CHARACTERIZATION OF Xanthomonas campestris pv. campestris AND MANAGEMENT OF CABBAGE BLACK ROT THROUGH SEED TREATMENT WITH ACTINOMYCETES IN EAST HARARGHE ZONE, EASTERN ETHIOPIA

Abstract

Black rot of cabbage is the most prevalent and destructive disease in many cabbage-growing area of Ethiopia. Even though cabbage black rot and the causal agent Xanthomonas campestris pv. campestris (Xcc) is known, no identification and characterization of Ethiopian isolates has been accomplished. In addition, biohazard effect of synthetic chemicals and economic infeasibility necessitated the search for relatively safe bioagents. Hence, experiments had designed with the specific objectives to 1) isolate and characterize Xcc from cabbage seed and leaves; 2) evaluate the effect of actinomycete isolates against Xcc on culture media and cabbage seed; and 3) detect their effect(s) on germination and vigor of cabbage seeds. A total of 15 farmer fields were selected and infected cabbage leaves were taken randomly and seed of cabbage were bought from Haramaya store shop. Leaf segment maceration and liquid plating methods had used to isolate bacteria from leaf and seed, respectively. A total of 54 isolates of bacteria were characterized as Xcc based on pathogenicity, cultural, morphological, biochemical and physiological characteristic of the isolates. The bacterium produced small, yellow, circular/oval, smooth and shining colonies in the yeast extract-dextrose CaCO3 (YDC) culture medium. The identities of the isolates had confirmed by the pathogenicity test and all isolates induced symptoms of black rot disease within 5-15 days after inoculation. All isolates were Gram-negative, motile and rod-shaped, did not produce florescence pigment on KB medium and grew at 25-35 oC. The biochemical test revealed that almost the entire determinative tests had positive reaction with the 90% of isolates. Xcc had detected in all seed samples and all sampled cabbage fields except at Babile. Totally seven (A1, A2, A3, A4, A5, A6 and A7) actinomycetes were isolated through serial dilution from soil and grown on starch casein agar then identified as actinomycetes based on their morphology. Their antibacterial activities on Xcc with zone of inhibition and number of colony counted and effect(s) on germination percentage and vigor index of cabbage seed had tested in completely randomized design (CRD) with three replications. From tested isolates, actinomycete isolated from forest soil (A5) showed the highest growth inhibitory (6.02 mm) and reduction on the viability of Xcc (0.3 CFU ml-1) as compared to others and control. All tested isolates of actinomycetes significantly (p≤0.05) improved cabbage seedling with 79% germination percentage and 86% of vigor index as compared to controls. Thus, the results suggest that application of actinomycetes, as seed treatment would be a good alternative to manage black rot of cabbage. However, the degree of effectiveness varied with isolates. Isolates A5, A3 and A2 were more effective than isolates from soil of cabbagexvi cultivated field on all tests. Therefore, soil from cabbage-cultivated field should not take for commercial use of actinomycetes as alternative management of cabbage black rot. In addition, this study implies that further investigation on molecular characterization of Xcc and the in vivo effect of actinomycetes in disease management.