Molecular characterization of Xanthomonas citri subsp. citri isolates and its resistance sources in citrus germplasm

Abstract

Bacterial canker caused by Xanthomonas citri subsp. citri is an important disease of citrus. Ten isolates were obtained from the 17 samples collected from different districts of Punjab. Pathogenicity test on Kinnow revealed that Xcc 6 isolated from Kagzi lime was the most aggressive isolate with per cent disease index of 50.28. Thirteen cultivars of citrus were screened with the Xcc 6 using the detached leaf method under laboratory conditions. Screening was also done under field conditions. The cultivar Limequat had shown a highly resistant reaction with no disease development under laboratory conditions as well as under field conditions. Similarly, mandarins except PAU Kinnow 1 which displayed moderately resistant reaction exhibited moderately susceptible response under field conditions. The least average size and number of lesions were exhibited by the cultivar PAU Kinnow 1 under laboratory conditions. The isolates displayed variable results for different biochemical tests. All the bacterial isolates were Gram negative, rod shaped and exhibited growth on nutrient agar medium. Antibiotic sensitivity test revealed chloramphenicol to be the most effective antibiotic. Molecular characterisation was conducted using 15 RAPD primers. Similarity coefficient ranging from 0.098 and 0.634 was obtained with the highest similarity between Xcc 3 from Ladhowal, Ludhiana and Xcc 4 from PAU, Ludhiana and minimum similarity between Xcc 6 from PAU, Ludhiana and Xcc 2 from Ferozepur with a similarity coefficient of 0.098. Dendrogram construction using Unweighted Pair Group Method with Arithmetic Mean analysis gave two separate clusters at 30 per cent similarity. Only Xcc 8 from Boparai Kalan, Ludhiana belonged to the second cluster while all the other nine fell in the first cluster.