Tree workers have been urged to follow the recommendations of the Forestry Commission's Keep it Clean campaign to ensure they do not unwittingly spread tree diseases on clothing and tools. "Don't allow your machines to carry a disease from one site to another," biosecurity officer James Roberts told a seminar at APF 2016 at the Ragley Estate in Warwickshire (15-17 September).
Workers should instead ensure that kit is regularly sterilised. "You can get a basic biosecurity kit for £30," said Roberts. He recommended two disinfectants - CleanKill for clothing and boots; and Propeller for tools.
Giving canker stain of plane as an example of a pathogen that spreads through material passing from tools to tree wounds, he said: "On a recent trip to look at the disease in Italy, we had to wear biosecurity suits that were then burnt." For those buying trees from a nursery, he urged: "Don't be afraid to ask where they're from and what their biosecurity measures are. If one turns out to have a disease, can they track down the others?"
Explaining the context for the campaign, he said: "The number of pests is on the rise, matching the increase in global trade. Some 280 species on the UK Plant Health Risk Register are 'of concern'. There are a good number of pests and diseases that are not yet known to be here but which we are looking out for."
The commission works with the Animal & Plant Health Agency and its specialist division the Plant Health & Seeds Inspectorate. "We have teams at docks," said Roberts. "We also do a lot of surveys, from aerial surveys to monitor Phytophthora ramorum to pheromone trapping of the oak processionary moth (OPM), and we now have citizen science too in the Observatree project."
Such work has to contend with "unknown unknowns", which have so far included the discovery of a pathogen new to science, Phytophthora kernoviae, in 2003. "Existing pests and diseases may act in a novel fashion, such as P. ramorum infecting larch from 2009," said Roberts. "Or they can turn up in places you don't expect, such as Asian longhorn beetle in Kent in 2013. We thought it was too far north for it to complete its life cycle." The site near Paddock Wood "will shortly be declared free of the pest", he added.
Counter measures are hampered by the fact that "nearly all the pests and diseases of concern are at some stage incredibly small and difficult to spot", he said. "They include the sporangia of P. ramorum at less than 50(mu)m to the egg plaques of OPM, which have more than 100 eggs in a 2-3sq cm area. They can also be carried in a range of media."
Giving updates on work to counter current tree pests and diseases, his colleague tree health officer Mick Biddle said: "Based on aerial surveillance we have a felling programme for larch infected with P. ramorum - we will serve a statutory notice on the landowner. We are noticing more collateral damage on hemlock, Douglas fir, bilberry, English oak and beech. But where control measures have been implemented to the full in the Forest of Dean, there have been no new cases for four years."
The related P. siskiyouensis "has been picked up in Dorset, having been previously unknown in the UK," he said. "If you notice a bleeding alder, please notify us, though there is a P. alni too." Meanwhile, another relative, P. plurivora, has been detected in park and street trees in Cornwall. "Some have already had to be felled before the cause was known. This is possibly an up-and-coming disease, though not yet quarantine-able."
The fungus Sirococcus tsugae causes needles of some cedars to turn pink and can cause twig and branch cankers, and it has also been found in western hemlock," said Biddle. "Please let us know if you see it." Oriental chestnut gall wasp (Dryocosmus kuriphilus) has been found in London's Richmond and Greenwich Parks this year. It lays eggs in buds of sweet chestnut. "It doesn't need to mate so you only need one for an outbreak," he warned. "We are surveying across the South East for this, but it's not by itself a tree killer."
OPM remains "a growing problem in London", added Biddle. "We have put out 900 traps to catch the males, which spread out more than the females. But wider surveys in local authority and Woodland Trust land have yet to show findings." Ash dieback "is on most sites I go to where there is ash regeneration", he said. "At Reepham, Norfolk, two-to-five per cent of ash trees have been found to be asymptomatic. We are monitoring this."