Allelochemicals Biologica... - Name

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CHUIHUA KONG
1940; Bailey and Mansfield, 1982). Anti-microbial allelochemicals or phytoalexins
have been extensively investigated and play important roles in plant defense (Dixon,
2001). Most of anti-microbial allelochemicals have relatively broad-spectrum activity
on pathogens and specificity is often occurred in cropping systems. Accordingly, an
understanding of the interactions between allelochemicals and pathogenic organisms
is essential in disease control in agro-ecosystems.
The structures and sources of many allelochemicals with anti-microbial activity
have been documented (Dixon, 2001; Grayer and Harborne, 1994; Harborne, 1999).
In this chapter, evidence for anti-microbial functions of allelochemicals from weed
Ageratum conyzoides L. and food crop Oryza sativa L. has been reviewed. Effects of
these allelochemicals on related pathogen management in the A. conyzoides
intercropped citrus orchard and the paddy ecosystem were discussed.
2. ALLELOCHEMICALS OF A. conyzoides AND THEIR EFFECT ON
RELATED PATHOGENS
2.1. Ageratum conyzoides L.
Ageratum conyzoides of the family Compositae (Asteraceae) is native to Central
America (Kossmann and Groth, 1993) Caribbean and Florida (USA). It has spread to
West Africa, Southeast Asia, South China, India, Australia and South America
(Okunade, 2002; Stadler et al., 1998). A. conyzoides is an annual erect, branched
herb growing 15 to 100 cm tall. Its stem is covered with fine white hairs, leaves are
opposite, pubescent with long petioles and include glandular trichomes. It has a shallow
tap root system. The inflorescence contains 30 to 50 pink or purple flowers arranged
in a corymb and are self-incompatible (Jhansi and Ramanujam, 1987). The fruit is an
achene with an aristate pappus and is easily dispersed by wind and animals fir. Seeds
are positively photoblastic and remain viable upto 12 months (Okunade, 2002). The
seeds germinate between 20-25°C. It prefers a moist, well drained soil but may tolerate
dry conditions (Ladeira et al., 1987). This species has great morphological variations
and appears highly adaptable to varying ecological conditions (Hu and Kong, 2002a).
It is a pioneer plant growing in ruined sites and cultivated fields and often becomes
dominant and forms a stand in natural community and is resistant to common insects
or diseases (Liang and Hunag, 1994). Although it is harmful to crops and invades
cultivated fields and interferes with the natural community compositions, it has been
used as folk medicine in several countries and it has anti-microbial, insecticidal and
nematicidal properties (Ming, 1999; Okunade, 2002). In Central America A.
conyzoides has been bred for many colours of flowers (Stadler et al., 1998). In South
China A. conyzoides is traditionally used as green manure in fields to increase the
crop yields, and usually is intercropped as understory in citrus orchards to suppress
weeds and control other pests (Liang and Hunag, 1994; Kong et al., 2004b). This
species appears to be a valuable agricultural resource (Ming, 1999).