First report of tomato bacterial speck caused by Pseudomonas syringae pv. tomato in Tunisia

Abstract

In spring 2016, severe spots were observed on nearly all tomato plants cvs. Amal, Kaouther and Malinche grown in several greenhouses in the region of Teboulba, on the eastern coast of Tunisia. Affected plants presented 1 to 5 mm, angular, necrotic spots surrounded by a yellow halo on young and older leaves (Fig. 1). When numerous lesions occurred on a single leaflet, they coalesced and a portion or the entire leaflet turned necrotic and died. Necrotic lesions were also observed on petioles, peduncles and stems of diseased plants. In some cases, a brown-black colouration was observed along the entire stem (Fig. 2). Similar necrotic symptoms were observed during spring 2017 on tomato seedlings grown in nurseries in the Kairouan governorate, in the centre of Tunisia (Fig. 3). Sixteen bacterial isolates were recovered from diseased plants of the three tomato cultivars and seedlings described above. Isolates were grown on tryptone casein soya agar medium and incubated at 28°C. All isolates were Gram negative, oxidase negative, produced levan and induced a hypersensitive reaction on tobacco leaves. These isolates were not fluorescent under ultraviolet light when grown on King's B medium. The pathogenicity of the 16 isolates was confirmed twice by spraying, until runoff, ten tomato seedlings (cv. Heinz 6691) at the four-true-leaf stage with each bacterial suspension adjusted to 10 8 CFU ml-1. Six days post-inoculation, small dark brown spots similar to those observed in naturally infected plants developed on leaves of all inoculated plants (Fig. 4). Negative control plants sprayed with either P. syringae pv. syringae (CFBP1392, isolated from Syringa vulgaris) or sterile distilled water were symptomless. Bacteria re-isolated from the necrotic lesions showed the same cultural and biochemical characteristics as those used for inoculation, thus fulfilling Koch's postulates.