used in parts of Europe
DEFRA, the RHS and the Forestry Commission have been issuing alerts about a disease that has not yet been discovered in the UK, but which, if it spread here, could pose a severe threat to our wild and cultivated flora. A bacterium, Xylella fastidiosa colonises vessels in the tissue xylem, through which plants transport water and nutrients.
This can result in stunting, wilting, withering, leaf scorch (dehydrated brown areas on foliage that ought to be fresh), premature leaf-fall, desiccation of buds and fruit and die-back of stems and branches. Affected plants may perish. Xylella inflicts this damage on a wide range of trees and shrubs, among them many valued for ornament and fruit.
It can also infect herbaceous species, which it leaves largely free of symptoms, but not of taint, as they become reservoirs of the pathogen and so potential sources of contagion.
The disease is not necessarily fatal for woody plants. Some seem able to live with it, successively replacing branches that die back and especially kinds that are stout-trunked, multistemmed or suckering. The victims’ regeneration and survival rates have been shown to improve with enhanced cultivation—feeding, watering, pruning, ameliorating the soil and treating the other diseases to which debilitated plants become susceptible.
However, no cure has yet been found for Xylella itself, nothing that will purge it from a plant’s system. The one small mercy is that the disease isn’t borne by wind, water or far-roving creatures. Its transmitters (vectors in plant pathology parlance) are cicadas, spittle bugs, leaf-hoppers, froghoppers and other sap-sucking insects that aren’t great travellers.
I remember being shown grape vines with Xylella on a visit to California in 1992. Since then, its various strains have progressed through the USA, attacking a broad spectrum of species. Apart from one case in Taiwan, it was believed confined to the Americas until 2013, when it was found to be responsible for ravaging olive groves in southern Italy.
Subsequently, it has been reported on a range of hosts in Corsica, the Balearics, mainland Spain, France and Germany. The EU response has been predictably authoritarian.
Scant regard is paid to growers who find that the disease doesn’t always prove fatal if one gives its sufferers extra care. Still less regard is paid to those who suspect that, as in the USA, we may just have to put up with Xylella, seeing it not so much as the green Black Death as a sad fact of life. Instead, con-
The disease doesn’t always prove fatal if plants get extra care
trol and prevention are the order of the day.
Of the measures proposed, two seem sensible: increased inspection of cultivated stock and stricter health-passporting for plants carried into and within the EU. That said, they’ll inevitably add to the cost and bureaucracy faced by suppliers and buyers and there’s no guarantee that they’ll be effective against a disease that can spread by other routes and that may, indeed, currently be far more widespread than is supposed.
Although it’s likely that such efforts will be shutting the stable door after the horse has bolted, it’s certain that other measures are already discarding a great many babies with the bathwater. In parts of southern Europe,
Xylella’s vectors have been subjected to mass insecticide, action that has turned a local, if deleterious, plant disease into an ecological disaster. Pity those Spanish groves where the cicada has sung its last and the nightingale starves. And it doesn’t even work: the woods decay, the fauna falls, but
Xylella endures. Equally unsound is the policy of fighting the disease by eradicating everything that might possibly succumb to it. Often, the destruction of infected plants will be the best practice, but it’s another thing entirely to condemn all the symptomless and very probably uninfected examples in the vicinity of any species in which Xylella has ever been found.
Two facts need to be drummed into would-be slashers-and-burners. First, as Defra says, the disease’s ‘incidence is highly sporadic and, often, trees surrounding a severely affected tree may remain free of the disease.’ Second, American scientists have observed that individuals and varieties of some Xylella-prone species appear to be resistant to it. Such resilience offers potential lifelines, but plants possessing it are likely to be consigned, without prior investigation, to the indiscriminate inferno that Europe calls a firewall.
Rather than gold-plating these measures, the UK Government proposes recasting them in solid 24-carat gold. Infected plants would be destroyed, but so would be all plants within a 100m (110-yard) radius of them that are capable of becoming infected. Given Xylella’s diverse, long and fast-growing list of known hosts, this would amount to razing parks, sizeable gardens and the greenery of whole neighbourhoods.
Proposals in respect of our horticultural industry are characterised by a cure-by-kill zealotry that recalls the footand-mouth tragedy. Restrictions on the movement of plants would be imposed for five years on any nurseries and garden centres within a 5km (threemile) radius of an outbreak.
Some imagine that Xylella will boost the UK nursery industry as, in the face of stricter plant-passporting, Continental imports will decline and we will buy British, but the above proposals could massively outweigh any such gain.
Moreover, UK growers know that, if their industry is to flourish as it deserves, they’ll need not only to continue receiving plants from abroad for development, but also, and above all, to sell vastly more of their own output into overseas markets. The costs and other difficulties associated with increased biosecurity would affect them as gravely as any foreign competitor or, more gravely, as they’d be starting from a weaker position, having enjoyed nothing like the governmental support that some EU member states give to their horticultural sectors.
We should be buying British in any event. As for the presumption that homegrown plants are Xylella-free, I’m afraid it probably won’t stay safe for long. The bacterium may be here already, but undetected. Its symptoms closely resemble those of several common diseases (something to bear in mind as the authorities call for citizen vigilance).
If not here yet, it will be: no free nation could tolerate or afford such controls as would keep it out—not even an island nation and least of all one whose renewed mantra is ‘worldwide trade’. When its presence is confirmed, we’ll need sensible debate and action in which the cure isn’t deadlier than the malady.
We’ve been somewhere like this before, and not long ago, when the serious diseases Phytophthora ramorum and Ash Die-back began to spread in Great Britain. With both, Country Life counselled against alarmist reactions and destructively draconian proposals. In both cases, time is proving us right.