Copper fungicides and antibiotics are frequently used to control bacterial pathogens in processing tomatoes in Brazil. However, failure of control is common and can be caused by the presence of resistant strains. In vitro tests were carried out to evaluate the sensitivity to copper, streptomycin and oxitetracyclin of 389 xanthomonads strains isolated from tomato plants with bacterial spot symptoms: 92 of X. axonopodis pv. vesicatoria (60 strains group A /race T1 and 32 group C /race T3), 93 of X. vesicatoria (group B/race T2) and 204 of X. gardneri (group D/race T2). The strains were obtained from commercial fields in the states of Goiás, Minas Gerais, Pernambuco, and Bahia from 1995 to 1998 and in 2000. Bacterial suspension aliquots of 5 ml were deposited on Nutrient-Agar medium amended with copper sulfate (at concentrations of 50 and 200 µg/ml), streptomycin sulfate (25 and 200 µg/ml) or oxitetracyclin hydrocloridrate (25 µg/ml). None of the strains was resistant to oxitetracyclin or to copper at 200 µg/ml of copper sulphate. However, strains differed in their sensitivity to 50 µg/ml copper and to streptomycin sulfates (both concentrations used). The frequencies of X. gardneri, X. axonopodis pv. vesicatoria (groups "A" and "C") and X. vesicatoria resistant strains to streptomycin (25 µg/ml of streptomycin sulphate) were, respectively, 98%, 38% and 2%, whereas to copper the frequencies were 48%, 4% and 74%, respectively. All strains of group C/race T3 were sensitive to streptomycin and 97% sensitive to copper. Ninety eight percent of 'X. gardneri' strains, 38% of X. axonopodis pv. vesicatoria and only 2% of X. vesicatoria were resistant to streptomycin (25 µg/ml of the product used) whereas the frequencies of copper resistant strains were 48%, 4% and 74%, respectively for the three species of Xanthomonas.
Lycopersicon esculentum; Xanthomonas campestris pv.; vesicatoria; chemical control
