Study the morphological and biochemical diversity of Xanthomonas campestris pv. campestris, the causal agent of black rot disease in brassica crops in Nuwara Eliya district.

Abstract

Severe outbreaks of black rot disease caused by Xanthomonas campestris pv. campestris (Xcc) were observed in brassica production fields in the world as well as in Sri Lanka. This pathogen enters plants through hydathodes and spreads within the leaf and stem vasculature. Current control methods for black rot have proven challenging due to its seed-borne nature, limited effectiveness of chemical treatments, and the absence of resistant cultivars. There are nine different Xcc races recognized worldwide. However, currently no records in Sri Lanka. Therefore, this study was done to find the morphological and biochemical diversity of Xcc in brassica crops. Xcc infected leaf samples were collected from different brassica crops representing seven agro ecological zones in Nuwara Eliya District. The pathogen was isolated with yeast dextrose calcium carbonate (YDC) medium. Their pathogenicity was confirmed with the cabbage plants. The colony morphology was observed on YDC medium and subjected to different biochemical tests. It was recorded high diversity of colony morphology including their shape (50% circular, 46% irregular and 4% filamentous), size (70% Large, 24% Medium and 6% in small), surface texture (54% glistening and smooth, 8% smooth and 10% rough), elevation (50% raised, 36% flat, 12% umbonate and 2% convex), opacity 88% opaque, 6% translucent and 6% transparent), margins (46% even, 20% irregular, 16% undulate and others showed entire colony type), texture (46% moist and viscous, 28% shiny and viscous, 16% dry, and others were sticky in nature). The predominant colony type was yellow, circular, large, smooth-glistening, raised, opaque, and viscous. Out of 51 isolates, 34 were catalase oxidation positive, KOH positive, hydrolyze starch, make liquefaction with gelatin and grew at 28°C. In the pathogenicity test, symptoms appeared 16, 19, and 24-days following inoculation. Isolates from radish, broccoli, and knol-khol exhibited higher pathogenicity. However, depending on the host and agro-ecological zone, the black rot pathogen of brassica crops demonstrates a variety of phenotypic features, biochemical properties, and virulence.