Allelochemicals As Biopesticides

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Extracts of many plants with anthelminthic or antimicrobial properties have been proven effective in reducing soil populations of plant-parasitic nematodes (Ferris and zheng, 1999). Experiments which evaluated plant species documented in Chinese traditional medicine to be anthelminthic against plant-parasitic nematodes identified 153 aqueous plant extracts with activity against nematodes (Ferris and Zheng, 1999). Within a 24-hour exposure period, seventy-three of the extracts killed either juveniles of M. javanica or mixed developmental stages of P. vulnus, or both (Ferris and Zheng, 1999). Plants containing efficacious components included both annuals and perennials, which ranged in type from grasses and herbs to woody trees, representing 46 plant families (Ferris and Zheng, 1999). This research illustrates the tremendous potential for discovery of new active allelopathic compounds for plant-parasitic nematode control. in fact, many of the allelochemicals described below were isolated from crops observed to be nematode suppressive as rotation or green manure crops.

4.1. Glucosinolates

Glucosinolates are compounds primarily found in plants in the family Brassicaceae and are described previously in this chapter with respect to green manure crops. Enzymatic decomposition of glucosinolates in plant tissue occurs rapidly and is primarily attributed to microorganisms in soil (Fenwick et al., 1983). Products of glucosinolate degradation include organic cyanides and isothiocyanates which in addition to being evaluated as active compounds in green manures and organic amendments, have been studied for their their direct toxicity to nematodes as biochemical pesticides. Lazzeri et al. (1993) studied the direct effects of purified glucosinolates on second-stage juveniles of the sugar beet cyst nematode Heterodera schachtii. Compounds were isolated from seeds and plant tissue of brassicaceous hosts of the nematode. None of the glucosinolates tested in their native form were nematicidal. However, when exposed to the enzyme myrosinase, several compounds including sinigrin, gluconapin, glucotropeolin, glucode-hydroerucin, and the entire group of glucosinolate compounds extracted from rapeseed exhibited various levels of nematicidal activity depending on concentration and length of exposure (Lazzeri et al., 1993).

Later studies by Borek et al. (1995) investigated the persistence of glucosinolate-derived allyl isothiocyanate and allylnitrile in six soils. They found that the two compounds differed with respect to the temperature, moisture conditions, and soil physical conditions that effected their transformation in soil, and that both compounds dissipated from soil at relatively rapid rates. Donkin et al. (1995) studied the toxicity of glucosinolates and their enzymatic breakdown products to Caenorhabditis elegans. They found that allyl isothyocyanate, one of the decomposition products of the glucosinolate sinigrin, was three times more toxic to the nematode C. elegans than corresponding glucosinolate itself.

4.2. Benzaldehyde (benzoic aldehyde)

Benzaldehyde occurs in seeds of bitter almond (Prunus dulcis). It is found naturally in several cyanogenic glucosides and is used in food and fragrances for its almondlike aroma and flavor (Harborne and Baxter, 1993). The value of benzaldehyde as a fungicide is well established (Flor, 1926), with the nematicidal activity more recently demonstrated. Benzaldehyde reduced populations of M. incognita in field microplots with no phytotoxicity to cotton at 0.18 -2.14 ml/kg soil (Bauske et al., 1994). The combination of chitin and benzaldehyde added to peat-based potting mix improved tomato transplant growth and reduced galling by M. incognita in greenhouse trials (Kokalis-Burelle et al., 2002). When tested in vitro against M. incognita eggs, benzaldehyde was 100% effective as an ovicide for this species of root-knot nematode (Kokalis-Burelle et al., 2002).

The direct effect of benzaldehyde on C. elegans chemotaxis kinetics was analyzed by Nuttley et al. (2001). An initial attractive response to 100% benzaldehyde was reported, followed by a strong aversion to the chemical. They determined this behavior to be mediated by two genetically separable response pathways. initially, upon exposure, the attraction response dominates but eventually gives way to a repulsive response. Oka (2001) found that with juveniles of M. javanica, immobilization and hatching inhibition in vitro were greater with benzaldehyde and furfural than with several other essential oils. Benzaldehyde and furfural also reduced galling on tomato in pot experiments where other aldehydes were not effective (Oka, 2001).

