Abstract
Symptoms of a bacterial disease caused by Xanthomonas campestris pv. vesicatoria were observed on tomato plants growing in Al-Kharj area of Saudi Arabia. Symptoms included irregular yellow-necrotic areas on tomato leaves and spotting on the stem. Five bacterial isolates were isolated from the infected tomato seedlings. The pathogen identity was confirmed biochemically, and physiologically, hypersensitive reaction occurred in tobacco plants and pathogenicity test was conducted. All concentrations tested of salicylic acid (SA) had inhibitory effects against X. c. pv. vesicatoria under laboratory conditions. Application of SA at 1.5 ppm reduced severity of bacterial leaf spot of tomato.