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Funded Project |
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Funding Program:
Regional IPM Grants (S-RIPM) |
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Project Title:
Synergistic activity in mixtures of copper and garlic-derived products to enhance control of bacterial spot of peach |
Project Directors (PDs):
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Lead State: GA Lead Organization: University of Georgia |
| Research Funding: $106,708 |
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Start Date: Oct-01-2007 End Date: Sep-30-2009 |
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Pests Involved: bacterial spot |
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Site/Commodity: peach |
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Summary:
This is a Research Project to develop an efficacious, more sustainable, and cost-effective tactic for managing bacterial spot of peach, caused by Xanthomonas arboricola pv. pruni. The Pest Management Strategic Plan for Eastern Peaches lists bacterial spot as a key factor limiting orchard productivity in the region. Currently 20 to 30% of the peach acreage in the South and mid-Atlantic area is planted to cultivars that are highly susceptible to the disease. In severe epidemics, as in 2005, nearly 100% losses can occur on such cultivars. Even when bacterial spot is suppressed successfully on less susceptible cultivars, significant financial losses occur due to the cost of control.
The current management program for bacterial spot consists of application of copper products during the pre-bloom period to eradicate initial inoculum, and use of the antibiotic oxytetracycline during the cover sprays to reduce infection of leaves and fruit. In susceptible cultivars, cover sprays of oxytetracycline at 7- to 10-day intervals are essential until 1 month before harvest. However, with such a large number of applications, there is the ever-present risk for antibiotic resistance development. Moreover, environmental health concerns have put the reliance on oxytetracycline under scrutiny. These concerns have made the search for alternative management strategies and tactics imperative. Coppers, applied at low rates during the cover sprays, can control bacterial spot at a level equivalent to oxytetracycline, but this is still insufficient under high disease pressure. There is no room for increasing copper rates during the cover sprays due to phytotoxicity concerns. Furthermore, coppers alone are not a viable alternative because of the risk of resistance development. Thus, other mechanisms of increasing control efficacy need to be sought. A recent discovery suggests a means by which this could be accomplished: basic research in Japan showed up to 40-fold increased antimicrobial activity of copper when applied in mixture with allicin, a natural allyl sulfur compound from garlic. In subsequent laboratory experiments, we documented a similar synergistic activity between copper and garlic-derived compounds relative to growth inhibition of X. arboricola pv. pruni. Our work focused on two commercially formulated pesticidal ingredients that are cheaper and more stable than allicin, viz. liquid garlic extract and a blend of diallyl sulfides (DADS).
The aim of this project is to follow up on our preliminary data by determining bacterial spot control, phytotoxicity on peach leaves, and level of synergism among copper, garlic extract, and DADS in greenhouse and field experiments. Potential impacts of this research include reduced reliance on the at-risk antibiotic oxytetracycline, reduced risk of resistance development to oxytetracycline and copper, and improved disease control at current application rates or reduced cost of control if the synergistic activity allows the mixture components to be applied at reduced rates. The project goes to the heart of the production agriculture focus area in the National IPM Roadmap by improving sustainability, efficacy, and effectiveness of pest management tactics for crops and commodities consumed by humans. Execution of the field trials in both Georgia and Pennsylvania will ensure applicability of the results to the key stone fruit production regions in the eastern U.S.
Objectives: The overall aim of this project is to follow up on our laboratory data by determining bacterial spot control, phytotoxicity on peach leaves, and level of synergism among copper, garlic extract, and DADS in greenhouse and field experiments. Specific objectives are: 1) In greenhouse experiments on peach seedlings, determine suitable mixture rates of copper and the two garlic-derived products to achieve optimal bacterial spot control with minimal phytotoxicity. 2) Evaluate disease control, phytotoxicity, and economic costs and benefits associated with using mixtures of copper and the two garlic-derived products for bacterial spot control during the cover sprays in the field. |
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Final Report: |