The effects of benzaldehyde combined with several organic amendments on soil microbial populations and plant-parasitic and nonparasitic nematodes were investigated by Chavarria-Carvajal et al. (2001). They found that benzaldehyde combined with most organic amendments reduced damage from parasitic nematodes and selected for predominantly gram-positive rhizosphere bacteria. When benzaldehyde was combined with root-knot nematode egg parasitizing isolates of the fungus Fusarium solani, increasing rates of benzaldehyde in soil reduced nematode penetration and infection of the host plant, and resulted in increased parasitism of M. javanica females by the fungus (Siddiqui and Shaukat, 2003). However, the increasing rates of benzaldehyde resulted in lower egg parasitism by the fungus (Siddiqui and Shaukat, 2003).

4.3. Furfural

The biological activity of furfural (also known as 2-furancarboxaldehyde, furaldehyde; 2-furanaldehyde, 2-furfuraldehyde, fural, furfurol) has also been recognized for decades (Flor, 1926). Furfural is the aldehyde of pyromucic acid and has properties similar to those of benzaldehyde. Furfural is a derivative of furan and is prepared commercially by dehydration of pentose sugars obtained from sugarcane, cornstalks and corncobs, husks of oat and peanut, and other agricultural waste products (Harborne and Baxter, 1993). Commercially available products for disease and nematode control are available in several countries including the United States. These products include Multiguard FFA (furfural (75%) + allyl isothiocynate (25%), Harborchem, Cranford, New Jersey), and Multiguard Protect (furfural (50%) + metham sodium (50%), Harborchem, Cranford, New Jersey).

Rajendran et al. (2003) reported improved plant growth and reductions in soil populations of M. arenaria and R. reniformis in groundnut with formulations of furfural compared to an untreated control, with no effect on free-living nematodes in soil. Spaull (1997) also found that free-living nematodes were relatively unaffected by furfural application while parasitic nematodes differed in their susceptibility with species of Paratichodorus and Xiphinema being more susceptible than Helicotylenchus and Tylenchorhynchus. Sipes (1997) found furfural to be as effective as 1,3-D for reduction of preplant soil nematode populations in pineapple production. Rodriguez-Kabana et al. (1993) found that furfural was an effective nematicide against M. arenaria, M. incognita, Heterodera glycines, and Pratylenchus spp. on squash, okra, and soybean. Furfural reduced populations of M. incognita in field microplots with no phytotoxicity to cotton at 0.18 -2.14 ml/kg soil (Bauske et al., 1994).

4.4. Thymol

Thymol (isopropyl-m-cresol) is a volatile, phenolic monoterpene produced by several plants including thyme (Thymus vulgaris L.) (Baerheim Svendsen and Scheffer, 1985). Thymol has well-known antiseptic, antifungal, and anthelminthic properties (Wilson et al., 1977) and is also used for food and fragrance applications (Bauer et al., 1990).

Research by Soler-Serratosa et al. (1996) using combinations of thymol and benzaldehyde for root-knot and cyst nematode control on soybeans showed that both compounds exhibited wide spectrum nematicidal activity with Meloidogyne spp. and Dorylaimid nematodes being more sensitive than cyst nematode and nonparasitic nematodes (Soler-Serratosa et al., 1996). In addition to the direct toxicity of these compounds to nematodes, it was hyopothesized that stimulation of beneficial microflora by the compounds or their products, altered host response, and a deleterious physico-chemical environment may all contribute to reduced gall formation (Soler-Seratosa et al., 1996).

4.5. Citral

Citral, is the aldehyde of geraniol, and occurs in the volatile oils of lemon grass, lemon, orange, limetta, and pimento (Harborne and Baxter, 1993). The flavor of lemon oil is largely due to its citral content, and the pure aldehyde may be used to increase the flavoring power of commercial samples of that oil (Harborne and Baxter, 1993). In research trials evaluating the nematicidal potential of citral compared to other allelopathic chemicals, citral was less nematicidal against M. incognita juveniles, and more phytotoxic to tomato than benzaldehyde in vitro, and when added to a peat-based potting mix (Kokalis-Burelle et al., 2002). When tested in vitro against root-knot nematode eggs, citral reduced egg viability by 80%, but also decreased tomato seed germination and growth in greenhouse trials (Kokalis-Burelle et al., 2002). However, when evaluated in soil, citral reduced populations of root-knot nematode juveniles, galling on roots, and increased cotton growth when applied at 0.1 -0.5 ml/ kg soil in the greenhouse, and at 0.18 -2.14 ml/kg soil in field microplots with no phytotoxicity to cotton (Bauske et al., 1994). This difference in phytotoxicity may be due to the difference in host plants tested or to the presence of microorganisms in soil compared to the relatively uncontaminated conditions that occur in vitro and in potting media.

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