Epidemiology and Control of Diseases of Fruit Crops in Western Oregon

NON-TECHNICAL SUMMARY: Fire blight is a serious disease of pear and apple that impacts productivity of orchards, and prohibits export of these fruits to countries that do not have the disease. Our first purpose is to improve control of fire blight of pear and apple. Our second purpose is to evaluate the risk of movement of the fire blight pathogen in association with pear fruit.

<P>

APPROACH: Objective 1: We will address hypotheses concerned with the addition of avirulent strains of E. amylovora to already effective antagonist mixtures for fire blight suppression. We propose to evaluate the role of avirulent strains of E. amylovora with specific mutations in the hrp gene cluster for their potential to influence the domination of epiphytic populations of the pathogen by an antagonist mixture. We hypothesize that avirulent strains of E. amylovora will effectively contribute to antagonist mixtures by enhancing preemptive occupation of microsites currently colonized by virulent strains (but not by P. fluorescens or P. agglomerans), and by enhancing the relative competitive ability of the mixture, particularly at higher temperatures when E. amylovora has a growth rate advantage over P. fluorescens and P. agglomerans. <P>Objective 2: We will 1) determine the frequency which viable cells of E. amylovora can be detected on mature fruit of DAnjou pear in growing districts of Washington and Oregon, and 2) evaluate in experiments the capacity of Erwinia amylovora to colonize or persist on pear fruit surfaces; apple fruit will be used as a comparative control in these experiments. Results of this study are expected to form a foundation for assessing the risk of introduction of fire blight into a new geographic region via shipments of commercial pear fruit.
<P>
PROGRESS: 2004/10 TO 2009/09<BR>
OUTPUTS: Experimental activities were conducted on the following objectives. <BR> 1. Integrated chemical and biological control of fire blight: We evaluated combinations of biological control agents with oxytetracycline or kasugamycin in orchard trials inoculated with an antibiotic-sensitive strain of Erwinia amylovora. <BR> 2. Early detection of the fire blight pathogen: Detection of DNA of Erwinia amylovora by a loop-mediated isothermal amplification protocol (LAMP) was evaluated in laboratory assays and field experiments.<BR> 3. Comparison of the epiphytic fitness of a pathogenic and an avirulent strain of Erwinia amylovora: We investigated the dynamics of growth suppression of E. amylovora by comparing the ability of virulent and avirulent strains of E. amylovora to compete with each other on stigmas of pear and apple, and to compete with a co-inoculated mixture of effective bacterial antagonists. <BR> 4. Evaluation of likelihood of co-occurrence of Erwinia amylovora with mature fruit of winter pear: Over a four year period, we evaluated the potential for co-occurrence of E. amylovora with mature symptomless winter pear fruit by inoculation experiments and by survey of commercial orchards.<BR> 5. Understanding impacts of blackberry rust in the Pacific Northwest: The following subobjectives were addressed. a) Determine when infection cycle of Phragmidium violaceum is initiated in spring; b) identify the kind of weather and host developmental stages that allow for initial host infection; c) develop an efficient chemical control program; d) examine the effects leaf wetness and temperature interactions on the infection frequency of P. violaceum. Workshop held: Convened the '11th International Workshop on Fire Bight', Portland, OR, August 2007. The workshop was attended by 108 international scientists. We made five research presentations at this meeting (see Abstracts). Dissemination: Management of blackberry rust, N. Willamette Horticultural Soc., Wilsonville,OR, Jan. 2007; Integrated control of fire blight; Orchard Pest and Disease Management Conference, Portland, OR, January 2007; Understanding the significance of the new rust on blackberry,Orchard Pest and Disease Management Conference, January 2007; Charactering risk of the fire blight pathogen to associate with healthy pear fruit; Pear Research Review, Hood River, OR, February 2007; Overview of disease concerns in caneberry production,Raspberry Blackberry Preseason Workshop, Aurora, OR,February 2007. <BR> PARTICIPANTS: Individuals: K. Johnson (principle investigator); Virginia Stockwell (assistant professor senior research); Teresa Sawyer (senior faculty research assistant); Todd Temple (faculty research assistant). Partner Organizations: Oregon State University Extension Service, USDA ARS, Fresh Pear Committee, Washington Tree Fruit Research Commission, PNW Center for Small Fruits Research, Oregon Raspberry and Blackberry Commission. Collaborators: Larry Pusey, USDA ARS Wenatchee; Walt Mahaffee, USDA ARS Corvallis; Tim Smith, WSU Extension, Wenatchee; Steve Castagnoli, OSU Extension, Hood River; Diane Kaufman, OSU Extension, N. Willamette, AREC. Training and professional development: a) see 'Dissemination' above;, b) K. Johnson teaches 'Introductory Plant Pathology' annually to 55 undergraduates and 10 graduates students; K. Johnson co-teaches 'Plant Disease Management' every other year to 10-12 graduate students. <BR> TARGET AUDIENCES: This project generates both basic and applied knowledge related to the biology and management of diseases that affect fruit crops grown in western Oregon. The target audience for applied research results includes: growers, farm advisors, extension specialists/agents, and crop consultants in the western region of the United States. The target audience for basic research results includes: scientists, regulators, and policy makers (national and international) who have an interest in the crop diseases and specific objectives undertaken in our research. <BR> PROJECT MODIFICATIONS: We continue to address fundable questions and objectives that relate to mitigating the impacts of diseases of fruit crops grown in western Oregon.
<BR> <BR>
IMPACT: 2004/10 TO 2009/09<BR>
Changes in knowledge: <BR> 1. Integrated chemical and biological control of fire blight: Biological and chemical methods of fire blight suppression appear to be complimentary, and consequently, an integrated strategy consisting of a biological control agent sprayed early in bloom, followed by oxytetracycline or kasugamycin treatment at mid- to late bloom provides improved disease control with a reduced number of antibiotic applications. Nonetheless, an integrated biological and chemical strategy involving kasugamycin requires further investigation to determine the feasibility of selecting biological agents with greater tolerance to this material, and within the chemical component, to evaluate mixtures of antibiotics where kasugamycin is applied at a lower rate. <BR> 2. Early detection of the fire blight pathogen: A loop-mediated isothermal amplification protocol (LAMP) represents a sensitive, low cost method for detection of E. amylovora. A LAMP assay can be subjected to quantitative monitoring of DNA amplification, and may be adapted for use in the field. When coupled with disease forecasting models, the ability to rapidly detect epiphytic populations of E. amylovora with LAMP could improve fire management. <BR> 3. Comparison of the epiphytic fitness of a pathogenic and an avirulent strain of Erwinia amylovora: Results were consistent with the hypothesis that virulent E. amylovora modifies the epiphytic habitat presented by the stigma through the expression of pathogenesis-related genes, which increases resources available to itself and coincidentally to nonpathogenic competitors. The degree of biological control achievable with an avirulent strain of E. amylovora likely is limited by its inability to utilize the stigmatic habitat to the same degree as a virulent strain.<BR> 4. Evaluation of likelihood of co-occurrence of Erwinia amylovora with mature fruit of winter pear: We conclude that epiphytic E. amylovora has similar survival characteristics on both pear and apple fruit, that it is not an endophytic within mature symptomless pear fruit, its presence is exceptionally rare on commercially-produced fruit, and that epiphytic survival of E. amylovora through a postharvest chilling period is unlikely given the unrealistically high population size required for persistence. <BR> 5. Understanding impacts of blackberry rust in the Pacific Northwest: We determined that the blackberry rust pathogen is very sensitive to environmental conditions during the transition from the overwintering phase (in teliospores) to the active phase (as infections producing urediniospores). The degree to which the pathogen can make this transition determines whether or not the disease will be important in a given season. Delayed dormant treatments with lime sulfur significantly impact this transition; the developed weather-based infection models aid timing of early season scouting efforts and fungicide treatments.