Leaf susceptibility of Macaronesian laurel forest species to Phytophthora ramorum

Phytophthora ramorum (Pr) is an invasive oomycete in Europe and North America and the causal agent of sudden oak death (SOD), which occurs along the coastal fog belt of California and southwestern Oregon, and it also causes sudden larch death in the UK. The Macaronesian laurel forest (MLF), a relict subtropical evergreen forest of the North Atlantic islands, shares climatic and some taxonomic affinities with those areas affected by SOD. To assess the disease risk, we tested the foliage susceptibility of MLF species and their capacity to sustain Pr sporulation and compared the climatic suitability with other areas where the pathogen is established. Detached leaves of 15 species were inoculated with zoospores and mycelium (through wounding) with five Pr isolates belonging to the EU1 and NA1 clonal lineages. MLF species showed diverse responses to Pr, ranging from extensive necroses on Viburnum tinus to asymptomatic sporulation on Picconia excelsa. Eleven species developed necrotic lesions to different degrees through zoospore inoculation while this increased to 13 species through wound treatment. Overall, small necrotic lesions (i.e. tolerance) were predominant, but Pr was rather aggressive to V. tinus, Arbutus canariensis and Ilex canariensis. Although the mean sporangial production was generally low (25-201 sporangia) in all species, the number of sporangia per leaf in five MLF species was similar to those reported for Umbellularia californica, a key host driving the SOD epidemics in California. Climatic suitability indexes in MLF areas were similar to those where SOD is found in California. Our results indicate a moderate to high risk of Pr establishment if the pathogen is introduced in the MLF.

Also available from bioRxiv at: https://doi.org/10.1101/2023.07.15.549153