Evaluation of essential oils against Ralstonia solanacearum causing bacterial wilt of solanaceous crops

The antibacterial activity of nine essential plant oil viz. spearmint (Spearmint viridis), neem (Azadirachta indica), marigold (Tagetes erecta), castor (Ricinus communis), calamus (Acorus calamus), olive (Olea europaea), turpentine (Syncarpia glomulifera), eucalyptus (Eucalyptus macrorhyncha) and garlic (Allium sativum) was investigated at 5, 10, and 20, mg/ml oil concentrations on the growth of tomato, brinjal and capsicum isolates of Ralstonia solanacearum alongwith streptocycline and copper oxychloride as check treatments in vitro by paper disc, spectrophotometer and plate count methods. Oil from neem restricted the maximum bacterial growth to 7.20 mm at 20 mg/ml concentration followed by eucalyptus oil (5.60 mm) and spearmint oil (5.50 mm) against tomato isolate when evaluated with agar plate method. But at 5 mg/ml concentration, the essential oil of garlic showed the maximum inhibition zone of 5.17 mm. Similar trend was observed in capsicum and brinjal isolates. All the nine essential oils were found effective to all the three isolates of R. solanacearum by spectrophotometer and plate count method. However, Neem oil was found most effective to all the three isolates of R. solanacearum followed by spearmint oil. The OD values of neem oil at 20, 10 and 5 mg/ml concentration were 0.35, 0.52 and 0.66, respectively with corresponding cfu/ml values of 2.62x10², 3.52x10² and 5.24x10², respectively. The essential oil from castor was least inhibitory to all the three isolates.