Pathogenic variability among five bacterial isolates of Xanthomonas campestris pv. vesicatoria, causing spot disease on tomato and their response to salicylic acid

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 identified was confirmed biochemically and physiologically, hypersensitive reaction occurred in tobacco plants and pathogenicity tests were conducted. Significant variability was shown among the bacterial isolates. Isolates T2, T4 and T5 were the most pathogenic isolates as expressed by the disease severity index. The highest severity index (57.8%) was observed by isolate T5, followed by the two isolates T4 (55.3%) and T2 (43.1%) two weeks post inoculation. In an attempt to control this disease, Salicylic acid was used to study its ability to inhibit the growth of X. campestris pv. vesicatoria in vitro, and its potentiality in suppressing bacterial spot of tomato under artificial inoculation conditions. All concentrations tested had inhibitory effects against X. campestris pv. vesicatoria. The inhibition zone increased with increasing concentrations. SA at 1.5 ppm caused the largest inhibition zone (40 mm) followed by 1 ppm (33 mm). Application of SA at 1.5 ppm reduced the severity of bacterial leaf spot of tomato. Application as foliar treatment was more effective than seedling treatment. SA was the most effective at 1.5 ppm as foliar treatment where the number of lesions/leaf and disease severity were 1.8 and 14.1, respectively.