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Are grey squirrels implicated in spreading P. ramorum?

Is the spread of ramorum disease linked to grey squirrels? 

Helicopter aerial surveys are being used to detect symptoms of dieback or crown colour change in trees. The cause of the symptoms then has to be established with ground surveys and, in the case of P. ramorum, is usually then confirmed by laboratory analysis.
The surveys have concentrated mainly on larch in the western region of Britain, where the climatic conditions favour P. ramorum. Ground surveys have confirmed that much of the dieback detected in larch through the aerial surveys is due to squirrel bark stripping damage.
No sites were confirmed infected by P. ramorum as well as damaged by squirrels. On some sites there are suggestions of the co-occurrence of squirrel damage and Phytophthora infection. However, this does not demonstrate cause and effect, and we should be very cautious about presuming or suggesting a link between damage by squirrels (grey or red) and Phytophthora disease.
The results from the aerial surveys demonstrate the usefulness of this as a tool to identify areas of dieback. They have identified the presence of extensive squirrel damage on larch in some areas, but because the surveys are regionally focused, this is not a country-wide view of where squirrel damage to larch is occurring.

 
P. ramorum entering trees via squirrel damage wounds

P. ramorum does not need to infect via wounds. In the field the spores of P. ramorum are produced on certain plants species known as ‘sporulating hosts’, i.e. those infected plants which produce P. ramorum spores. Rhododendron and larch are significant sporulating hosts. Infected larch needles generate huge numbers of P. ramorum spores just before the needles are shed in autumn, but none appears to be produced in or on infected bark. Infection through bark results from contact with hundreds, perhaps even thousands, of spores. This is only likely to occur when the spores are washed from infected needles in rainfall and collected in water pockets on twigs or thicker bark surfaces. Interestingly, Phytophthora spores are thought to act together to achieve infection: it is a group action, and the number of spores in the group has to be large enough to succeed.

Tree damage by grey squirrels and susceptibility to infection

The only time that we have had evidence of wounds associated with Phytophthora infection has been with birch (growing under or close to infected larch), when blaze marks placed on the birch trees were the clear point of entry for P. ramorum as the lesion spread out from the blaze wound. However, for wounds to act as entry points for pathogens they almost always have to be very fresh (i.e. only hours or maybe a couple of days old) before the build-up of anti-microbial substances makes them unsuitable for pathogen invasion. Therefore, in the case of the infected birch, the blaze wounds would have been made during the time of sporulation by P. ramorum on larch, and very probably coinciding with a rainy period during autumn.


Grey squirrel damage on larch and access to Phytophthora spores

We have no evidence that suggests that squirrel damage gives good access to Phytophthora spores on larch. It is probably the reverse, because wounding to larch results in rapid and copious resin flow, which is likely to make it difficult for spore germination and infection to occur.  We also know that P. ramorum does not require a wound to infect, and is effective at infecting trees with intact bark. This tends to occur most frequently in thinner bark species (e.g. beech and grand fir) or on parts of trees where the bark is thinner (at the top or on smaller-diameter branches or twigs), but infection is not exclusively through thin bark. 
Our observations suggest that infection on larch occurs in twigs or smaller-diameter branches and then, once in the phloem tissue, P. ramorum progresses down the twigs and branches to the main stems. Sometimes there are discrete infection points on major branches or even trunks. However, these often coincide with branch forks or on the underside of branches where spores are likely to have been concentrated on to parts of the tree where water has collected and infection has resulted.

Link between bark stripping on larch and liability to P. ramorum infection

Larch trees suffering from squirrel bark stripping could become more susceptible to 'normal routes' of infection, particularly if the damage is severe and results in extensive resinosis. This is becuase this can exhaust trees, thereby making them succumb to infection more readily. However, multiple infections on individual trees caused by P. ramorum are usually the result of  high levels of spore inoculum generated from surrounding infected foliage. In general, mature trees can survive considerable bark damage (and heavy root loss) before exhibiting stress (other than girdling of smaller stems). 

P. ramorum spread by grey squirrels

To be involved in the spread of spores from one tree to another in a way that results in infections, squirrels would have to chew or brush against spores on needles, and transfer sufficient quantities to result in bark infection. Squirrels might well pick up spores on their coats, especially during wet or misty periods when sporulation occurs on needles. However, that does not mean that the infection cycle is completed and the spores reach a susceptible host and cause infection.

Phytophthora sporangia are short-lived and do not persist long in a dry environment. They would have to be rubbed on to suitable host material to result in infection, or at the very least be washed on to the soil, where they could then spread via water films and watercourses. Timing, susceptibility of a tree or tree part, and the number of spores and how they are delivered, all play a part in successful infection. 

Link between beech damage by squirrels and P. ramorum infection

Despite the fact that beech is heavily damaged by grey squirrels and does not have a resin response (hence wounds should theoretically be easier to infect) we do not see significant levels of P. ramorum on beech. When beech trees do become infected, the predisposing factor is their closeness to infected rhododendron or larch, and squirrel damage has never shown any sign of making beech trees more likely to suffer P. ramorum infection.

Spread of P. ramorum by other animals   

It is generally accepted that vehicles, people, birds and mammals might play a part in the spread of P. ramorum, hence the recommended biosecurity protocols when visiting woodlands. The Forestry Commission funded a study by the British Trust for Ornithology and the University of Aberdeen to see whether migratory birds spread Phytophthora, particularly if they are roosting on plants that are infected, sporulating hosts. The study involved swabbing the flanks and feet of birds to try to detect any Phytophthora present. No phytophthora was isolated from any of the swabbed birds. Studies on other bird species in the USA have been inconclusive.

Grey squirrel control to prevent the spread of disease

Grey squirrels remain a serious threat to our woodlands in their own right, and we encourage woodland managers to control grey squirrel populations on their land for the full range of benefits that control can help to achieve. These benefits include red squirrel and biodiversity conservation and timber crop protection, as well as possibly reducing the number of entry pathways for tree diseases.

Summary

Research by Forest Research, Fera and researchers in the USA shows that P. ramorum is adapted for very effective spread via moist winds and water, both locally (within ~100m) and over distances of 1-3 km (less frequent). Wounding is not needed for infection: indeed it might be counter-productive, particularly with larch. Animal and bird studies in the USA to confirm spread followed by infection have been inconclusive to date. Spread via animals moving Phytophthora-infested soil and/or leaf litter seems likely, because the spores would be protected in a moist matrix, and there is some anecdotal evidence to support this.

B  Mayle & J Webber

Last updated: 10 February 2012

Last updated: 19th September 2017
Grey squirrel on a birch tree