natural-products-in-plant-pest-management

64 J.C. Pretorius and E. van der Wattspore germination and the effective control of grape downy mildew( Plasmopara viticola). Among the plant extracts, those of Chloris virgata, Dalbergiahupeana, Pinus massoniana, Paeonia suffruticosa and Robinia pseudoacaciainhibited spore germination of the pathogen significantly. An in vivo leaf disctest showed that the infected leaf discs, treated with these five plant extracts,exhibited no disease symptoms. Their effects were the same or better thanthat of the traditional fungicide, liquid Bordeaux.Pandey et al. (2002), similarly, compared the antifungal potential of leafextracts from 49 angiosperms, collected in Uttar Pradesh, India, with commercialfungicides, by screening them against Helminthosoporium sativum(Cochliobolus sativus). The leaf extract of Mangifera indica completely inhibitedthe mycelial growth of the test fungus while four plant species, A. sativum,Azadirachta indica, Lawsonia inermis and Matricaria chamomila (Chamomillarecutita) showed more than 90% inhibition. On assaying different parts ofMangifera indica, the leaf and seed extracts were found to possess the highestactivity and, together with a leaf extract of Matricaria chamomile, performedbetter than the commercial fungicides.An approach to screen for plants with antimicrobial activity against plantpathogens that often results in success is to exploit indigenous knowledge onmedicinal plants and to screen these known plants for likely candidates. Therationale behind this approach is to screen traditional medicinal plantsknown in a specific area for their antimicrobial properties instead of randomlychoosing potential candidates from the long list of currently knownflowering plants, conifers, ferns or bryophytes. Rajiv et al. (2002) conducted astudy to screen for the most effective extracts out of 15 medicinal plantsagainst Helminthosporium nodulosum (Cochliobolus nodulosus) causing blight infinger millet. These included Impatiens balsamina, Solanum nigrum, Tageteserecta, A. sativum, A. indica, Datura metel, Emblica officinalis (Phyllanthusemblica), Eucalyptus citriodora, Euphorbia pulcherrima, Lantana camara, Menthaarvensis, Mimosa pudica, Nerium indicum (N. oleander), Ocimum sanctum(O. tenuiflorum) and Ricinus communis. Extracts were sprayed on the pottedfinger millet plants at 15, 30, 45, 60 and 75 days after sowing. Crude extractsof S. nigrum and I. balsamina showed the highest mycelial growth inhibition,followed by T. erecta. Overall, the crude extract of S. nigrum recorded the bestresult in vitro but was found inferior to the I. balsamina extract in in vivo tests(Rajiv et al., 2002).Besides large-scale screening programmes, data from quite a number ofsmaller projects that included the screening of one or more plants were publishedduring the past decade. In most cases these smaller projects includedthe isolation and identification of active compounds involved, even if onlythe chemical group level. Although large-scale screening programmes areimportant to at least identify plant orders, families, genera or species withpromising potential, the more concerted approach is probably preferable.Only a few approaches are mentioned here.In their study of antimicrobial properties of plant extracts, Orlikowski(2001a) used grapefruit extract (GE; Biosept 33 SL) to control Phytophthoraspp. Amendment of peat with GE at a concentration of 165 μg cm –3 resulted