Antibiotic activity of some plant extracts on pseudomonas syringae pv. Phaseolicola and Xanthomonas Campestris pv. Phaseoli.

Abstract

Several plant extracts were screened for antibacterial activity against Pseudomonas syringae pv. phaseolicola and Xanthomonas campestris pv. phaseoli in vitro. Extracts from Eucalyptus citriodora Hook, Nothoscordum inodorum L., Cupressus lucistanica Mill, Tagetes minuta L. and Santolina chamaecyparissus L. were found to be active on the two phytobacterial pathogens. The volatile oil of E.citriodora produced a significantly (P = 0.05) wider zones of growth inhibition on P. syringae pv. phaseolicola and X. campestris pv. phaseoli when compared to the other plant extracts. The growth inhibition zones produced by the volatile oil of E. citriodora measured 32.83 mm and 24.50 mm on X. campestris pv. phaseoli and P. syringae pv. phaseolicola respectively. There was an interaction between the test pathogens and the extract .- . type. This was demonstrated by the significantly (P = 0.05) high sensitivity of X. campestris pv. phaseoli to the different plant extracts when compared to P. syringae pv. phaseolicola. Further investigations were carried out to determine the minimum inhibitory concentration of the volatile oils from E. citriodora and T. minuta on P. syringae pv. phaseolicola and X. campestris pv. phaseoli. T. minuta extract caused total growth inhibition on the two phytobacterial pathogens at 0.20% concentration while E. citriodora extract caused complete growth inhibition on P. syringae pv. phaseolico/a and X. campestris pv. phaseoli at (xviii) 0.78% and 0.39% concentrations respectively. Fractionation of the volatile oils from E. citriodora, T. minuta, C. lucistanica and S. chamaecyparissus was carried out and the resultant fractions tested for antibacterial activity against P. syringae pv. phaseolicola and X. campestris pv. phaseoli. The crude extracts of E. citriodora and T. minuta had a significantly (P = 0.05) higher activity than the hexane and ether fractions on P. syringae pv. phaseolicola. The test for minimum inhibitory concentration of the ether fraction of E. citriodora indicated that it lies between 1.56% and 3.13% concentrations. The ether fraction of T. minuta did not produce a zone of growth inhibition when diluted beyond 6.25% concentration. Generally, the activity of ether fraction of E. citriodora with respect to its activity against X. campestris pv. phaseoli and P. syringae pv. obeseoiicote was significantly (P = 0.05) higher than that of T. minuta. A test was carried out to establish the stability of the active compounds in T. minuta and E. citriodora crude extracts when subjected to various v 0 temperatures. All the temperature regimes viz., Dc 1Oc, o o coo o 0 0 o 0 20c 30c 40c 50c 60c 70c 80c 90c 100c and 121c tested did not affect the activity of the two plant extracts under study with respect to their activities against P. syringae pv. phaseolicola and X. campestris pv. phaseo/i. The active fractions were not denatured by o temperatures as high as 121c . For instance the activity o of E. citriodora extract subjected to 121c on P. syringae (x Ly ) pv. phaseolicola was not significantly (P=0.05) lower than o that subjected to Oc In vivo test for antibacterial activity on P. syringae pv. phaseolicola and X. campestris pv. phaseoli indicated that streptomycin sulphate and copper oxychloride reduced halo blight infection significantly (P = 0.05) when compared to bean seedlings receiving foliar spray of E. citriodora or T. minuta extract under green house conditions. No significant (P = 0.05) difference was observed in halo blight and common blight infection among bean plants sprayed with streptomycin sulphate or copper oxychloride and those seed dressed with 0.59% concentration of crude extract of E. citriodora or 0.15% concentration of crude extract of T. minuta. Phytotoxicity tests for different concentrations of ether fractions and crude extracts of T. minuta and E. citriodora were conducted using fourteen day old bean plants of cv. Rosecoco-GLP-2 in the greenhouse. Plants which were sprayed with 6.25%, 9.38% and 12.5% concentrations of ether fraction of T. minuta extract showed defoliation two weeks after spraying. Bean plants sprayed with 3.13% concentration of ether fraction of E. citriodora and 0.20% concentration crude extract of T. minuta showed some brown spots on the leaves. No signs of phytotoxicity were observed on plants sprayed with 0.78%, 0.59% and 0.39% concentrations of crude extract of E. citriodora. Field trials for the control of halo blight and common blight of beans was carried out using 0.59% concentration crude extract and 1.17% concentration ether fraction of E. citriodora. A significantly (P = 0.01) higher halo blight infection was observed among treatments which had artificially inoculated seeds when compared to those obtained from small scale farmers receiving similar treatments. Bean plants sprayed twice with 0.59% concentration of crude extract of E. citriodora showed a significantly (p=O.05)lower halo blight severity when compared to the untreated control. Dressing bean seeds and later spraying bean plants with E. citriodora volatile oil at seedling stage and at pod filling stage reduced halo blight severity when compared to bean plants which were treated (dressed or sprayed) with the ether fraction from the same plant extract. Halo blight incidence in dressed inoculated seeds and foliar sprayed seedlings was not significantly (P = 0.05) different from that of the untreated control. Copper oxychloride and streptomycin sulphate reduced infection by common blight significantly (P = 0.05) compared to E. citriodora extracts. The ether fraction and crude extracts from E. citriodora did not significantly (PO.05) lower common blight infection (applied as seed treatment or foliar spray) when compared to the untreated control. Yield assessment indicated that bean seeds which were dressed with the volatile oil from E. citriodora produced a high yield. For instance, there was a significantly (P=0.05) higher yield/hectare from plots (xxi) planted with dressed inoculated seeds when compared to those from plots planted with similar seeds which were foliar sprayed only. Although the activity of these plant extracts was lower than that of streptomycin sulphate, they can be exploited as natural bactericides for controlling halo and common bacterial blights of beans.