Environmental Factors Driving the Recovery of Bay Laurels from Phytophthora ramorum Infections: An Application of Numerical Ecology to Citizen Science

Abstract

Phytophthora ramorum is an alien and invasive plant pathogen threatening forest ecosystems in Western North America, where it can cause both lethal and non-lethal diseases. While the mechanisms underlying the establishment and spread of P. ramorum have been elucidated, this is the first attempt to investigate the environmental factors driving the recovery of bay laurel, the main transmissive host of the pathogen. Based on a large dataset gathered from a citizen science program, an algorithm was designed, tested, and run to detect and geolocate recovered trees. Approximately 32% of infected bay laurels recovered in the time period between 2005 and 2015. Monte Carlo simulations pointed out the robustness of such estimates, and the algorithm achieved an 85% average rate of correct classification. The association between recovery and climatic, topographic, and ecological factors was assessed through a numerical ecology approach mostly based on binary logistic regressions. Significant (p < 0.05) coefficients and the information criteria of the models showed that the probability of bay laurel recovery increases in association with high temperatures and low precipitation levels, mostly in flat areas. Results suggest that aridity might be a key driver boosting the recovery of bay laurels from P. ramorum infections.

The Destructive Tree Pathogen Phytophthora ramorum Originates from the Laurosilva Forests of East Asia

“…Taking into account P. ramorum ’s dependence on rain and fog events for dispersal and infection and its etiology on infected larch, rhododendron and California bay laurel in Europe and North America [ 99 , 100 , 101 , 102 , 103 , 104 , 105 ], we suggest in the Asian laurosilva forests P. ramorum may specialise in colonising seasonally juvenile and senescing foliage during the monsoon rains, with new infections initiated by inoculum from quiescent infections on attached leaves or from the leaf litter. As demonstrated by our isolations, P. ramorum is also a successful coloniser of freshly fallen leaves in the forest streams.…”

Canker and decline diseases caused by soil- and airborne Phytophthora species in forests and woodlands

“…Foliar infections, in contrast to bark cankers, do not usually result in plant death, thus plant species that have susceptible foliage only are not eliminated, enabling continuous inoculum production ( Garbelotto et al 2017 , Lione et al 2017 ). Due to its common occurrence in coastal forests and the high sporulation capacity of P. ramorum on its leaves, California bay laurel is the main driver of inoculum build-up and, hence, the epidemic in California, whereas infected tanoak leaves are the most important source of inoculum in south-western Oregon ( Davidson et al 2005 , Rizzo et al 2002 , 2005 , Hansen et al 2008 , Peterson et al 2015 ).…”

The role of passive surveillance and citizen science in plant health

“…Since 2008 a project in California and Oregon has run “SOD blitzes”, these events trained volunteers to identify P. ramorum symptoms and use a mobile mapping tool. Volunteers were then engaged to survey local forests and urban parks, with the resulting data improving the accuracy of predictive maps of the current distribution (Lione et al 2017 ). Citizen science has also been used to monitor distribution, abundance, of pest species such as the horse-chestnut leaf-miner ( Cameraria ohridella ) in the UK (Pocock and Evans 2014 ).…”