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.

[1]  B. Shearer,et al.  Distribution, reproduction, and movement of Phytophthora cinnamomi on sites highly conducive to jarrah dieback in South Western Australia , 1983 .

[2]  T. Swiecki,et al.  Evaluation of Stem Water Potential and Other Tree and Stand Variables as Risk Factors for Phytophthora ramorum Canker Development in Coast Live Oak 1 , 2002 .

[3]  D. Rizzo,et al.  Phytophthora ramorum as the Cause of Extensive Mortality of Quercus spp. and Lithocarpus densiflorus in California. , 2002, Plant disease.

[4]  Peter J. M. Bonants,et al.  Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum , 2001 .

[5]  V. Barnett,et al.  Applied Linear Statistical Models , 1975 .

[6]  W. D. Billings,et al.  North American Terrestrial Vegetation , 1988 .

[7]  J. H. Bowers,et al.  Relationship between inoculum level of Phytophtora capsici and mortality of pepper , 1991 .

[8]  J. M. Duniway,et al.  Dispersal of Phytophthora capsici and P. parasitica in furrow‐irrigated rows of bell pepper, tomato and squash , 1995 .

[9]  E. Hansen,et al.  Phytophthora species in oak forests of north-east France , 1999 .

[10]  M. Garbelotto,et al.  First Report of Foliar Infection of Rosa gymnocarpa by Phytophthora ramorum. , 2004, Plant disease.

[11]  B. Pavlik,et al.  Oaks of California , 1993 .

[12]  C. Mitchell.  Trophic control of grassland production and biomass by pathogens , 2003 .

[13]  B. L. Welch.  ON THE COMPARISON OF SEVERAL MEAN VALUES: AN ALTERNATIVE APPROACH , 1951 .

[14]  M E Woolhouse,et al.  Population Biology of Multihost Pathogens , 2001, Science.

[15]  David M. Rizzo,et al.  Sudden oak death: endangering California and Oregon forest ecosystems , 2003 .

[16]  N. Osterbauer,et al.  Surveying For and Eradicating Phytophthora ramorum in Agricultural Commodities , 2004 .

[17]  R. Schlub.  Epidemiology of Phytophthora capsici on bell pepper , 1983, The Journal of Agricultural Science.

[18]  J. C. Hickman,et al.  The Jepson Manual: Higher Plants of California , 1993 .

[19]  S. Thomson,et al.  Occurrence of Phytophthora species and other potential plant pathogens in recycled irrigation water. , 1974 .

[20]  David M. Rizzo,et al.  Phytophthora ramorum and Sudden Oak Death in California: II. Transmission and Survival 1 , 2002 .

[21]  D. Rizzo,et al.  Non-oak native plants are main hosts for sudden oak death pathogen in California , 2003 .

[22]  D. L. Mcintosh.  THE OCCURRENCE OF PHYTOPHTHORA SPP. IN IRRIGATION SYSTEMS IN BRITISH COLUMBIA , 1966 .

[23]  O. K. Ribeiro,et al.  Phytophthora diseases worldwide , 1998 .

[24]  D. Rizzo,et al.  First Report of Phytophthora ramorum on Douglas-Fir in California. , 2002, Plant disease.

[25]  E. Hansen,et al.  Sudden Oak Death Caused by Phytophthora ramorum in Oregon. , 2002, Plant disease.

[26]  H. Levene.  Robust tests for equality of variances , 1961 .

[27]  S. Werres,et al.  First studies on the pairing of Phytophthora ramorum , 2003 .

[28]  B. Shearer,et al.  Dispersal of Phytophthora cinnamomi through lateritic soil by laterally flowing subsurface water , 1993 .

[29]  William D. Ritts,et al.  SPREAD OF AN INVASIVE PATHOGEN OVER A VARIABLE LANDSCAPE: A NONNATIVE ROOT ROT ON PORT ORFORD CEDAR , 2002 .

[30]  D. Benson,et al.  Nematodes ― a threat to ornamental plants in the nursery and landscape , 1985 .

[31]  P. H. Tsao,et al.  Phytophthora: Its Biology, Taxonomy, Ecology, and Pathology , 1988 .

[32]  J. Ristaino,et al.  New Frontiers in the Study of Dispersal and Spatial Analysis of Epidemics Caused by Species in the Genus Phytophthora. , 2000, Annual review of phytopathology.

[33]  David M. Rizzo,et al.  Sudden Oak Death and Associated Diseases Caused by Phytophthora ramorum , 2003 .

[34]  Rudolf Schmid,et al.  North American Terrestrial Vegetation , 2000 .

[35]  L. L. Wilson,et al.  Splash dispersal of Colletotrichum acutatum and Phytophthora cactorum by short-duration simulated rains , 1992 .

[36]  E. Hansen,et al.  First Report of A1 Mating Type of Phytophthora ramorum in North America. , 2003, Plant Disease.

[37]  Maggi Kelly,et al.  Landscape Dynamics of the Spread of Sudden Oak Death , 2002 .

[38]  C. Brasier,et al.  Production of gametangia by Phytophthora ramorum in vitro. , 2004, Mycological research.

[39]  M. J. Griffin,et al.  Cocoa black pod: a reinterpretation. , 1984 .

[40]  L. Madden,et al.  Splash dispersal of Phytophthora cactorum from infected strawberry fruit , 1985 .

[41]  E. Hansen,et al.  Managing Port-Orford-Cedar and the Introduced Pathogen Phytophthora lateralis. , 2000, Plant disease.

[42]  C. Campbell.  The Importance of Dispersal Mechanisms in the Epidemiology of Phytophthora Blights and Downy Mildews on Crop Plants , 1999 .

[43]  D. Rizzo,et al.  First Report of Phytophthora ramorum on Coast Redwood in California. , 2002, Plant disease.

[44]  D. Rizzo,et al.  AFLP and phylogenetic analyses of North American and European populations of Phytophthora ramorum. , 2004, Mycological research.

[45]  R. Kunimoto,et al.  Dispersal of Phytophthora palmivora sporangia by wind-blown rain , 1974 .