Archivio Journal of Plant Pathology

Abstract:

Failure to control tomato bacterial spot caused by Xanthomonas spp. with antibiotics and copper-based products is frequently reported. Also, in some pathosystems, isolates that have acquired resistance to one chemical agent may display altered fitness when compared to sensitive isolates. Therefore, the objectives of this study were (i) to evaluate the sensitivity of 94 isolates of Xanthomonas spp. to copper and streptomycin, and (ii) to compare the competitive ability of selected copper-sensitive and -resistant isolates of X. perforans in planta. Isolate reaction to each chemical was divided into four classes (S, sensitive; MS, moderately sensitive; R, resistant; and HR, highly resistant). While no isolates were recorded as HR to copper sulphate, 100% X. gardneri and 84.9% X. perforans were copper-R. Copper sensitive isolates were found only in X. perforans and X. euvesicatoria. Streptomycin-R and HR isolates predominated, except among X. perforans which had a majority sensitive or MS isolates. Six X. perforans isolates with contrasting responses to copper were inoculated to tomato plants, individually and in mixed inoculations for aggressiveness and competitiveness assessments. In vivo symptoms caused by S or MS isolates were significantly more severe than those caused by R isolates, indicating reduced fitness of the latter in the absence of copper sulphate. In one out of the three mixed inoculations the copper-R isolate was not recovered, further suggesting fitness impairment of copper-R isolates in the absence of selection pressure.