First Report of Phytophthora ramorum causing Japanese Larch dieback in France N Schenck, C Saurat, C Guinet, C Fourrier-Jeandel, L Roche, A Bouvet, … Plant Disease

Abstract

Phytophthora ramorum Werres, De Cock & Man in’t Veld, an oomycete known in the USA as the causal agent of Sudden Oak Death, has spread across Europe since the early 2000s. It is responsible for damage and death to a wide range of plant species, including mature trees. In 2009 it was identified on Japanese larch (Larix kaempferi) in South-West England (Webber et al., 2010) and since, it has caused severe damages and losses to Larix spp. in the United Kingdom and the Republic of Ireland. There are two lineages of the oomycete EU1 and EU2 found in Europe (King et al., 2015), EU2 being the more aggressive. The symptoms on larch include necrosis and loss of needles, wilting of shoots, dieback of branches and death, often with abundant resin bleeding on trunks and branches. As sporulating hosts, Larix spp. may disperse P. ramorum over long distances. In May 2017, wilting, yellowing/reddening needles and branch mortality was observed on mature Larix kaempferi (about fifty years old) in the forest of Saint-Cadou, Finistère, in the far North western part of France (3° 59’ 49.2’’ W ; 48° 22’ 22.4’’ N). Approximately, 4027% of the trees were affected in May, and 42% later in September 2017. The presence of P. ramorum was suspected, and was first confirmed by testing samples collected from trunks and branches with necrosis and resin bleeds, using the specific conventional PCR method developed by Ioos et al. (2006). The oomycete was also isolated in pure culture, using a Phytophthora selective medium (PARB[H]). The features observed, such as a coralloid mycelium, the presence of numerous, thin-walled chlamydospores (up to 75 µm large) and deciduous, semi-papillate sporangia arranged in clusters, matched those reported for P. ramorum . In June 2017, the presence of P. ramorum was confirmed in another larch stand in Hanvec, Finistère (4° 12’ 45.0” W ; 48° 20’ 10.8” N), using the same identification techniques. In this stand, the prevalence was not precisely estimated, but was deemed much lower than in Saint-Cadou. Based on the analysis of Cox1 partial sequence and the PCR-RFLP pattern described by Van Poucke et al. (2012) on Cox1, the P. ramorum isolates collected in these two forests could be assigned to the EU1 lineage. This is the first report of P. ramorum affecting Japanese larch in France and in mainland Europe. Until now it had only been detected on shrubs in nurseries, green spaces, and in rare circumstances in the natural environment on understory vegetation (rhododendron) in Normandy and Brittany, but not in the vicinity of the infected larch stands. The presence of this pathogen in the natural environment represents a major threat for larch trees, but also for the other potential forest host trees in this region, such as sweet chestnut and might have a severe impact on both forest and ornamental tree species. Research is in progress to learn more about this outbreak, the possible origin of the inoculum, the extension of the disease and its progression.