Transmission of Phytophthora ramorum in Mixed-Evergreen Forest in California

Abstract

During 2001 to 2003, the transmission biology of Phytophthora ramorum, the causal agent of sudden oak death, was studied in mixedevergreen forest, a common forest type in northern, coastal California. Investigation of the sources of spore production focused on coast live oak (Quercus agrifolia) and bay laurel (Umbellularia californica), dominant hosts that comprised 39.7 and 46.2% of the individuals at the study site, respectively. All tests for inoculum production from the surface of infected coast live oak bark or exudates from cankers were negative. In contrast, sporangia and chlamydospores were produced on the surface of infected bay laurel leaves. Mean number of zoospores produced from infected bay laurel leaves under natural field conditions during rainstorms was 1,173.0 +/- SE 301.48, and ranged as high as 5,200 spores/leaf. P. ramorum was recovered from rainwater, soil, litter, and streamwater during the mid- to late rainy season in all 3 years of the study. P. ramorum was not recovered from sporadic summer rains or soil and litter during the hot, dry summer months. Concentrations of inoculum in rainwater varied significantly from year to year and increased as the rainy season progressed for the two complete seasons that were studied. Potential dispersal distances were investigated for rainwater, soil, and streamwater. In rainwater, inoculum moved 5 and 10 m from the inoculum source. For soil, transmission of inoculum was demonstrated from infested soil to bay laurel green leaf litter, and from bay laurel green leaf litter to aerial leaves of bay laurel seedlings. One-third to one-half of the hikers tested at the study site during the rainy season also were carrying infested soil on their shoes. In streamwater, P. ramorum was recovered from an unforested site in pasture 1 km downstream of forest with inoculum sources. In total, these studies provide details on the production and spread of P. ramorum inoculum in mixed-evergreen forest to aid forecasting and managing disease transmission of this environmentally destructive pathogen.

Pathogenicity and sporulation of Phytophthora pinifolia on Pinus radiata in Chile

“…Scale bars are: a=50 μm, b-d= 25 μm and e-f=10 μm correlate with the onset and seasonal variation of rainfall, but it was also remarkably closely linked to the amount of rainfall in a particular period of time. The same patterns have also been observed with P. ramorum in California and Oregon, which has a Mediterranean climate with a distinct wet and dry season, and sporulation of the pathogen with subsequent plant infection appear to be primarily restricted to the rainy season (Davidson et al 2005;Rizzo et al 2005).…”

The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

“…Both Pythium and Phytophthora have frequently been recovered from lakes, streams and irrigation ponds by using baits or molecular methods. Recent work with Phytophthora ramorum, an introduced pathogen and causal agent of sudden oak death, has documented movement in soil and streams in natural ecosystems (Davidson et al 2005). However, most soilborne pathogens move and spread directly through the soil profile as mycelium.…”

Scientific Opinion on the Pest Risk Analysis onPhytophthora ramorumprepared by the FP6 project RAPRA

“…The pathogen may jump from infected habitat patches to other suitable patches due to movement of infected plants, soil or water. Evidence indicates that infested soil is inadvertently moved around by people (Davidson et al, 2005) and can be a source of infection for plants . Additionally, genetic studies combined with spatial autocorrelation analyses show that the pathogen can be effectively moved several miles from a source (Mascheretti et al, 2008 .…”

“…The natural distribution of European larch is mainly restricted to the Alps and the Carpathians (Figures 4 and 5). Although frost may not be expected to be a limiting factor for P. ramorum survival (Tooley et al, 2008), there is ample evidence both from the field and experimental settings (Davis et al, 2008) that the most favourable environmental conditions occur in the presence of rainfall and with temperatures around 20 ºC (Davidson et al, 2005;. In their updated US national risk map for P. ramorum using climate, host and pathways data, Koch and Smith (2008) excluded areas with minimum temperature below 0 ºC for at least 150 days (Kliejunas, 2010).…”

“…63: P. ramorum epidemiological knowledge Rhododendron, the plant considered to be the main host in Europe, is rarely killed by the pathogen This Member State comment overlooks the role played by Rhododendron in building up P. ramorum inoculum, which can then lead to infection of many other native European plant species. The California forest epidemic and associated oak mortality is known to be driven by sporulation on bay laurel (Davidson et al, 2005) which does not seem to suffer significantly because of infection by the pathogen (DiLeo et al, 2009). A paper by Cobb et al (2010) indicates that in presence of a foliar sporulating host (bay laurel) and a deadend host (oak), P. ramorum actually results in an increase in the presence of the foliar host.…”