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Results From report submitted by the PI to USDA CRIS report system PROGRESS: 2007/09 TO 2010/08 OUTPUTS: Objective 1: Synergistic interactions between copper (Cu) and garlic-derived compounds were characterized in depth. Cu, allyl disulfide (AD), diallyl sulfide mixture (DADS), or garlic extract (GAE) were applied to nutrient broth either alone or in combination at their respective ED10, ED25, and ED50 values. A suspension of X. arboricola pv. pruni was added to give a final concentration of 50,000 CFU/ml. Bacterial growth inhibition was measured after 48 h, and synergism in the mixtures was determined using the Gowing equation. In a second set of experiments ("killing curve method"), Cu, the three garlic-derived compounds, and X. arboricola pv. pruni were applied to nutrient broth as described above; aliquots of the suspensions were dilution-plated onto nutrient agar after 48 h; and CFUs were counted 48 h later. Mixtures that reduced log(CFU) by more than 2-fold compared with the more effective singular compound were considered to act synergistically. Objective 2: A total of seven field trials was conducted in Georgia and Pennsylvania between 2008 and 2010 to evaluate the performance of Cu-DADS mixtures and other oxytetracycline alternatives applied during the cover sprays in reducing leaf and fruit infection by X. arboricola pv. pruni across a range of peach or nectarine cultivars. Early-season (dormant through bloom) copper sprays were omitted to allow build-up of bacterial inoculum for the test. In addition, plots were spray-inoculated at bloom, petal fall, and/or shuck-split with a bacterial suspension of the pathogen. The trials also included Cu alone, DADS alone, an oxytetracycline (OTC) standard, and an untreated check. Furthermore, an oxytetracycline-Cu mixture as well as the experimental antibiotics gentamicin or kasugamycin (the latter in mixture with either Cu, captan, or Penncozeb) were evaluated in select trials. There were between 6 and 8 cover spray applications for each treatment. In three trials, between one and five soil drench applications (without foliar sprays) of the resistance inducer acibenzolar-S-methyl were evaluated as an alternative type of bactericide treatment. These applications were made in a volume of 0.5 or 0.75 L/tree with a concentration of active ingredient between 0.01 and 0.04%. Leaf and fruit disease incidence or severity as well as phytotoxicity were assessed in each trial. In untreated plots in 2009 and 2010, custom shelters were used to protect immature fruit from bacterial splash and infection starting at shuck-split. Developing fruit were exposed by removing the shelters at various stages of fruit development. In separate experiments on peach seedlings outdoors, the effect of temperature, natural sunlight, simulated rainfall, and three ultraviolet (UV) protectants and one sticker-extender (lignin, titanium oxide, oxybenzone, and Nu Film-17) on the longevity of OTC on the leaf surface were quantified. Numerous grower presentations and field visits related to bacterial spot management were made in the two participating states. An MS student and a postdoc working on the disease were mentored at Penn State and the Univ. of Georgia, respectively. PARTICIPANTS: Harald Scherm, University of Georgia, Athens (PI; overall project coordination, participation in field trials in Georgia); Phillip M. Brannen, University of Georgia, Athens (co-PI; conducted outreach in Georgia); Henry K. Ngugi, Penn State University, Biglerville (co-PI; conducted field trials and outreach in Pennsylvania). PARTNER ORGANIZATIONS: USDA-ARS Fruit and Tree Nut Research Lab, Byron, GA (provided access to field sites). COLLABORATORS: M. Jefferson Cook, IV, Taylor County Extension Coordinator, Butler, GA (assisted in field trials in Georgia). TRAINING/ PROFESSIONAL DEVELOPMENT: Rock S.C. Christiano, University of Georgia, Athens (postdoc; planned, conducted, and analyzed all research under objective 1); Sarah J. Bardsley, Penn State University, Biglerville (MS student; assisted in field trials in Pennsylvania). TARGET AUDIENCES: The primary target audience for this project consists of commercial stone fruit growers in the Southeast and the mid-Atlantic region of the United States. Knowledge transfer has occurred annually in educational sessions at the Cumberland-Shenandoah Fruit Workers Conference; the Mid-Atlantic Fruit and Vegetable Convention; the FREC field day in Biglerville, PA; grower meetings in Adams, Franklin and Lancaster Counties in Pennsylvania; the Middle Georgia and North Florida-South Georgia Peach Updates; and numerous farm and packinghouse visits in both states between 2008 and 2010. Overall, at least 150 fruit growers were educated on bacterial spot management and specifically on use of low rates of Cu and other OTC alternatives during cover sprays. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period. IMPACT: 2007/09 TO 2010/08 Objective 1: The observed antibacterial activities in the Cu-garlic mixtures were greater than the expected antibacterial activities (calculated based on the assumption of additive action of Cu and the garlic product) in every single case (Cu at 3 concentrations combined with each garlic product at 3 concentrations), indicating synergism. In 19 of these 27 combinations, synergism was statistically significant. The relative levels of synergism between the three garlic derivatives were similar and ranged from 18 to 42%. Mixtures of Cu with AD, DADS, and GAE reduced log(CFU) by 7.3, 2.0, and 2.1, respectively. These results are expected to spark further research to determine the chemical or physiological mechanism(s) of the synergistic interaction with regard to bacterial disease control. Objective 2: Although disease levels and the ranking of different treatments varied across the seven trials, several robust conclusions could be drawn with regard to the efficacy of OTC alternatives. Cu at low rates provided disease suppression equivalent to or better than OTC. Furthermore, gentamicin alone as well as kasugamycin in mixture with Cu or captan had significant activity against X. arboricola pv. pruni. This suggests that Cu compounds and the fungicide captan may have a synergistic activity on kasugamycin