Effectiveness of Selected Chemicals in Inhibiting Pseudomonas syringae pv. tomato in vitro and in Controlling Bacterial Speck. Karen C. Conlin, Former Graduate Assistant, Department of Plant Pathology, University of Georgia, Athens 30602. S. M. McCarter, Professor, Department of Plant Pathology, University of Georgia, Athens 30602. Plant Dis. 67:639-644. Accepted for publication 22 November 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-639.

In laboratory, growth chamber, and field studies, streptomycin sulfate and certain fixed coppers alone and in combination with recommended fungicides (mancozeb and chlorothalonil) were toxic to Pseudomonas syringae pv. tomato (PST) in vitro and significantly reduced severity of bacterial speck on foliage and fruit of Chico III tomato plants. These chemicals also reduced epiphytic populations of PST on field-grown plants. Of 30 fungicides tested in vitro, only the organic sulfur compounds, especially those containing manganese ethylenebisdithiocarbamate (Mn EBDC), were toxic to PST. In vitro tests suggested a synergistic action between EBDC fungicides and several copper compounds although combined treatments did not always give better disease control than the fixed coppers alone. Mn EBDC compounds (maneb, maneb plus ZnSO₄, and mancozeb) alone at field rates were highly toxic to PST in vitro. Mancozeb alone significantly reduced lesion counts in the greenhouse but gave poor control in the field. Our results suggest that both streptomycin and cupric hydroxide are useful in a preventive control program for bacterial speck on tomato transplants grown in southern Georgia.