Larch is the third most important conifer species in the UK, accounting for 10% of conifer plantations. However timber forecasting models expect that due to the effect of the invasive pathogen Phytophthora ramorum since 2009, future restocking trends will result in larch accounting for only 0.5% of conifer area in Wales and 2% in England and Scotland. Research from a collaboration between the Department of Biology and Forest Research show how certain climatic conditions affect the spread of the disease and could help predict future disease expansions, informing the development of management strategies for larch in the UK. Lead author Dr Heather Dun said,
''The loss of larch as a viable commercial species in the UK increases the vulnerability of the forestry industry, which already relies on a small number of timber-producing species.''
Phytophthora ramorum is an invasive oomycete pathogen that has been causing significant mortality of larch trees (Larix spp.) in the United Kingdom since 2009. This is the first multiyear study of the natural infection processes of P. ramorum on Japanese larch (Larix kaempferi) and the factors influencing disease progression. The research involved field surveys in south-west Scotland, which suffered an extensive epidemic in spring 2013 allowing detailed examination of how P. ramorum infects individual trees and spreads across a site over an extended time period. In addition, a marked expansion of the disease in spring 2018 enabled scientists to consider how environmental conditions may influence outbreaks.
The study spanned three growth seasons across multiple sites, enabling a better understanding of the processes and patterns of progression of infection. Observations suggest that initial shoot infections take place in autumn, possibly by the pathogen extending from infected needles into the shoot at bud points. Rapid lesion expansion then occurs the following spring.
Because summer precipitation was higher than the 19-year average in the year preceding each of the two disease expansion years in south-west Scotland, the scientists hypothesize that heavy rain might have promoted sporulation of P. ramorum from infected needle litter and led to early infection of the needles. Other environmental conditions such as low March temperatures, easterly storms, and late season snow likely aided infection by stressing the trees. Co-author Prof. John Mackay says of the research
''The outcomes of the study have been disseminated to forest managers and we hope they will be able to predict future disease expansions following particularly high rainfall over the preceding summer. This could result in a planned increase in capacity for felling and timber processing at the best time.''
John added that there is also interest from the forestry industry in finding resistance to P. ramorum and developing a breeding programme, which would benefit from the better understanding of the natural infection processes that the current study has provided.
To read more about this research, published in Plant Pathology, visit: bsppjournals.onlinelibrary.wiley.com/doi/10.1111/ppa.13821.
