Use of Allelopathic Crops in Biological Control

Allelopathy includes not only plant-plant, plant-microorganism interactions but also plant-insect interaction (Durtn-Serantes et al. 2002). Plants are able to produce compounds that act as repellents for herbivorous pests and as attractants for antagonistic organisms, e.g., predators and parasitoids. However, some insects are able to detoxify the chemical and so they can feed on the plant (Capinera 2005). For example, secondary metabolites from barley such as gramine and hordenin help in defence of the plant against armyworm (Mythimna convecta Walker) larvae and fungus (Drechslera teres (Sacc.) Scoem.). In some cases, allelochemicals can affect pests, microorganisms and plants at once. Ageratochromene of Ageratum conyzoides L. acts as anti-juvenile hormone on insect and inhibits the growth of the pathogens like phytopthora root rot of pepper (Phytophthora capsici L.), but also suppress ryegrass (Lolium spp.) and barnyardgrass (E. crus-galli (L.) P. Beauv.) (Shiming 2005). This type of allelopathy can help in organic pest management. In Africa the farmers use Napier grass (Pennisetum purpureum Schum.) and desmodium legumes (silverleaf D. uncinatum (Jacq.) DC. and greenleaf Desmodium intortum (Miller) Urban) for control of stemborers (Busseola fusca Fuller, Chilo partellus Swinh.) and striga (Striga spp.) in maize fields. Desmodium is planted between the rows of maize as a repellent crop. Napier grass is grown around the field as a trap crop because it is preferred to maize for oviposition by stemborers, and it is subsequently removed and used as fodder. This method is called the "push-pull" strategy (Cook et al. 2007). In South China intercropping of Ageratum conyzoides L. in citrus orchards is practiced that effectively suppresses microbes, insects, and growth of some weeds through the release of volatile allelochemicals (ageratochromene, monoterpenes, sesquiterpenes, flavones and others) (Kong et al. 2004). Crop rotation with marigold (Tagetes spp.) (plant density of about 20 plants m-2) holds damage from root-lesion nematodes (Pratylenchus penetrans Cobb) below the economic threshold (Reynolds et al. 2000; Ball-Coelho et al. 2001). Marigold residues are as well toxic to same fungi (Weaver et al. 1994).

Sometimes the strategy used in weed control could be effective against pests and pathogens. Isothiocyanates from cruciferous green manure are potentially useful in controlling Fusarium oxysporum Schltdl. pathogens (Smolinska et al. 2003), Gaeumannomyces graminis (Sacc). Arx & D. L. Olivier pathogen and they are toxic to eggs of the black vine weevil (Otiorhynchus sulcatus Fabricius) (Angus et al. 1994). Biofumigation with broccoli (biomass incorporated into soil) controls Meloidogyne incognita (Kofoid & White) Chitwood nematode and positively effects saprophytic nematodes (Roubtsova et al. 2007). Production of biocidal pellets to be used as organic treatments in addition or in alternative to biocidal green manure are reported and discussed, because dried plant material, after water addition, showed, in vitro, a good fungitoxic activity on Pythium ssp. and Rhizoctonia solani Kühn (Lazzeri et al. 2004).

Combination of two different methods helps to protect plants from weeds and pathogens and pests at once. The combination of solarization and organic amendment (millet residues) can be an effective alternative to pesticides for managing charcoal rot disease of cowpea (Vigna unguiculata (L.) Walp.) with heavy infestations of Macrophomina phaseolina (Tassi) (Ndiaye et al. 2007). When wheat or barley soil amendments were combined with heating, nematode galling (Meloidogyne inkognita (Kofoid & White) Chitwood) was reduced by 99-100% and recovery of pathogens Sclerotium rolfsii Sacc. and Pythium ultimum Trow was reduced by 84-100% (Stapleton 2006).

14.13 Conclusion

Though allelopathy is too complex a phenomenon and its application is not easy because of different effects in diverse environment, it is very promising from an ecological point of view. Allelopathy should find main use in management of organic agriculture based only on biological processes and cycles with the effort to attain ecological balance. However, allelopathy utilization is possible in conventional, low input or sustainable agricultural systems. Suitable use of allelopathic crops could reduce the pesticide application and so to reduce the environmental and food pollution, decrease costs in agriculture, improve food security in poor regions and soil productivity, increase biodiversity and sustainability in the agro-ecosystem.

Allelopathy plays an important role in many preventive agricultural methods such as use of cover crops, mulches, green manure or intercropping. Roots of allelopathic plants or their decomposing residues release compounds in the soil that are toxic to weeds. Annual, biennial, or perennial herbaceous plants in a pure or mixed stand can be grown for these purposes. Decomposition time of plant residues and amounts of biomass are important factors of weed control by mulching. Biofumigation is the name for one type of allelopathy that includes effects of the chemicals, i.e., highly toxic isothiocyanates, produced by Brassica green manure. However, the weed-suppressive effect is influenced by species, planting date, seeding rate and method, weather and other factors and the balance of allelopathic crops in the crop rotations is necessary due to possible autotoxicity.

These days, allelopathic plants as catch crops or trap crops found utilization in plant protection of tropical regions against parasitic weeds. They do not eliminate the parasite completely but decrease the seed bank in the soil. Other applications of allelopathy for weed control include the use of plant residues as a natural herbicide agent, e.g., water extracts, pellets, flours, by-products of crop processing, etc. The strategies for use of volatile compounds as soil fumigants are developed.

Newly investigated pollen allelopathy could effectively reduce the reproductive ability of wind pollination of annual weeds. Pollen of allelopathic species could be artificially dusted on the stigmatic surface of other plants. This phenomenon needs study and field testing yet.

Allelopathic compounds act as repellents for herbivorous pests, so the same strategy used in weed control could be effective against pests and pathogens. Only allelopathy is not possible to use the complete control of weeds, pests or diseases; it is necessary to combine it with other methods of plant protection.

For next development of allelopathy utilization, especially such ways as breeding for stronger allelopathic potential is very hopeful. Hybridization could be a promising method of breeding. However, allelopathic activity was identified as a quantitative trait and therefore this characteristic is affected by both genetic effects and environmental conditions. The main disadvantage of the application of allelopathy is considerable variability in the dependence on environment. Therefore all results achieved in laboratory should be compared with effects of allelopathic crops in field conditions.

Nowadays majority of the obtained experimental results and knowledge about allelopathy come from conventional agriculture therefore more research is needed in conditions of organic farming. Future research should also be oriented on the long-term impacts of allelopathic plant characteristics on weed and on pest and fungi populations and integration of allelopathic crops with the plant protection management.

This work was supported by the Ministry of Education of the Czech Republic MSM 6007665806.

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