5.2.14
CONTROL OF BACTERIAL SPOT ON TOMATO IN THE GREENHOUSE AND FIELD WITH BACTERIOPHAGES

JB JONES1, GC SOMODI1, LE JACKSON2 and BK HARBAUGH1

1University of Florida, Gulf Coast Research and Education Center, 5007 60th St. E., Bradenton, FL 34203, USA; 2AgriPhi Inc., 160 North Main, Logan, UT 84321, USA

Background and objectives
Bacterial spot of tomato, incited by Xanthomonas campestris pv. vesicatoria (Xcv), is a devastating disease of tomato in Florida and can cause significant yield losses when weather conditions are optimal [1]. As a result of chemical control being less than efficacious when environmental conditions for disease development are optimal [2], it was essential to identify other control strategies for controlling this disease. During the past year, we have tested bacteriophages for control of the bacterial spot pathogen on overhead-irrigated tomato transplants [3]. The objective of this study was to demonstrate the efficacy of bacteriophages for bacterial spot control, starting in the greenhouse and extending into the field.

Materials and methods
'Agriset' tomato seedlings were transplanted into a soilless mix in plug trays and treated immediately as follows: (i) seedlings received overhead irrigation with water alone; (ii) seedlings received overhead irrigation with water and were sprayed twice weekly with copper-mancozeb; (iii) seedlings were irrigated with water containing a mixture of phages specific against strains of tomato races 1 and 3. The final concentration of phage was 107 p.f.u./ml. Phages were used in all irrigations. The flats were set up in a randomized complete block. The centre plant in each flat was inoculated with a mixture of T1 and T3 strains of Xcv. The bactericide copper hydroxide was applied in combination with mancozeb. The seedlings were grown in preparation for planting in the field. Prior to transplanting to the field, disease incidence was assessed in each flat. The transplants were set in the field in a split-plot design. Transplants for each block were taken from the Speedling flats in the greenhouse. Each replicate of transplants from the greenhouse was set into each of the following treatments in the field: (i) no foliar spray; (ii) copper-mancozeb applied every 4-5 days; and (iii) bacteriophage mixture applied 2-3 times per week between 5 and 6.30 a.m. All plots were sprayed weekly with the fungicide Bravo (chlorothalonil) for control of target spot and other fungal diseases. Disease severity ratings were made three times during the season and then the area under the disease progress curve (AUDPC) was determined for each plot. Two pre-harvest plant vigour ratings also were made. Fruit were harvested from each plot 23 October, 4 November and 10 November 1997. All data were analysed statistically. Greenhouse data were analysed as a randomized complete block design and the field data were analysed as a split-plot design.

Results and conclusions
The incidence of bacterial spot was significantly less on copper-mancozeb- and bacteriophage-treated greenhouse seedlings than on control seedlings. However, the copper-mancozeb and bacteriophage treatments did not differ significantly from each other. In the field, plants treated with copper-mancozeb or bacteriophage had significantly lower AUDPC values than the control. The greenhouse treatments had no affect on the field AUDPC values. When vigour ratings were assessed, plants in bacteriophage-treated plots had greater vigour than the copper-mancozeb or control treatments. Plants in field plots which received bacteriophage had significantly greater number and weight of extra large fruit than plants receiving the copper-mancozeb or no bactericide. This accounted for a 25% yield increase in plots receiving the bacteriophage treatment as compared to plots receiving the copper-mancozeb treatment. Bacteriophages appear to be effective in reducing disease severity of greenhouse-grown seedlings and field plants, and in increasing yield of extra large fruit.

References
1. Pohronezny K, Volin RB, 1983. HortScience 18, 69-70.
2. Jones JB, Jones JP, 1985. Proceedings of the Florida State Horticultural Society 98, 244-247.
3. Somodi GC, Jones JB, Jackson LE, 1997. Phytopathology 87, in press.