natural-products-in-plant-pest-management

66 J.C. Pretorius and E. van der Wattused, is scheduled to be phased out, because it was defined by the MontrealProtocol of 1991 as a chemical that contributes to the depletion of the ozonelayer (Bowers and Locke, 2000). Presently, pre-plant soil fumigation and fungicideapplications are used to control wilt diseases. Subsequently, extractsfrom a number of plants have been identified as possible alternatives tomethyl bromide against soil-borne pathogens.Due to environmental and safety concerns associated with pesticides, aswell as the need for a replacement of methyl bromide, Bowers and Locke(2000) investigated the effect of several formulated plant extracts and essentialoils on soil populations of F. oxysporum. Fusarium wilts are some of themost widespread and destructive diseases of many major ornamental andhorticultural crops (Bowers and Locke, 2000). Treatment of the soil with 10%aqueous emulsions of the formulated extracts of a chilli pepper extract andessential oil of mustard mixture, a cassia tree extract and clove oil reducedpopulations of Fusarium by 99.9, 96.1 and 97.5%, respectively, 3 days after soiltreatment. The same formulations also suppressed disease development inthe greenhouse and resulted in an 80–100% plant stand after 6 weeks. Theobserved reductions in the pathogen population in soil and increase in plantstand in the greenhouse indicated that these natural plant products may playimportant roles in future biologically based management strategies for thecontrol of Fusarium wilt diseases.Similar results were observed with neem tree (Azadirachta indica) extractsagainst soil-borne fungi (Ume et al., 2001). It was shown that extracts fromthe leaves and seed kernels possess antifungal activity against Sclerotium rolfsii(Corticium rolfsii). Both leaf and seed extracts showed some effect againstdifferent growth stages of the fungus, but the effects were fungistatic ratherthan fungitoxic. The non-polar extracts of the seed kernels were reported tobe more effective than those rich in polar terpenoids such as azadirachtinand an aqueous leaf extract was also more effective than the kernel-derivedmaterial. Neem is perhaps the most useful traditional medicinal plant inIndia and each part of the tree has some medicinal property that has made itcommercially exploitable (Kausik et al., 2002).Amadioha (2002) evaluated the antifungal activities of different extractsof neem, both in vitro and in vivo. The oil extract from seeds as well as waterand ethanol leaf extracts of the plant were effective in reducing the radialmycelial growth of Cochliobolus miyabeanus in culture and in controlling thespread of brown spot disease in rice. However, the oil extract was found tobe the most effective, followed by the ethanol leaf extract, in inhibiting thegrowth of the pathogen in vitro and in controlling the development of thedisease in vivo. The oil and ethanol extracts compared favourably with carbendazim(Bavistin) at 0.1% active ingredient and had the potential to controlthe brown spot disease of rice in vivo. Additionally, Bohra and Purohit(2002) studied the effects of the aqueous extracts of 17 plant species collectedfrom Rajasthan, India on a toxigenic strain of Aspergillus flavus. The neemextract recorded the highest mycelial growth inhibition of the fungus.The antimicrobial potential of plant extracts has also been reportedagainst the highly resistant fungi that cause soil-borne damping-off disease