Screening of essential oils on rice pathogen isolated from Madagascar

Four predominant isolated pathogens have been identified during survey of rice diseases in three regions in Madagascar. These pathogens were the bacteria Xanthomonas oryzae pv. oryzae and Pseudomonas fuscovaginae and the fungi Alternaria sp. and Curvularia lunata. These pathogens are causing significant yield reduction on rice, which is the staple food in this country. After the identification of those pathogens, the objective of this work was to identify appropriate control measures against them. Plants or plant extracts has been traditionally used to control human and plant diseases. Nowadays, the problem of residues of phytopharmaceutical product, reinforce the need for research on the development of natural plant extracts to control plant diseases. Madagascar has many endemic plant of interest and we have evaluated the antimicrobial properties of essential oils from Malagasy plants. The main steps of this study is the in-vitro screening of essential oils to control the pathogens isolated on rice in Madagascar and to test the phytotoxicity on rice plants of essential oils of interest. Thirty nine essentials oils from Madagascar had been tested for the first step. Activity of essentials oils on Pseudomonas fuscovaginae, Alternaria sp. and Curvularia lunata have been tested using broth media and on agar medium for Xanthomonas oryzae pv. oryzae. The growth of the pathogen (bacteria and conidia) in the presence of the essential oil was evaluated and compared to a control. Essential oils that showed interesting antimicrobial effect on these pathogens were tested for their phytotoxicity on rice plants. Briefly, one to two drop(s) of essential oils were applied on leaves of three weeks rice seedling, three leaves per plant. Seven essential oils of the 39 tested showed interesting antimicrobial activity with a percentage of growth inhibition of bacterium of more than 70% of at least one bacterial pathogen. One essential oil inhibited the growth of the two bacterial pathogens. For fungal pathogens, six essential oils presented a percentage of germination inhibition of fungal spore of more than 70% of at least one fungal pathogen. One essential oil inhibited the spore germination of the two fungal pathogens. It is interesting to note that these 6 essential oils have also an effect against at least one bacterial pathogen. Therefore, a total of seven essential oils were tested for their phytotoxicity on rice plants. Three essential oils, such as clove leaf, clove clow and bract, tested were phytotoxic as they induced necrotic spots on rice leaves after 48 h. These results confirm that some essential oils present an antimicrobial activity on plant pathogens. However some of them are phytotoxics on rice plants. The in-situ confirmation on plant of this activity against plant pathogens is currently ongoing.