since this antibiotic is generally not effective against bacterial spot when applied alone. Results with the Cu-DADS tank-mix were inconsistent across trials, with no significant improvement in disease suppression (compared with Cu alone) in most cases. Acibenzolar-S-methyl drench applications also performed inconsistently, with significant disease suppression in one trial and limited efficacy in two trials. In the shelter experiments, fruit exposed continuously had the highest disease severity while fruit sheltered from infection throughout most of the trial had the least amount of disease. From the beginning of the season, disease severity and fruit susceptibility declined exponentially. Thus, it should be possible to scale back bacterial spot sprays starting early June. In the study on OTC longevity on the leaf surface, constant temperatures up to 40C did not affect OTC degradation on leaves. In contrast, OTC residue decreased rapidly in natural sunlight in the absence of rain, declining by 43.8, 77.8, and 92.1% within 1, 2, and 4 days after application, respectively. Use of shade fabric, simulating overcast sky, reduced OTC degradation significantly but did not extend OTC persistence beyond 7 days. OTC residues during the 7-day exposure period were negatively correlated with solar radiation, but not with temperature. UV protectants and Nu Film-17 were ineffective in improving OTC persistence outdoors. Simulated rainfall drastically lowered OTC residue after 2 min, and levels were near the detection limit after 60 min of continuous rain. Over the past 3 years, data collected from this project have led to the major revision of the sections on bacterial spot management in the Pennsylvania Tree Fruit Production Guide and the Southern Peach, Nectarine, and Plum Pest Management and Culture Guide. PUBLICATIONS (not previously reported): 2007/09 TO 2010/08 1. Christiano, R.S.C., Reilly, C.C., Miller, W.P., and Scherm, H. 2010. Oxytetracycline dynamics on peach leaves in relation to temperature, sunlight, and simulated rain. Plant Dis. 94:1213-1218. 2. Ngugi, H.K., Bardsley, S.J., Lehman, B.L., and Jarjour, B. 2010. Evaluation of alternatives to oxytetracycline for control of bacterial spot of peach during cover sprays, 2009. Plant Disease Management Reports 4:STF023. |
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Outcomes N/A |
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Impacts From report submitted by the PI to USDA CRIS report system Objective 1: The observed antibacterial activities in the Cu-garlic mixtures were greater than the expected antibacterial activities (calculated based on the assumption of additive action of Cu and the garlic product) in every single case (Cu at 3 concentrations combined with each garlic product at 3 concentrations), indicating synergism. In 19 of these 27 combinations, synergism was statistically significant. The relative levels of synergism between the three garlic derivatives were similar and ranged from 18 to 42%. Mixtures of Cu with AD, DADS, and GAE reduced log(CFU) by 7.3, 2.0, and 2.1, respectively. These results are expected to spark further research to determine the chemical or physiological mechanism(s) of the synergistic interaction with regard to bacterial disease control. Objective 2: Although disease levels and the ranking of different treatments varied across the seven trials, several robust conclusions could be drawn with regard to the efficacy of OTC alternatives. Cu at low rates provided disease suppression equivalent to or better than OTC. Furthermore, gentamicin alone as well as kasugamycin in mixture with Cu or captan had significant activity against X. arboricola pv. pruni. This suggests that Cu compounds and the fungicide captan may have a synergistic activity on kasugamycin since this antibiotic is generally not effective against bacterial spot when applied alone. Results with the Cu-DADS tank-mix were inconsistent across trials, with no significant improvement in disease suppression (compared with Cu alone) in most cases. Acibenzolar-S-methyl drench applications also performed inconsistently, with significant disease suppression in one trial and limited efficacy in two trials. In the shelter experiments, fruit exposed continuously had the highest disease severity while fruit sheltered from infection throughout most of the trial had the least amount of disease. From the beginning of the season, disease severity and fruit susceptibility declined exponentially. Thus, it should be possible to scale back bacterial spot sprays starting early June. In the study on OTC longevity on the leaf surface, constant temperatures up to 40C did not affect OTC degradation on leaves. In contrast, OTC residue decreased rapidly in natural sunlight in the absence of rain, declining by 43.8, 77.8, and 92.1% within 1, 2, and 4 days after application, respectively. Use of shade fabric, simulating overcast sky, reduced OTC degradation significantly but did not extend OTC persistence beyond 7 days. OTC residues during the 7-day exposure period were negatively correlated with solar radiation, but not with temperature. UV protectants and Nu Film-17 were ineffective in improving OTC persistence outdoors. Simulated rainfall drastically lowered OTC residue after 2 min, and levels were near the detection limit after 60 min of continuous rain. Over the past 3 years, data collected from this project have led to the major revision of the sections on bacterial spot management in the Pennsylvania Tree Fruit Production Guide and the Southern Peach, Nectarine, and Plum Pest Management and Culture Guide. |
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