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United States Department of Agriculture

Agricultural Research Service


Location: Foreign Disease-weed Science Research

2009 Annual Report

1a. Objectives (from AD-416)
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 members of this group is key to developing improved management practices and pest risk assessments which will allow damage caused by such diseases to be reduced. Our first objective is to determine basic knowledge of the biology, taxonomy, ecology, and pathology of selected Oomycete pathogens as the basis for development of improved control/management strategies and pest risk assessments. Our second objective is to determine the response of selected host species to inoculation with selected Oomycete pathogens to determine susceptibility of plant species of agricultural and commercial importance as well as naturally occurring plant species. Our proposed studies will focus on sudden oak death caused by Phytophthora ramorum and brown stripe downy mildew of maize caused by Sclerophthora rayssiae var. zeae. Attainment of our objectives will benefit U.S. agriculture by contributing new knowledge regarding biology, infectivity, spread potential, and survival ability of destructive Oomycete pathogens thus facilitating the development of improved regulatory, management, and control strategies.

1b. Approach (from AD-416)
Research will be conducted utilizing specialized containment facilities to investigate critical biological factors required to develop improved identification, detection, monitoring, and management strategies for selected Oomycete plant pathogens. We will obtain isolates of selected pathogens and perform containment greenhouse, growth chamber, and laboratory experiments to elucidate their biology, taxonomy, ecology, and pathology. Pathogenicity studies will be performed on selected host plant species to determine levels of susceptibility to emerging Oomycete pathogens.

3. Progress Report
To identify potential biocontrol agents for the sudden oak death pathogen, Phytophthora ramorum, ten candidate microorganisms were applied to the foliage of Rhododendron cuttings which were then inoculated with P. ramorum. We compared the resulting necrosis to that produced on cuttings treated with a fungicide. Three isolates displayed good protection, with one isolate not significantly different than the fungicide. Work is continuing to investigate the rate of antagonist needed for protection, the length of time the antagonist is effective, and whether the antagonists are effective against different lineages of P. ramorum. Screening of 25 different species and cultivars of Viburnum for their susceptibility to P. ramorum has been completed. Results showed a wide range in susceptibility with no species or cultivar being completely resistant, and that evergreen species or cultivars that are more susceptible than deciduous species. Sprouted acorns of six Quercus species, common to the eastern U.S., were tested for their susceptibility towards P. ramorum. All species were susceptible to P. ramorum zoospores with as little as 1 hour exposure time. A study has been completed on the survival of P. kernoviae oospores in sand at different temperatures. Results showed that even at 30 degrees C, oospores were viable after 1 year in sand. A study was also completed determining the optimal method for production of viable P. kernoviae oospores. An assay developed for quantifying inoculum present in runoff from Phytophthora-infected-plants was used for basic epidemiology studies and applied risk assessments. Work continued on a long-range study of root-infected Rhododendrons. Work continued on evaluating the longevity of chlamydospores of P. ramorum incubated in soil at different temperatures and assayed by direct plating and polymerase chain reaction (PCR). Work continued on evaluating a soil biofumigant as a means of remediation of P. ramorum-infested soil. Work also continued on research to determine whether soil constituents can influence spore germination of P. ramorum chlamydospores. Final experiments were completed in a project to adapt and optimize a previously described mitochondrial based real-time PCR assay for use with a Cepheid SmartCycler. Work was also completed on a study to assess risk of spread of P. ramorum on common container weeds of nursery plants. The effect of moisture period (up to 6 days dew) on foliar disease development in northern red oak and chestnut oak was assessed along with the propensity of P. ramorum to form sporangia and chlamydospores on these two potential host species. Smaller lesion areas were produced on northern red oak compared with chestnut oak but on a per-leaf basis, the two species did not differ. Northern red oak sustained a far higher density of spores per lesion than did chestnut oak, and significantly more chlamydospores were produced on northern red oak than on chestnut oak. Knowledge of the sporulation potential of P. ramorum on these two valuable Eastern forest species enhances our ability to predict the epidemiological impact should the disease become established in the Eastern US.

4. Accomplishments
1. Runoff defined as a mechanism of disease spread for nursery plants infected with Phytophthora ramorum. P. ramorum causes sudden oak death, a disease that has killed thousands of trees in California and Oregon and threatens the nursery industry through quarantines due to movement of the pathogen on nursery plants. We determined that in addition to causing above ground foliar symptoms, P. ramorum has an active soil phase which must be considered in regulatory and control efforts. We developed an assay for quantifying numbers of pathogen spores present in runoff from infected plants, determined which environmental conditions favor root infection and production of inoculum, and related the number of spores in runoff to the resulting root infection. The assay was used to evaluate the risk associated with spores in runoff for containerized nursery stock and for native plant species and thus helped answer high-priority questions posed by the Nursery Committee of the California Oak Mortality Task Force. Results from runoff studies of native plant species will also help the U.S. Forest Service assess risk of P. ramorum to east coast ecosystems.

2. Determined conditions for infection by Phytophthora ramorum on Rhododendron. Phytophthora ramorum is an important pathogen which has killed thousands of oaks in California and Oregon. We investigated the temperature and moisture conditions that allow Phytophthora ramorum to infect Rhododendron. Disease occurred over a wide range of temperatures, although amounts of disease were minor at the temperature extremes. Moisture periods of 24 h and 48 h resulted in the greatest number of diseased leaves. The results help define conditions which lead to epidemic development and indicate that P. ramorum has the potential to become established in parts of the U.S. that are outside its current range. The information has been transferred to APHIS and U.S. Forest Service scientists for use in the development of pest risk assessments and risk maps to better understand the potential for establishment of P. ramorum outside of its current range.

Review Publications
Widmer, T.L. 2008. Comparing necrosis of Rhododendron leaf tissue inoculated with Phytophthora ramorum sporangia or zoospores. Plant Disease. 93:30-35.

Tooley, P.W., Browning, M.E. 2009. Susceptibility of some common Eastern forest understory plant species to Phytophthora ramorum. Plant Disease. 93:249-256.

Tooley, P.W., Browning, M.E., Kyde, K.L., Berner, D.K. 2009. The effect of temperature and moisture period on infection of Rhododendron Cunningham’s White by Phytophthora ramorum. Phytopathology. 99:1045-1052.

Last Modified: 2/23/2016
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