Foreign Disease Program

Biology and Epidemiology of Emerging Plant Pathogenic Oomycetes

Oomycete plant pathogens cause a wide range of serious diseases of great concern to U.S. agriculture, and some are of recent emergence or reemergence.  Obtaining knowledge of the biology, taxonomy, ecology, and pathology of selected diseases caused by Oomycetes is key to developing best management practices and pest risk assessments which will allow damage caused by such diseases to be reduced. 

 

There exist more than 500 species of Oomycetes, which include the water molds and downy mildews. Oomycetes are filamentous protists, which were once classified as fungi because of their filamentous growth and because they feed on decaying matter.  A search of the fungal database maintained by the USDA at Beltsville, MD indicated that at least 37 Phytophthora species exist which are not present in the U.S.  Species such as Phytophthora kernoviae, which is present in the U.K. and causing widespread damage have not yet arrived in the U.S..  Other newly described species such as P. foliorum  have been found on ornamental species in California and Tennessee, and caused problems in identification due to its cross-reactivity with PCR-based assays for P. ramorum.  One of the goals and directives of our research Unit, centered around use of our BL-3 level containment facility, is to keep informed about pathogens of imminent threat to U. S. agriculture and have resources in place to address the research needs of the agricultural community when these threats become severe and/or new pathogens gain entry into the U.S

Our objectives are to determine basic knowledge of the biology, pathology, and epidemiology  of selected Oomycete pathogens as the basis for development of improved control/management strategies.  Our studies focus largely on sudden oak death caused by Phytophthora ramorum.  P. ramorum, a newly described species found since 1993 to attack ornamentals in Germany and the Netherlands.  The same pathogen was observed since the mid-1990s to be responsible for death of tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), and California black oak (Q. kelloggii) in coastal regions just north and south of the San Francisco Bay in California.  Losses of at least three species of native oaks in California caused by P. ramorum since 1995 have reached epidemic levels.  Due to the ever-widening host range of the causal pathogen Phytophthora ramorum and uncertainty of the threat potential it poses to oak species nationwide, state and federal regulations have been put in place that restrict movement of P. ramorum hosts out of infested areas of California.  Canada has also invoked quarantine measures that restrict importation of certain plant hosts into the country.  These quarantines may serve to restrict movement of rhododendrons and other ornamental species.

We also study other emerging Phytophthora species such as P. kernoviae, which is currently killing trees in the United Kingdom and a concern to US regulatory authorities.  We also have tentative plans to study brown stripe downy mildew of maize caused by Sclerophthora rayssiae var. zeae when material becomes available from overseas collaborators.

Attaining our research objectives will provide benefits by giving us a greater understanding of the biology, survival, spread, and pathogenic potential of destructive Oomycete pathogens.  Accomplishing our objectives will allow us to be prepared with in-depth knowledge of pathogen biology, taxonomy, genetics, ecology, and pathology of emerging Oomycete pathogens that can be used to develop novel and effective exclusion, control, and management strategies.

Dr. Paul Tooley is lead scientist on this project, and conducts a research program to elucidate some of the basic biology, ecology, and epidemiology of Phytophthora ramorum.  Dr. Tooley has over 25 years of experience working with Phytophthora diseases, most notably P. sojae on soybeans, P. infestans on potatoes, and now P. ramorum.  Research projects on P. ramorum conducted in his program now include studies of the biology of chlamydospores of P. ramorum , sporulation potential of P. ramorum, and inoculum threshold determination.  He works closely with support scientist Marsha Browning.

Dr. Nina Shishkoff joined FDWSRU in 2003 and has a high degree of expertise in mycology and substantial experience working on powdery mildews and other fungal pathogens.  Her research goals are to determine the role of the soil phase of Phytophthora ramorum in disease epidemiology.  She is studying the soil phase of P. ramorum and the ability of runoff from P. ramorum-infected plants to cause infection on nearby uninfected plants.  She is also characterizing products that will allow disinfestation of gravel beds and soil infested with P. ramorum.  

Dr. Timothy Widmer joined the Unit in 2006 after working for a number of years at the ARS European biological control laboratory in Montpelier, France on biological control of weeds and Phytophthora megakarya.  He had worked previously with Drs. Jim Graham and Dave Mitchell at the University of Florida on Phytophthora on citrus bringing knowledge of this group of pathogens when he joined our Unit. Dr. Widmer is studying ospores, the long-term survival structure of P. kernoviae and characterizing their viability and germination.  He is also determining whether P. ramorum survival, sporulation, and infectivity can be reduced by the application of specific antagonistic microorganisms.  Dr. Widmer works closely with Biological Science Laboratory Technician Steven Dodge.