Biocontrol of Xanthomonas axonopodis causing bacterial spot by application of formulated phage

Currently phages used as an alternative method for biocontrol plant pathogenic bacteria. However, the efficacy of phage is greatly reduced due to its short longevity on leaf surface of plant. Formulated phages [0.5% corn flour (CF) and 0.5% skim milk (SM)] significantly increased phage longevity on leaves surface were tested in greenhouse and open field trials during the growing season 2014. The leaves of pepper showed halo blight symptoms were tested for the presence of phages specific of Xanthomonas axonopodis by spot test. The phages were isolated by single plaque assay from the infected pepper leaves with spots symptoms. The isolated phages produced plaques with 3 to 5 mm diameters translucent spreading halo. The morphological phages particles were isometric head with 70 to 75 nm across with short contractile tails. The formulated isolate phages (CF and SM) were reduced the disease severity to 20.5 and 18.3% under greenhouse and 24.5 and 32.2% under open field conditions respectively, compared with unformulated phages. While, copper mancozeb reduced the disease severity to 45.3 and 39.5% (greenhouse) as well as 60.7 and 35.4% (open field) respectively. The skim milk and corn flour were applied for disease control trials and increased the phage longevity at 20 to 50 hrs and phage population 10⁶, 10⁵ (skim milk) to 4 x 10⁴ and 5 x 10³ plaque forming unit (PFU) in greenhouse and open field respectively . These formulated phages may be useful as a tool to efficient biocontrol of halo blight disease.