The invasive, fungal-like Phytophthora ramorum causes Sudden Oak Death (SOD), which is responsible for destroying tens of thousands of oak trees in the US. SOD has reached epidemic levels in the coastal forests of northern California and southwestern Oregon, killing large numbers of oak trees and infecting a wide range of other host plants. Analysis of landscape changes seen in aerial photographs of northern California, over a 58-year period revealed a significant increase in woodland areas. This forest expansion has led to increased numbers of bay laurel trees, which act as hosts for P. ramorum, resulting in an increase in the occurrence and spread of SOD.
In a unique study the influence of change in land-use on the spread of this disease has been assessed. By examining aerial photographs of northern California taken in 1942 and 2000, researchers observed that, in 58 years, oak woodland areas had increased by 25 per cent, while grassland and chaparral (shrubland) areas had decreased by 34 per cent and 51 per cent, respectively.
Forest expansion may be due to current US policy, which aims to suppress forest fires in order to protect wildlife and human property. Yet, forests fires are a natural part of the local ecosystem, essential for regeneration and cleansing the woodlands of parasitic insects and disease.
A feature of the observed forest expansion was a change in land-cover including an increase in the number of bay laurel trees. Non-lethal P. ramorum infection of this host plant leads to spore production and transmission of the disease to other trees. In 2000, forests also had a higher density of trees. This reduces the temperature of the forest understory (the area immediately above the forest floor which consists of tree trunks, saplings and small ground plants), by shading the forest floor from the sun. Researchers found that disease levels were highest in these areas, where cooler conditions favour disease spread. This suggests that management strategies such as low-intensity prescribed fire and thinning of bay laurel in infected and susceptible sites, should be explored.
Trees in Europe are susceptible to P. ramorum and infected trees have been identified in the UK and in the Netherlands1. A pest risk analysis for P. ramorum has been developed2 and this will be updated based on the findings of the RAPRA project3, aiding development of strategies for controlling potential outbreaks in Europe.