Most of the local, short-range (mostly less than 5-10 m) tree-to-tree movement of inoculum within infested stands is accomplished by rain splash, assisted by wind (Davidson and others 2005). P. ramorum is spread over larger distances by a number of agents, each of which is capable of acting over various spatial scales (table 1-6). Most of these long-distance transport methods can only deliver low numbers of spores to a given target. Hence, spores transported long distances may initiate foliar infections, but are unlikely to be present in numbers required to initiate trunk cankers.
Table 1-6—Agents that may be involved in long range dispersal of Phytophthora ramorum
| Agent | Effective range | Notes |
|---|---|---|
| Human activities1 | Local2 to many kilometers to global | Many mechanisms are known, ranging from transport of infested soil on shoes and tires, transport of infested plant materials (e.g., bay leaves) via vehicles, and movement of infected nursery stock (local to international) in commerce. Human activities were responsible for the original transport of P. ramorum to California |
| Wind | Local to hundreds of meters, rarely kilometers | In some topographic situations, water drops containing sporangia may be transported tens to hundreds of meters (Turner and others 2008). Turbulent airflow may occasionally transport spores greater distances, up to several kilometers (Hansen 2008). Infected leaves or leaf fragments sent aloft by strong winds may be transported tens to perhaps hundreds of meters. |
| Watercourses | Local to kilometers | During the rainy season, inoculum can enter watercourses directly from overhead foliage or from surface runoff below infected foliage. Inoculum can also be produced in both rainy and dry periods on infected bay leaves that fall into watercourses. P. ramorum has been detected kilometers downstream from known inoculum sources on land (Davidson and others 2005). Inoculum has been found in both small streams and in sizeable rivers. Inoculum in watercourses may be splashed directly onto low hanging foliage, moved via animals that visit the water, applied to soil or plants when water is pumped for irrigation, or be deposited on foliage during flood events. |
| Other vectors | Local to kilometers (?) | Possible movement by animal vectors is very difficult to document and may occur only rarely, but could explain anomalous spread occasionally seen in local and longer-range (kilometers) situations. infested soil may be moved by vertebrate hoofs. Birds or rodents may transport inoculum on their bodies or via collected nest construction materials. Insects have not been shown to vector P. ramorum, but snails feeding on infested debris can excrete viable P. ramorum spores (Parke and others 2008). |
1Transport by humans is well documented. Conclusive data on the other modes of long-distance dispersal are currently lacking, though observations and some data suggest that they are possible.
2Local range is considered to be about 10-100 meters.