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Characterization of Xanthomonas isolates causing black spot on tomato in South Africa

Shutt Vou: University of Pretoria

<div>Black spot of tomato, caused by seed and seedling-borne black spot <em>Xanthomonads</em> (BSX), is prevalent in South Africa and can cause substantial yield losses of the crop. Multilocus sequence analysis (MLSA) of two housekeeping genes (<em>dnaK</em> and <em>gyrB</em>) was used to resolve the taxonomic classification of 30 <em>Xanthomonas</em> strains previously identified as belonging to this genus. Pathogenicity tests on a susceptible tomato cultivar, Red Khaki, and matured green tomato fruits were also undertaken and susceptibility of strains to both CuSO<sub>4 </sub>and streptomycin was tested <em>in vitro</em>. Plants and fruit were monitored at weekly and on daily intervals, respectively. Disease symptoms were scored and a percentage infection derived for the seedlings and tomato lesion diameters measured with a ruler. Of the 30 isolates, seven species of <em>Xanthomonas</em> were identified, viz: <em>X. euvesicatoria, X. gardneri, X. perforans, X. vesicatoria, X. campestris</em> pv. <em>campestris, X. arboricola</em> pv. <em>populi</em> and <em>X. arboricola</em> pv. <em>juglandis</em>. Five species, <em>X. euvesicatoria, X. gardneri, X. perforans, X. vesicatoria </em>and <em>X. campestris</em> pv. <em>campestris,</em> caused black spots on both leaves and fruits of the susceptible tomato cultivar. This study thus showed that all four BSX species previously reported to be responsible for black spot disease of tomato elsewhere are present in South Africa. Also, this is the first report of <em>X. campestris</em> pv. <em>campestris</em> causing black spots on tomato leaves. All four of the isolates used in pathogenicity tests were resistant to both CuSO<sub>4</sub> and streptomycin, therefore other management options will need to be explored.</div>