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Management of larch stands affected by Phytophthora ramorum

Implications for the establishment and spread of Heterobasidion annosum

This note considers the risk of infection by Heterobasidion annosum (H. annosum) fungus associated with creating larch stumps and chemically killing standing larch trees.

Recomendations:

  • serious consideration be given to applying protective treatment to all larch stumps created during phytosanitary fellings
  • chemical killing of larch infected by Phytophthora ramorum (P. ramorum) should currently be reserved for exceptional cases

Summary

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. By October 2013 about 16,000 hectares of larch trees in the UK (approximately 12% of the standing forest stock of the species concerned) had been felled or were under notice to be felled due to P. ramorum infection.

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 and Western hemlock, is currently recommended. However, unless the stumps created during the clearance of infected larch stands are treated to prevent infection by H. annosum, the future employment of such areas for growing 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 utilised for productive forestry in the UK. The use of stump treatment in GB forests has always been at the discretion of the owner / manager, so it is therefore possible that areas might have been left untreated.

It is strongly recommended that forest managers responsible for phytosanitary fellings who are not routinely employing stump treatment at present should give serious consideration to doing so in future. 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, more consideration has inevitably 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 control of 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 post-treatment is currently under investigation. There is also the possibility that, if herbicide treatment was employed upon any scale, the number of moribund larches created could additionally provide a breeding resource for large larch bark beetle (Ips cembrae) (where present), and result in 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, it is recommended that such treatment be reserved for exceptional cases where felling is not feasible, and 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 creation of untreated larch stumps and with chemical killing of trees

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 on the behaviour of the fungus in relation to certain hosts remains fragmentary. The majority of experimental studies on H. annosum in Europe have concentrated upon species of pine and spruce (and, to a lesser extent, fir), and data relating to infection of 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, a limited investigation of first-rotation crops of hybrid, European and Japanese larch in southern Sweden which had been thinned without the use of stump treatment 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), supporting the view that larch stumps are highly susceptible to infection by H. annosum.

b) Infection of chemically killed trees by H. annosum

There is evidence indicating that herbicide treatment of standing trees can result in enhanced susceptibility to infection by H. annosum. It originates largely from surveys and investigations of Norway spruce and Sitka spruce crops in Northern Ireland subjected to chemical thinning by the “hack and squirt” method during the 1980s (Schaible, 1993; Johnston et al., 2003). The appearance of fruiting bodies on a proportion of stems only 2 to 3 years after treatment, and evidence that fruiting by H. annosum was occurring within most treated compartments within 4 years of herbicide application (whilst no fruiting of H. annosum was noted in equivalent stands which had been mechanically respaced), led to the suspension of chemical thinning in Northern Ireland in late 1991. Controlled experiments conducted by Seaby & Schaible (2000) subsequently 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). Whilst application of urea to stem cuts treated with glyphosate resulted in some reduction in colonisation by H. annosum, 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 currently be unsafe to presume that herbicide-treated stems of larch will display lower susceptibility to infection and colonisation by H. annosum than those of Picea sitchensis.

More information in relation to the efficacy of different herbicide treatments for killing of standing trees, and their effects on the susceptibility of treated stems to infection by H. annosum, is clearly required. To this end, a new Forest Research project has been initiated with financial support from Forest Enterprise and FC Scotland. The project will investigate the impacts of different tree-killing treatments (glyphosate applied either via “hack and squirt” or Ecoplugs, together with an improved ring-barking treatment developed by Trees for Life) on subsequent infection by H. annosum in larch and Scots pine.

Steve Hendry & Joan Webber, Forest Research

Further Guidance and Advice

References

Johnston D, Cameron AD & Woodward S (2003). Incidence of Heterobasidion annosum infection in chemically thinned stands of Norway spruce (Picea abies (L.) Karst.). Forestry 76: 363-366

Hendry SJ (2007). Stump treatment – your decision support system. FC internal guidance document.

Redfern DB, Pratt JE, Gregory SC & MacAskill GA (2001). Natural infection of Sitka spruce thinning stumps in Britain by spores of Heterobasidion annosum and long-term survival of the fungus. Forestry 74: 53-71.

Rishbeth J (1959). Stump protection against Fomes annosus I Stump treatment with creosote. Annals of Applied Biology 47: 519-528.

Rönnberg J, Mårtensson S & Berglund, M (2007). Incidence of butt rot in first rotation Larix stands and justification for stump treatment. Proceedings of the 12th International Conference on Root and Butt Rots of Forest Trees. p. 86-89.

Schaible R. (1993). Heterobasidion annosum in chemically thinned plantations. Forest Research Note 1/93. Department of Agriculture for Northern Ireland, Belfast. 2pp.

Seaby DA & Schaible RC (2000). Preventing Heterobasidion annosum from colonizing chemically thinned Sitka spruce stems. Forestry 73: 351-359.

Last updated: 21st June 2016