Control of bacterial spot on tomato in the greenhouse and field with h-mutant bacteriophages.

A mixture of host-range mutant (h-mutant) bacteriophages specific for tomato race 1 (T1) and race 3 (T3) of the bacterial spot pathogen, Xanthomonas campestris pv. vesicatoria (Doidge) Dye was evaluated for biological control of bacterial spot on 'Sun- beam' tomato ( Lycopersicon esculentum Mill.) transplants and field-grown plants for two seasons (Fall 1997 and Fall 1998). Foliar applications of bacteriophages were compared with similar applications of water (control) and of copper/mancozeb bactericides, the commonly used chemical control strategy for tomato seedling and field production. In 1997, the incidence of bacterial spot on greenhouse-grown seedlings was reduced from 40.5% (control) to 5.5% or 0.9% for bactericide- or bacteriophage-treated plants, respectively. In 1998, the incidence of bacterial spot was 17.4% on control plants vs. 5.5% and 2.7% for bactericide- and bacteriophage-treated plants, respectively, although these differences were not statistically significant at P ≤ 0.05. Applications of bacteriophages to field-grown tomatoes decreased disease severity as measured by the area under the disease progress curve (AUDPC) by 17.5% (1997) and 16.8% (1998) compared with untreated control plants. Preharvest plant vigor ratings, taken twice during each field season, were higher in the bacteriophage-treated plants than in either bactericide-treated plants or nontreated controls except for the early vigor rating in 1998. Use of bacteriophages increased total weight of extra-large fruit 14.9% (1997) and 24.2% (1998) relative to that of nontreated control plants, and 37.8% (1997) and 23.9% (1998) relative to that of plants treated with the chemical bactericides. Chemical names used: manganese, zinc, carboxy- ethylene bis dithiocarbamate (mancozeb). thwarted genetic approaches for providing stable resistance in commercial cultivars of tomatoes and peppers (Jones et al., 1995; Kousik and Ritchie, 1996; Pohronezny et al., 1992). Copper-containing bactericides have been used to control bacterial spot for many years, but their effectiveness has been ham- pered by the emergence of copper-resistant strains of the pathogen (Jones et al., 1991; Marco and Stall, 1983). The development of bacteria resistant to antibiotics and chemicals and concomitant reduction in efficacy of these bactericides has sustained interest in devel- oping alternative control strategies to combat bacterial diseases in vegetable crops. Bacteriophages (phages) are viruses that only invade bacteria. They are generally spe- cies or subspecies specific, do not infect nontarget, beneficial bacteria, and also are nontoxic to humans. As early as 1926 (Moore, 1926), bacteriophages were used as a biocontrol agent and were effective in con- trolling Stewart's disease in corn (Thomas, 1935). Using bacteriophages, Civerolo and Kiel (1969) reduced bacterial spot disease of peach seedlings caused by Xanthomonas oryzae by 86% to 100%. However, use of bacteriophages was abandoned because of the emergence of bacterial mutants resistant to the bacteriophages used and the develop- ment of successful control measures with antibiotics and other chemicals. An approach that uses a mixture of host-range mutant (h-