The effects of ocimum gratissimum, ipomoea batatas And brassica oleracea var botrytis extracts on the Growth and disease-causing capability of ralstonia Solanacearum in solanum tuberosum

Abstract

Irish potato (Solanum tuberosum L.) is a major cash and food crop worldwide. The production of this crop has in a great way been negatively affected by the extensive spread of bacterial wilt that is caused by Ralstonia solanacearum with over 50% crop yield losses reported in the last two decades. In addition, no bactericide has been developed to combat the disease and exercise of integrated control measures have not resulted to a sustainable solution. Scientists are putting a lot of efforts in research to get a solution that is effective, environmental friendly and that will enable the capacity of food production be improved to cater for the increasing human population. Plants have been shown to contain principles which are active against pathogens and among them are Brassica, Ipomoea and Ocimum species. It is on this basis that this research was aimed at determining the efficacy of locally available plants extracts in controlling the bacterial wilt disease in potatoes. The study was carried out in Maseno University Botany & Horticulture Department laboratory and the institution's Botanic Garden in 2007. The pathogen was isolated from infected potato tubers and cultured on nutrient agar medium. Ethyl acetate and methanol extracts of leaves of Ocimum gratissimum, Ipomoea batatas and Brassica oleracea var. botrytis and essential oils of. Ocimum gratissimum were evaluated for their efficacy at concentrations of 0.4, 0.2, 0.1, 0.05 and 0.025 mg/ml dissolved in dimethylsulphoxide in inhibiting in vitro growth of wilt bacteria. The experiment was extended to greenhouse where aqueous extracts of the three plants at concentrations of 2.5, 5 and 10% were evaluated for their efficacy in controlling the development of bacterial wilt symptoms on inoculated potato plants with control plants being inoculated with no treatment subjected to them. The extracts were also evaluated for their effect on different potato plants parameters. The experimental set up was randomized completely block design with 12 treatments and three replications. Data collectedwas subjected to Analysis of Variance (ANOVA) using SAS statistical package and the effects declared significant at 5% level. Linear correlation was used to compare the relationship between variables. The study has proved that the three plants contains principles that are active against wilt bacteria with essential oils from Ocimum gratissimum at O.4mglml exhibiting highest activity (10.lmm inhibition diameter) and Ipomoea batatas methanol extract at 0.025mglml exhibiting zero inhibition. The laboratory study has also shown a highly significant (P < 0.0001) interaction among plants, solvents and the various concentrations in the inhibitory activity. Aqueous extracts of the three plants have also exhibited varying levels of controlling wilt symptoms and promoting potato growth. The extracts of Ocimum at 10% and Brassica at 5% were the most effective in controlling the development Of wilt symptoms with average wilting index of 1.33 meaning less than 50% wilting occurrence. Potato plants treated with Brassica extracts at 10% exhibited an abnormal character of its leaf tips drying and thus .affecting their growth. In assessing the effect of potato wilting, the interaction between the plant and various concentrations was shown not to be significant (P> 0.05) in the first four days after inoculation. However it was significant (P<O.001) in the following three days and highly significant (P< 0.0001) after and until the end of the study period. The crude extracts had a significant effect on the growth parameters of potato. Based on the findings of this study the three plants contains compounds that are active against the growth of wilt bacteria and we recommend further evaluation of the crude extracts to determine the actual active ingredients which can be used to develop an effective biocontrol agent for this disease.