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Critical Research and Extension to Reduce Losses Due to Strawberry Powdery Mildew (Podosphaera Aphanis)

Gadoury, David
New York Agricultural Experiment Station
Start date
End date
Our long-term goal remains to develop an advanced understanding of pathogen biology, ecology and epidemiology, to strategically redesign IPM programs for strawberry powdery mildew and thereby improve the efficacy, efficiency, and sustainability of these programs.

Our specific research objectives are:

  1. Map the geographic distribution of cleistothecia, and identify the pathogen, host, and environmental factors favoring their production, and develop means to reduce or prevent their formation and survival.
  2. Quantify the seasonal distribution of ascospore release by P. aphanis, and relate the availability of ascosporic inoculum and susceptible host tissue to optimal timing of control measures to suppress powdery mildew.
  3. Quantify development of ontogenic resistance in leaves and berries of commercially relevant strawberry cultivars and develop phenological models for use in IPM programs.
  4. Extension and Outreach: Produce and make effectively available to stakeholders a set of revised recommendations for the management of strawberry powdery mildew based upon a comprehensive approach incorporating sanitation, ontogenic resistance, and deployment of control measures commensurate with risk of infection from known or monitored sources of inoculum.
Outputs include the following:
  1. By December the first results will indicate (i) initial distribution of ascocarps and mating types,and (ii) reasons for presence or absence of ascocarps. Confirmatory experiments will be conducted from Sept-Dec 2011. By spring 2012, we should know where cleistothecia form, and why.
  2. By Dec 2012 the cardinal temperatures for vegetative and generative growth of the pathogen will be defined, and preliminary models formulated. Impacts of varying degrees of fungicidal suppression in high tunnel trials will be defined by Dec 2012. Feasibility of fungicide-free production should also be indicated by this time.
  3. By spring 2013 we will have indicators of (i) general extent and magnitude of cryptic development in nurseries, (ii) general extent of resistance to DMI and strobilurin fungicides, (iii) management options for nurseries that reduce risks of distributing plants harboring fungicide-resistant isolates.
  4. By Nov. 2011, efficacy of thermotherapy treatment should be known from trials. Confirmatory and refined experiments will being in Nov. 2011 and conclude by Nov. 2012. Guidelines for use of thermotherapy should be available by spring 2013. 5. Advisory models based on berry phenology would be available for the 2012 growing season, with refinement from additional data in 2013.
  5. The engaged stakeholder network has already been established through the surveys conducted prior to this proposal submission. The forum for discussion and feedback will be in operation by Dec 2010. The first research results will be included in the management recommendations for the 2011 growing, and will be updated "on the fly" as new results are obtained and verified. The REU program will "graduate" two summer scholars in August of 2011, 2012, and 2013.
More information
Non-Technical Summary: Choose a crop that is highly susceptible to a major disease, with extraordinarily high quality standards, and severe economic markdowns for minimal damage. Then add the twin impacts of regulatory issues and fungicide resistance to your management challenges. Finally, throw in great uncertainty regarding how to best deploy your few remaining fungicides. This is what makes strawberries a "Crop At Risk". Our national survey of stakeholders documented (i) grower-reported losses due to powdery mildew, (ii) unsatisfactory disease control, and (iii) increased potential loss as production shifts towards high tunnel systems. Despite decades of research on strawberry powdery mildew, there are fundamental aspects of pathogen biology, ecology, and epidemiology that are misunderstood, and adversely affect control programs. For example, the APS Compendium of Strawberry Disease states: "Cleistothecia, when produced, do not appear to function as perennation bodies." However, we demonstrated (Phytopathology 100:246-251) that cleistothecia (spherical overwintering structures containing infectious spores) are functional. Our recent work on host resistance and pathogen survival as mycelium revealed additional opportunities to redirect and refocus control measures. Our goal is to develop an advanced understanding of pathogen biology, ecology, and epidemiology, to strategically redesign IPM programs for strawberry powdery mildew and thereby improve their efficacy, efficiency, and sustainability. By design, the project is a seamless blending of research and outreach, and uniquely incorporates an engaged national network of stakeholders to refine the project "on the fly". The project personnel are well poised to accomplish this goal and our proposed work is enthusiastically supported by the stakeholder community.

Approach: Objective I: We will build upon survey contacts to further engage stakeholders in the research by soliciting samples of leaves from stakeholders representing a broad geographic range of the continental US. Parallel studies of Florida isolates will be carried out from multiple plantings in Florida by personnel at the Gulf Coast Research and Extension Center. Detached leaves and potted plants in controlled environment chambers will be used to examine the effect of temperature on asexual spore germination and infection, latent period, colony expansion, sporulation productivity, and colony longevity. Models developed from this data will be validated in parallel field experiments. We will also document the scope and magnitude of cryptic infections in nurseries by collecting and maintaining in strict isolation, samples of purportedly disease-free planting material from cooperating commercial nurseries to obtain baseline data on the prevalence of cryptic powdery mildew contamination of planting stock. We will employ isolation methods (short-term growth of plant material in vitro) similar to those we used in earlier studies of overwintering mycelium on persistent leaves. We will conduct a time/temperature factorial experiment to eradicate S. aphanis from transplants using aerated steam. We anticipate that fungicide use might be significantly reduced and perhaps eliminated in the first year of production when mildew-free transplants are used. Objective II: We will study the airborne ascospore dose in strawberry field plantings from earliest snowmelt to complete exhaustion of the ascospore supply. Three Burkard volumetric spore samplers will be installed in a heavily infected strawberry planting at NYSAES in Geneva NY. Because the ascocarps of S. aphanis contain only a single ascus, we anticipate that airborne dose may be near or below the threshold of detection of the Burkard sampler (approximately 10 spores/m**3). To insure that we sample sufficient spores to recreate a statistically and biologically meaningful distribution for diverse release events, we will employ a method that we developed and successfully tested for a similar powdery mildew: E. necator. Objective III: We will repeat our previously described experiments for ontogenic resistance of (i) emergent, (ii) expanding, and (iii) expanded leaves, of several SD and DN cultivars. Objective IV: We will build upon the foundation of engaged stakeholders identified in our 2010 survey. In effect, this network will function as the "beta testers" for preliminary modifications of disease management programs. By design, the information flow will be two-way: from the project to the stakeholders, and from stakeholders back to the project personnel. Extension deliverables generated by the project will be distributed through a stakeholder network consisting of both electronic and live distribution components. The electronic distribution component will involve the NASGA survey respondents, thereby encompassing producers, nursery/industry members, and extension/education specialists.

Funding Source
Nat'l. Inst. of Food and Agriculture
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Sanitation and Quality Standards
Bacterial Pathogens