Allelopathy is basically defined as the influence or the impact of a plant on the growth of the other plant and microorganisms by releasing certain chemical compound. The allele chemicals could have a promontory as well as inhibitory effect depending upon their concentration. These allele chemicals are usually the byproducts of the plant metabolic processes and can be produced by leaves stems and roots. They can be released in the environment as root exudates, decomposition or by leaching from plant residues. Presently, in the conventional agriculture we are using large quantities of the pesticides which are not only resulting in the environmental pollution but are also causing the pesticide resistance. The use of allelochemicals solely or in combination with the pesticides can be a good alternative approach.
Various practices such as cover crops, crop rotations, mulching and the mix application of the allelochemicals with the herbicides can be used as effective tools in controlling weeds through Allelopathy. The rice-wheat cropping system is the major cropping system of Indo-Pak which is heavily infested with the weeds. The growing of the allelopathic crops such as sorghum and pearl millet after wheat and before the transplanting of the rice can offer effective weed control in rice for 40 to 45 days. In general the crops sown after sorghum experience less weeds because of the fact that roots of the sorghum release certain allelochemicals which suppress other plants. Another option in this aspect is the use of allelopathic crops as cover crops. Cover crops are basically grown to control weeds, for water and soil conservation and for the purpose of the suppressing insect pests and diseases. Spider lily can be used as a cover crop to suppress the weeds. It can also be incorporated in the soil as the mulch. The leaves of this plant contain a special chemical named lycorine which have a suppressive effect on the roots, shoots of the rice weeds. Mulching is another tactic which is used to reduce the germination and the seedling growth of the weeds. It has been reported that the application of the mint marigold as the mulch can reduce the germination of the several problematic weeds in the rice.
The water soluble allelochemicals can be used as the foliar spray for the control of the weeds and to improve the yield of the crops. It has been reported that the application of the sorghum extract at various rates have shown a suppressive impact on the weeds in the wheat field along with improving the yield of the wheat. Various studies have indicated that application of the sorghum water extract has been found good in controlling weeds upto 30-50% in wheat, Rice, maize and cotton. The application of the sorghum extract in combination with that of the sunflower, sesame and tobacco has also shown satisfactory results in weed control in wheat especially wild oat. The rate of the herbicides can be reduced greatly if they are applied in combination with the allelochemicals. For example it has been reported that the application rate of isoproturon was decreased by 50 to 60 percent when it was mixed with sorghum extract (12 L/ha).
Along with being used as the effective weed suppresser the allelochemicals can be successfully employed in suppressing the insect pests and diseases. It has been reported that the oil of the Neem seeds exhibit a suppressive effect on the nymph and adults of the strawberry aphid. Along with this certain reports also supports the suppression of the white fly by Neem extracts. Similarly, the extracts from the California pepper tree has a suppressive effect on the adults of elm leaf beetles by killing about 97% of their population at a concentration of the 4.3 and 4.7% w/v. The use of the certain allelochemicals have also been found effective in controlling the insects in the vegetables grown in the kitchen gardening. The allelochemicals have also been found excellent in the control of the causative agents of the certain diseases. Root exudates of the Chinese chive has been found effective in controlling bacterial blight in tomato. Similarly, the water extracts of wheat, canola, brassica and other crops have been reported effective in controlling certain disease.
By keeping in view the above examples we can conclude that in the use of allelochemicals in the pest control can be a good practices in the coming decades as the concerns are rising about the environmental degradation and pesticide resistance due to the unjudisous use of the pesticides.