Considerations for using bacteriophages for plant disease control.

review Bacteriophage 2:4, 208–214; October/November/December 2012; © 2012 Landes Bioscience Considerations for using bacteriophages for plant disease control Jeffrey B. Jones,1,* Gary E. Vallad,2 Fanny B. Iriarte,1,3 Aleksa Obradović,4 Mine H. Wernsing,1 Lee E. Jackson,5 Botond Balogh,1,6 Jason C. Hong1,7 and M.Timur Momol1 Plant Pathology Department; University of Florida; Gainesville, FL USA; 2Plant Pathology Department, Gulf Coast Research & Education Center; University of Florida; Wimauma, FL USA; 3Department of Plant Pathology; Kansas State University; Manhattan, KS USA; 4Faculty of Agriculture; University of Belgrade; Belgrade, Serbia; 5 Layton, UT USA; 6Nichino America Inc., Apollo Beach, FL USA; 7United States Department of Agriculture Agricultural Research Service–U.S. Horticultural Research Laboratory; Fort Pierce, FL USA 1 Keywords: bacteriophage, biocontrol, phage, tomato The use of bacteriophages as an effective phage therapy strategy faces significant challenges for controlling plant diseases in the phyllosphere. A number of factors must be taken into account when considering phage therapy for bacterial plant pathogens. Given that effective mitigation requires high populations of phage be present in close proximity to the pathogen at critical times in the disease cycle, the single biggest impediment that affects the efficacy of bacteriophages is their inability to persist on plant surfaces over time due to environmental factors. Inactivation by UV light is the biggest factor reducing bacteriophage persistence on plant surfaces. Therefore, designing strategies that minimize this effect are critical. For instance, application timing can be altered: instead of morning or afternoon application, phages can be applied late in the day to minimize the adverse effects of UV and extend the time high populations of phage persist on leaf surfaces. Protective formulations have been identified which prolong phage viability on the leaf surface; however, UV inactivation continues to be the major limiting factor in developing more effective bacteriophage treatments for bacterial plant pathogens. Other strategies, which have been developed to potentially increase persistence of phages on leaf surfaces, rely on establishing non-pathogenic or attenuated bacterial strains in the phyllosphere that are sensitive to the phage(s) specific to the target bacterium. We have also learned that selecting the correct phages for disease control is critical. This requires careful monitoring of bacterial strains in the field to minimize development of bacterial strains with resistance to the deployed bacteriophages. We also have data that indicate that selecting the phages based on in vivo assays may also be important when developing use for field application. Although bacteriophages have potential in biological control for plant disease control, there are major obstacles, which must be considered. Bacterial Diseases in Agriculture Bacterial pathogens are associated with plant diseases in temperate, sub-tropical and tropical environments and can account for *Correspondence to: Jeffrey B. Jones; Email: Submitted: 09/23/12; Revised: 01/28/13; Accepted: 02/01/13 http://dx.doi.org/10.4161/bact.23857 208 major economic losses to agricultural production. Disease control for many bacterial-incited diseases is challenging.1 Major challenges associated with control of members of the proteobacteria include: pathogen diversity; the inability to identify durable resistance in the host plant to the target pathogen; the pathogen’s ability to reach high populations in a relatively short period of time when conditions are conducive for disease development; and lack of effective chemical control. For most plant diseases, including bacterial incited diseases, an integrated management strategy is essential, combining proper cultural practices, biological control, bactericides or plant activators, where applicable, and plant resistance.2,3 Chemical control has been a major component of plant disease management, especially for diseases caused by bacteria. Unfortunately, bacterial plant pathogens have been more recalcitrant to chemical treatments than their fungal counterparts. Those disease management approaches that have relied heavily on chemicals alone have had limited success. Chemical control of bacterial diseases has traditionally consisted of bactericides such as antibiotics and copper-based compounds. For many years, copper has been used as a chemoprotectant more extensively than any other chemical for the control of bacterial plant diseases; however, copper resistance has been identified and characterized in many plant pathogenic bacteria and is primarily associated with plasmids although there is chromosomal associated copper resistance.4–10 Antibiotics, although used less extensively than copper, have also been used as part of a management strategy for various bacterial diseases. The aminoglycoside antibiotic, streptomycin, has been in use since the 1950s.11 As a result of overuse, streptomycin-resistant strains became prevalent in a very short period of time (i.e., within several years), which in turn limited its efficacy for managing bacterial spot of tomato and pepper.11 Streptomycin has also been used for many years for the management of fire blight of apple and pear,12 and a number of other bacterial plant pathogens.13 The efficacy of streptomycin for control of fire blight lasted much longer than for bacterial spot of tomato and pepper because the streptomycin resistance was associated with a plasmid which required acquisition by sensitive strains. Recent advances in the development of chemical compounds that stimulate plant defenses have offered another promising approach Bacteriophage Volume 2 Issue 4 REVIEW review for disease control. Often referred to as plant defense activators, these chemical compounds mimic phytohormones that in turn induce systemic acquired resistance (SAR) in the plant. These materials have integrated well into existing management strategies because of their unique mode of action, and have shown success for managing several bacterial diseases, including bacterial speck and spot of tomato and pepper,2,14,15 Xanthomonas leaf blight on onion16 and fire blight on apple.17 Due to their effect on plant physiology, some negative effects on yield have been associated with plant activators in certain plant species,15,16 while with other pathosystems they have been relatively ineffective.18 Although improved performance has been possible in some cases through the optimization of rates and application intervals,19 or making applications directly to plant roots rather than a traditional foliar spray.20 Regardless of their efficacy, plant activators alone have not provided sufficient control of bacterial diseases and clearly require integration with other effective compounds. Biological Control Biological control has been a desirable strategy for controlling plant di (...truncated)