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The Plantsman
Time to avert disaster
David Ingram calls for immediate action from the trade to prevent further imports of plant disease
‘... if growers seized the initiative and introduced a voluntary code of practice immediately, disaster might be averted’
Image: Tim Sandall
While governments worry about illegal immigrants, aliens of a very different kind - plant pathogens - slip into the country unnoticed. In his article (Preventing invasive pathogens: deficiencies in the system, pp54-57) Clive Brasier draws attention to the dire consequences of this for natural ecosystems and cultivated plants. Before considering the best way forward, I will summarise the scientific principles underlying the problem.
Scientific principles
Natural plant communities consist of many species and populations, including potential pathogens, all occupying different niches in space and time. During evolution some organisms have acquired the ability to cause disease in particular hosts. In Mexico, for example, where relatives of the potato evolved, the fungus Phytophthora infestans, cause of blight, can be found on wild Solanum populations. It does not, however, normally cause catastrophic epidemics, the success and survival of both host and fungus populations being ensured because each is genetically diverse: in the former for resistance to infection and in the latter for the ability to cause disease. Through co-evolution, a balance has been achieved such that each may grow and reproduce to give rise to the next generation.
Epidemics occur when circumstances intervene to upset the natural balance. In horticulture, for example, genetically uniform populations of plants are grown to facilitate display and good husbandry. If a pathogen mutates to overcome the resistance of an individual in such a population, it will then spread unchecked, for all individuals will be equally susceptible.
Alternatively, a new pathogen to which indigenous plants have no resistance may be introduced from elsewhere, as in the case of Dutch elm disease across the northern hemisphere and ramorum dieback (sudden oak death) in California and Europe, the latter probably introduced on nursery stock.
Hybrid threat
The threat from any alien species removed from its native habitat is potentially dangerous - cane toads in Australia, zebra mussels in Ireland and Japanese knotweed in Britain, for example. These dangers are massively increased if hybridisation occurs between an aggressive alien species and a relatively benign but widespread native one. This already appears to have happened with the new Phytophthora species now affecting Alnus. The problem is exacerbated with plant pathogens because they have high mutation rates and are capable of dispersing large numbers of spores rapidly over wide areas. With global change, aliens and hybrids may remain undetected for long periods, only becoming a problem with a change of climate favouring their growth and dispersal.
Action now
I endorse Brasier’s assertion that urgent action is required. At the back of our minds is the spectre of a catastrophe in the UK as great as the epidemic of jarrah die-back, caused by Phytophthora cinnamomi, that destroyed so much of the World Heritage status Gondwana-rich native vegetation of western Australia. Some may argue, irresponsibly in my view, that new plant diseases will pose such severe selection pressures that novel resistances will arise in hosts and that new equilibria between hosts and pathogens will soon be achieved. Yes, but only over enormously long evolutionary time scales. Moreover, with some new, aggressive diseases such as ramorum dieback, it is possible that eradication of a host may occur in islands such as these, with no possibility of recovery. We need to face up to the fact that time is not on our side.
No to laissez-faire
Another argument that I cannot accept is that we should regard all exotic introductions as simply part of the Darwinian process, with the weakest going to the wall and the fittest taking the natural world forward into a glorious future. If you are happy to lose all our native elms, oaks and beeches, to be replaced by exotics such as eucalyptus, so be it, but that kind of laissez-faire attitude is not for me. The natural world in the UK is too fragile to withstand the constant onslaught of introductions resulting from human folly and negligence. With the increasing international trade in plants it is time to assess whether the existing systems for preventing the introduction of alien pathogens are sufficient.
It is our problem
It might be asserted that the problem is the concern of conservationists, not gardeners. I contest this cynical view: conservation of the natural world must be the concern of us all, not something that can simply be delegated to specialists. This is clearly recognised by the Royal Horticultural Society (RHS) in its conservation and environmental guidelines for gardeners.
Moreover, garden plants are already being killed in large numbers by exotic pathogens. Also, it is likely that importers of alien pathogens on garden plants may, quite rightly, be held financially liable for the damage they cause, as chemical polluters are. In the end we shall all pay, as the cost of plants increases to compensate.
Let’s get on with it
Something must be done, urgently, but what? Brasier’s proposals are based mainly on scientific principles which the present procedures lack. Some are relatively inexpensive, such as updating risk protocols to cover pathogen genotypes, varieties and unnamed taxa, and regularly reviewing plant health policies and protocols. So let’s get on with them now. Effecting others, such as the more rapid identification of high-risk alien pathogens, and the provision of better scientific intelligence and forecasting based on intuitive, insightful and strategic research, will necessitate significantly increased funding. This will require considerable scientific pressure and political lobbying. Moreover, because of the nature of scientific research, the strategy will take a very long time to bear fruit. While lauding the approach, I fear that it may be so long-term as to be a case of closing the door after the horse has bolted.
Code needed now
Other proposals, if adopted now, would have an immediate effect: regulating all plant introductions as we do animal introductions; normally allowing the import of most plant material only in the form of meristem cultures or seeds (not entirely safe or realistic?); and encouraging local commercial propagation of plants rather than relying on imports.
These proposals would involve a major financial investment from growers, and therefore, ultimately, gardeners, as well as fundamental changes to commercial practices. Changes in legislation would also be needed and investment by government in implementing and policing the new regulations.
All this could take time but if growers seized the initiative and introduced a voluntary code of practice immediately, disaster might be averted. With the long-term reputation and financial health of the nursery trade at stake, why wait? Now there’s a challenge for the Horticultural Trades Association and the Institute of Horticulture.
Challenge for RHS
Finally, Brasier argues for a Rio-style international agreement to provide for the scientific testing of the risks posed by newly identified pathogens both in the country of origin and, more importantly, in the countries at risk. This must be on the agenda of the next Earth Summit in 2010 but that is a long time off.
So why doesn’t the RHS take a lead by organising a meeting of all interested parties to debate the issues raised by Brasier and to formulate a plan of action?
If such a meeting is not called, a unique opportunity to help protect the UK’s natural ecosystems, the horticultural trade and our gardens will have been lost, perhaps for ever.
Prof David Ingram VMH is a plant pathologist and conservationist and Master of St Catharine’s College, Cambridge
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