First report of Bacterial Speck of Tomato caused by Pseudomonas syringae pv. tomato in AJK, Pakistan

Abstract

Tomato (Lycopersicon esculentum L.) is the second most cultivated vegetable crop. World production of tomato is about 177 million metric tons from 4.78 million ha, and Pakistan contributed an annual production of 2.9 million metric tons from 0.15 million ha (FAO, 2021). The bacterial speck of tomato caused by Pseudomonas syringae pv. tomato has devastated tomato cultivars in terms of their production and quality in Poonch, AJK, Pakistan. In a systemic field survey of tomato growing areas of Poonch division including Rawalakot, Bagh and Hajira, 23 fields were selected randomly with an approximately 10km apart from each other in the temperate climate ranging temperature from 18-22 ℃ with humidity more than 70%. It was observed small black spots with yellow haloes on tomato leaves and on fruits small, dark slightly raised lesions were observed (Mensi et al., 2018). On the basis of symptomological parameters average disease incidence of 42% was observed. Twelve isolates were recovered from the collected leaf, twig and fruit samples on King’s B medium while incubation temperature was 27±2 ℃. All the isolates were gram found gram negative and produced levan when grown on sucrose supplemented medium. Hypersensitive reaction of all the isolates was positive on tobacco leaves and were non-fluorescent under UV light. Pathogenicity test was performed on tomato seedlings inoculated with bacterial suspension having concentration of 108 CFU/ml with five replications and for control treatment sterile distilled water was used. Small dark brown to black spots similar to the symptoms found in the field were observed while no symptoms appeared in the control thus confirming the Koch’s postulates (Valenzuela et al., 2022). The amplification of 16s rRNA and gyrB gene of two isolates was done using 16s rRNA universal primer and an housekeeping gene primer respectively by polymerase chain reaction as described by Karlson et al. (1993) and Yamamoto et al. (2000). The amplicones of 16s rRNA gene were sequenced (GenBank Accession Nos. MK591077 and MK591130) and found 99% identical to the 16s rRNAgene sequences from Pseudomonas syringae pv. tomato. Pathogen was also confirmed by amplification of specific gene sequence gyrB gene (GenBank Accession Nos. MK608712 and MK608713) and successfully amplified 890bp gene sequence confirming the 99% identity upon BLAST. To best of our knowledge it is the first report of Pseudomonas syringae pv. tomato in AJK, Pakistan and was previously reported in Chile (Valenzuela et al., 2022), Tunisia (Mensi et al., 2018), California (Arredondo and Davis, 2000) and in several other countries in the world. The results of the current study will devise further epidemiological studies and quarantine measures to limit the outbreak of this disease on tomato.