Expansion of natural infection of Japanese larch by Phytophthora ramorum shows trends associated with seasonality and climate

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. Field surveys in south-west Scotland, which suffered an extensive epidemic in 2012 from a highly virulent new lineage (EU2), allowed detailed examination of how P. ramorum infects individual trees and spreads across a site over an extended time period. A marked expansion of the disease in spring 2018 allowed us to consider how environmental conditions influence outbreaks, with summer precipitation and spring storms found to be significant factors. Field observations revealed that buds on fine shoots appeared to be the primary infection sites with infection occurring between late autumn and spring, and rapid lesion extension during flushing. Infection and spread of P. ramorum were not consistent within or between sites and it is likely that site-specific factors influence the spread of infection and subsequent mortality. The climatic conditions identified in this study could help predict future disease expansions and inform the development of management strategies for larch in the UK.