Phytophthora ramorum (Sudden Oak Death (SOD)).
Authors: Garbelotto Matteo , Frankel Susan J. 
Published: June 2021

Abstract

Phytophthora ramorum is considered an invasive species due to its ability to spread, persist, and reproduce in new environments. Its rapid life-cycle, propensity to reproduce asexually and splash dispersal via windblown rain, plus its ability to survive through harsh climatic conditions, are elements favouring this species' potential invasiveness. Spread potential in forests has been elucidated by several studies in California and Oregon employing population genetics approaches. Results have consistently shown that scale of spread of naturalized endemic pathogen populations in natural ecosystems is limited to a few hundred metres and, occasionally, during extremely wet years, spread may reach a few (3-5) km (Mascheretti et al., 2008; Mascheretti et al., 2009; Eyre et al., 2013). Spread events at scales larger than those reported above appear to be associated either with the movement of infected plant parts, normally from large wild infestations, or with the introduction of infected plants, normally from infested ornamental nursery plant stock (Croucher et al., 2013). Spread scales from the hundreds of metres to the few kilometres apply to pathways that involve only foliar hosts, in particular California bay laurels and tanoaks, and are clearly positively correlated with rainfall (Eyre et al., 2013). However, spread from foliar hosts such as California bay laurels, tanoaks and ornamental rhododendrons to stem hosts such as oaks and tanoaks occur at the much lower scale of 10 to 20 metres and are strongly associated with the occurrence of episodic and above average rainy years (Cobb et al., 2012; Garbelotto et al., 2017). Given the limited spatial scale of dispersal of P. ramorum, its spread is strongly driven by structure and composition of individual forest stands and is projected to increase as the density of infectious foliar hosts increases (Cobb et al., 2010; Meentemeyer et al., 2015). Monocultures of Japanese larch in the UK, stands with high proportion of tanoaks in Oregon and California, and oak woodlands with an abundance of California bay laurels have all been the hardest hit systems. Presence of contiguous forests (Condeso and Meentemeyer, 2007), genetics of host populations (Dodd et al., 2005; Hayden et al., 2011), microclimate (Anacker et al., 2008; DiLeo et al., 2014) and climate (Meentemeyer et al., 2004; Venette and Cohen, 2006; Ireland et al., 2013; Meentemeyer et al., 2015) are all know to drive the spread in ecosystems invaded by P. ramorum. In spite of the theoretical tolerance of the pathogen to both high and low temperatures, models validated by extensive field sampling in regions infested by NA1 populations indicate high maximum temperatures strongly limit the spread of the pathogen (Meentemeyer et al., 2015) and may even cause significant reversion from positive to negative infection status in foliar hosts (Lione et al., 2017). High temperatures have also been shown to make water populations of the pathogen not viable (Eyre et al., 2015). In the lab, exposure of Petri dishes to 55°C for 1 hour, to 45°C for 4 hours or to 40°C for 24 hours has blocked pathogen growth (Swain et al., 2006), but survival of the pathogen has been reported up to 1 week at 55°C for infected California bay laurel leaves (Harnik et al., 2004), and in other trials the pathogen has been shown to survive in pant tissue between 40°C (2 days) and -20°C (4 days) (Tooley et al., 2008). Finally, the pathogen's broad host range on popular, nursery grown, ornamental plants, and the non-lethal, nondescript nature of the disease on most of the foliar hosts allows for long-term dispersal.