The disease risks associated with larch trees (Larix species) infected with Phytophthora ramorum (P. ramorum) do not end when the trees are felled. This is because the tree stumps left can be infected with other damaging organisms as they decompose.
Notable among these is Heterobasidion annosum (H. annosum) fungus, the fruiting bodies of which are pictured above on the trunk of a Scots pine (Pinus sylvestris).
It can infect several species of conifer trees which might be considered as replacements for larch, causing root and butt rot. In some circumstances it can kill the tree, or weaken the trunk so that it becomes vulnerable to windblow. In most cases it spoils or devalues the timber.
This means that if it is left to colonise the stumps unimpeded, H. annosum can make it difficult to establish a replacement crop of healthy conifer species on the site.
We therefore recommend that:
- forest managers seriously consider applying protective treatment to all larch stumps created during phytosanitary fellings; and
- they reserve chemical killing of larch infected by P. ramorum for exceptional cases.
The material below is adapted from a paper by Steve Hendry and Joan Webber of Forest Research. It considers the risk of infection by H. annosum associated with creating larch stumps and chemically killing standing larch trees.
H. annosum was formerly known as Fomes annosus, and the disease it causes is often still called Fomes or Fomes butt rot.
Action to control the spread of P. ramorum within and between forests in the UK since 2009 has primarily taken the form of rapid detection and felling of infected larch stands.
The choice of tree species for restocking such areas is already curtailed. This is because avoidance of planting larches, and caution in planting other conifers which are susceptible to infection by P. ramorum, such as Douglas fir (Pseudotsuga menziesii) and Western hemlock (Tsuga heterophylla), are currently recommended.
However, unless the stumps created by clearing infected larch stands are treated to prevent H. annosum infection, the future use of such areas for any productive conifer crops might be relatively short-lived. Larch stumps are highly susceptible to infection by H. annosum, and will transmit the pathogen to standing trees of most of the conifer species used for productive forestry in the UK. The use of stump treatment in British forests has always been at the discretion of the owner or manager, so it is therefore possible that areas might have been left untreated.
We therefore strongly recommend that forest managers responsible for phyto-sanitary fellings seriously consider routinely treating stumps. This is to minimise problems with the establishment of new conifer crops in the short term, and to ensure production unaffected by root and butt rot in the longer term.
As the area of larch in Britain with confirmed P. ramorum infection has increased, consideration has been given to means by which inoculum production by the pathogen might be eliminated without the need for immediate felling of infected trees. The availability of such means would address concerns over the need to fell infected larch in areas where major forest operations are either difficult or dangerous. They could also be employed to buffer roading and harvesting programmes against short-term extremes of demand driven by unpredictable annual fluctuations in disease severity.
In the absence of effective fungicide treatments for P. ramorum in forest stands, the only intervention which might substitute for felling under certain circumstances would be chemical killing of infected trees.
Whether killing would be sufficiently rapid following herbicide application to prevent an extended period of inoculum production by P. ramorum is under investigation.
It is also possible that, if herbicide treatment were employed on any scale, the number of moribund larches created could additionally provide a breeding resource for large larch bark beetle (Ips cembrae) (where present). There is a risk that this could lead to sufficiently large populations of the beetle to trigger attacks on healthy trees.
Moreover, herbicide treatment of standing spruces has been found to result in heavy infection by H. annosum, and such infection might also be expected to occur in larches which are chemically killed.
Although chemical killing of larches infected by P. ramorum should not be ruled out, we recommend that it be reserved for:
- exceptional cases where felling is not feasible; and
- cases where the risk associated with continued production of P. ramorum inoculum from infected trees over-rides the other risks highlighted above.
Evidence for the risk of H. annosum infection associated with untreated larch stumps and chemical killing
a) Infection of larch stumps by H. annosum
Although a large body of research evidence relating to the biology and ecology of H. annosum exists, information about the behaviour of the fungus in relation to certain hosts remains fragmentary. The majority of experimental studies on H. annosum in Europe have concentrated on pine and spruce species (and, to a lesser extent, firs). Data relating to larch stumps is relatively scarce.
However, two sources of information indicate a high degree of susceptibility of larch stumps in Britain to spore infection by H. annosum.
Rishbeth (1959) reported the results of experimental studies into protectant treatments of conifer stumps. He noted, in relation to an experiment carried out on a mixed stand of Douglas fir and European larch in East Anglia subject to natural infection by basidiospores of H. annosum, that
“European larch (Larix decidua Mill.) showed a high natural stump infection with Fomes annosus , which was controlled by the above-mentioned treatments, excepting the one longest delayed.”
A review of the history of second- (and occasionally third-) rotation conifer crops in Scotland which displayed a high incidence / severity of H. annosum infection revealed that the previous crop had frequently consisted of a larch species which received no prophylactic treatment against the pathogen (Hendry, unpublished). These indications of susceptibility to infection were sufficiently strong for Hendry (2007) to make the following recommendation on treatment:
“The disease risk is always high for pure pine or larch crops or for mixtures of these species. You must always treat pine and larch stumps……..”.
Subsequently, there was a limited investigation of first-rotation crops of hybrid (L. eurolepis), European and Japanese (L. kaempferi) larch in southern Sweden which had been thinned without stump treatment. It revealed that the incidence of H. annosum infection in the remaining standing trees varied from 20% to 53% at 40 to 50 years after planting (Ronnberg, Martensson & Berglund, 2008). This supported the view that larch stumps are highly susceptible to H. annosum infection.
b) H. annosum infection of chemically killed trees
There is evidence that herbicide treatment of standing trees can result in enhanced susceptibility to H. annosum infection. It originates largely from surveys and investigations of Norway spruce (Picea abies) and Sitka spruce (P. sitchensis) crops in Northern Ireland subjected to chemical thinning by the “hack and squirt” method during the 1980s (Schaible, 1993; Johnston et al., 2003).
Fruiting bodies were observed on a proportion of stems only 2 to 3 years after treatment. There was also evidence that fruiting by H. annosum was occurring within most treated compartments within 4 years of herbicide application. (By contrast, no fruiting of H. annosum was noted in equivalent stands which had been mechanically respaced). These findings led to the suspension of chemical thinning in Northern Ireland in late 1991.
Controlled experiments were subsequently conducted by Seaby & Schaible (2000). They indicated that glyphosate treatment of Sitka spruce by the “hack and squirt” method resulted in greatly enhanced colonisation of stems by H. annosum (65.7% of the cross-sectional area occupied by the fungus versus 4.5% for untreated controls). Application of urea to stem cuts treated with glyphosate resulted in some reduction in colonisation by H. annosum. However, the proportion of the cross-sectional area of the stem occupied by the fungus was still high in relation to the controls (28.4% versus 4.5%).
The susceptibility of Sitka spruce stumps to infection by H. annosum spores is variable, and post-infection colonisation of such stumps is frequently not extensive (Redfern et al., 2001). The limited evidence available (see above) therefore indicates that larch stumps are likely to be more susceptible to infection than those of Sitka spruce. It would be unsafe to presume that herbicide-treated stems of larch will display lower susceptibility to infection and colonisation by H. annosum than those of Sitka.
Our Tree Health Diagnostic and Advisory Service can help with diagnosing and treating H. annosum infection.