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Funded Project

Funding Program: Regional IPM Grants (S-RIPM)

Project Title: Management Alternatives for Bacterial Spot of Peach

Project Directors (PDs): Harald Scherm [1] Henry K Ngugi [2] Phillip M Brannen [3]

Lead State: GA Lead Organization: University of Georgia

Research Funding: $47,400

Start Date: Jul-01-2004 End Date: Jun-30-2006

Pests Involved: bacterial spot

Site/Commodity: peach

Summary: The recently published Pest Management Strategic Plan for peaches in the eastern U.S. identified bacterial spot, caused by Xanthomonas arboricola pv. pruni (Xap), as a key factor limiting orchard productivity. In recent years, consumer demand for red-skinned peaches has led to the planting of large acreages with cultivars developed in California; these turned out to be highly susceptible to the disease in eastern production regions. In severe epidemics favored by warm and humid weather, control is almost impossible and nearly 100% loss can occur. Even when bacterial spot is managed successfully on less susceptible cultivars, significant losses occur due to the cost of control. To make matters worse, in-season management of the disease during the cover sprays depends almost exclusively on the antibiotic oxytetracycline (Mycoshield); human health and environmental concerns have put. this antibiotic under scrutiny, and there is an ever-present risk for resistance development in the pathogen population due to frequent applications. Thus, there is a critical need for management alternatives that improve control and reduce over-reliance on a single antibiotic for in-season use. Xap overwinters in protected sites such as leaf scars and buds, and possibly epiphytically in cracks on bark, but the relative importance of these different survival niches has not been quantified previously. Bacteria in the overwintering sites multiply in early spring and spread to the developing leaves and fruit. Here, we propose a three-pronged approach to optimize disease control: 1) Quantify the relative importance of overwintering sites of Xap as a basis for developing better targeted management tactics that reduce in-season disease pressure. For example, if leaf scars are the main overwintering sites, efforts to improve management should focus on preventing leaf scar colonization in the fall, perhaps in conjunction with treatments that result in a less protracted defoliation of trees. Conversely, if the incidence of overwintering in buds is high, efforts should focus on eradication of bacteria in buds, e.g., by applying early-season copper sprays in combination with an effective surfactant that facilitates deeper penetration of the active ingredient into the buds. 2) Evaluate alternatives to Mycoshield for use during the cover sprays; these will include two non-antibiotic bactericides, a plant resistance activator, a film-forming antitranspirant, and a newly formulated systemic copper product. 3) Isolate and characterize bacteriophages of Xap and assess their potential as biocontrol agents. Inter alia, this will involve high-throughput screening of UV protectants for their ability to improve persistence of the phages outdoors, which has been a key problem with phage therapy in the past. While not targeted toward short-term implementation, this last objective will be critical for sustainable management of bacterial spot in the longer term. Objectives: Our overall goal is to clarify the overwintering habits of Xanthomonas arboricola pv. pruni (Xap) to develop informed strategies for reduction of initial inoculum and to develop management alternatives that improve disease control and reduce over-reliance on a single antibiotic for in-season disease management. Specific objectives are: 1) Quantify the relative importance of potential overwintering sites of Xap as a basis for developing better targeted fall and/or spring management tactics that reduce in-season disease pressure. 2) Evaluate the efficacy of alternatives to Mycoshield (oxytetracycline) for control of bacterial spot during the in-season cover sprays.

Final Report:

Results PROGRESS: 2004/07 TO 2006/06 I. Overwintering study: Xanthomonas arboricola pv. pruni-specific primers, originally developed at North Carolina State University, were incorporated into a BIO-PCR format for detecting the pathogen in symptomless host tissue. To elucidate the relative importance of different overwintering niches, host tissue samples were collected from two peach orchards with a history of the disease in fall after leaf drop, in winter during the dormant phase, and in early spring at bud swell in each of 2 years. The samples included leaf scars, terminal buds, floral buds, vegetative buds, current-season bark, previous-growth bark, and cankers when present. Samples were extracted and subjected to enrichment, DNA extraction, and PCR analysis. Results indicated that the incidence of positive detections decreased during the winter, and that it was greatest in cankers and vegetative buds and lowest in bark and floral buds. - II. Alternatives to Mycoshield (oxytetracycline): A total of six field tests to evaluate alternative bactericides as a replacement for oxytetracycline during the cover sprays were carried out on susceptible peach cultivars at locations in Georgia and South Carolina during 3 years. Bactericide applications commenced at petal fall or shuck split at weekly to biweekly intervals and ended ~1 month before harvest. Leaf spot intensity, disease-associated defoliation, and fruit disease intensity at harvest were assessed. Specific treatments and disease pressure varied across sites and years, but the following general conclusions could be drawn: 1) copper products can be applied effectively and safely during the cover sprays as an alternative to or in rotation with Mycoshield utilizing a rate of 0.5 oz/acre metallic Cu; 2) higher application rates of copper at the petal fall application caused notable leaf phytotoxicity, from which the trees recovered rapidly; 3) STBX-016 (copper sulfate pentahydrate), a locally systemic copper, tented to be more effective but also caused more phytotoxicity than fixed coppers; 4) repeated applications of Agri-Fos (potassium salts of phosphorous acid) caused leaf phytotoxicity and were ineffective against bacterial spot, although numerical disease reductions were observed for Mycoshield-Agri-Fos rotations; 5) Mycoshield applied in mixture with Nu-Film 17 as an 'extender' (spreader-sticker and UV protectant) tented to be more effective than Mycoshield alone; and 6) unconventional products such as 1st Biowash 1227 (tall oil fatty acids), Garlic GP (garlic oil), Oxidate (hydrogen dioxide), and Wilt-Pruf (pineolene) were ineffective in reducing disease levels. IMPACT: 2004/07 TO 2006/06 Bacterial spot disease is a key factor reducing orchard productivity in the southeastern United States. In this project, we quantified the relative importance of different host niches for pathogen overwintering, which will aid in the development of fall andspring management tactics that specifically target the most important overwintering sites. In addition, we identified reduced-risk alternatives to or rotation partners for the antibiotic oxytetracycline, the current mainstay for in-season management of the disease. PUBLICATIONS (not previously reported): 2004/07 TO 2006/06 1. Scherm, H., Savelle, A.T., Cook, M.J. IV, Reilly, C.C., and Hotchkiss, M.W. 2006. Evaluation of alternatives to oxytetracycline for control of bacterial spot of peach during the cover sprays, 2005. Fungicide and Nematicide Tests 61:STF10. 2. Ngugi, H.K., Brannen, P.M., Watson, T., and Scherm, H. 2005. Evaluation of alternatives to oxytetracycline for control of bacterial spot of peach during the cover sprays, 2004. Fungicide and Nematicide Tests 60:STF009.

Outcomes N/A

Impacts From report submitted by the PI to USDA CRIS report system. Bacterial spot disease is a key factor reducing orchard productivity in the southeastern United States. In this project, we quantified the relative importance of different host niches for pathogen overwintering, which will aid in the development of fall andspring management tactics that specifically target the most important overwintering sites. In addition, we identified reduced-risk alternatives to or rotation partners for the antibiotic oxytetracycline, the current mainstay for in-season management of the disease.

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