Allelochemicals Biologica... - Name

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potential when applied as seed treatments (Akhtar and Mahmood, 1995; 1997) and
bare-root treatments (Akhtar and Mahmood, 1993; 1994) leading some to conclude
that compounds found in neem may act as inducers of resistance to some nematodes
including M. incognita and Rotylenchulus reniformis (Siddiqui and Alam 1988). Other
nematode-suppressive mechanisms of compounds derived from neem include
antifeedent, repellent, deterrent, growth disruption, juvenile toxicant, and ovicidal
properties (Akhtar, 1998). Testing of neem-based products and development of
application techniques for plant-parasitic nematode control is increasing in western
countries. There are currently several neem-based pesticides available in the United
States for use on certain greenhouse and ornamental crops, with many more available
for use in India as insecticides (Akhtar, 2000). A comprehensive review of the
nematode suppressive potential of neem products is provided by Akhtar (2000).
4. ALLELOCHEMICALS AS BIOPESTICIDES
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
NANCY KOKALIS-BURELLE AND RODRÍGO RODRIGUEZ-KÁBANA 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