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Allelochemicals: Biological Control of Plant Pathogens and Diseases, 193 -206. © 2006 Springer. Printed in the Netherlands.

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).

2.2 The essential oil of A. conyzoides and their biological activities on related pathogens

A. conyzoides has a wide range of secondary metabolites including flavonoids, chromenes, benzofurans and terpenoids. Among these secondary metabolites, some are allelochemicals inhibiting the growth of other organisms (Okunade, 2002; Pari et al., 1998). Usually, A. conyzoides can produce and release volatile chemicals into the environment. The concentration of its released volatiles is so high that the unpleasant odor can be smelled in the fields. Therefore, most investigations have focused on chemical components of its essential oil (Albersberg and Singh, 1991; Ekundayo et al., 1988; Menut et al., 1993; Wandji et al., 1996). It was found that ageratochromenes and their derivatives, monoterpenes and sesquiterpenes were the major components of the essential oil from A. conyzoides (Kong et al., 1999; 2002a; Pari et al., 1998).

The allelopathic potential of volatile allelochemicals from A. conyzoides has been reported in our previous papers (Kong et al., 1999; 2002a; 2004a). Anti-microbial effects of the essential oil from A. conyzoides have been confirmed for a long time (Biond et al., 1993; Dixit et al., 1995; Rao et al., 1996). Table 1 showed that several fungal pathogens, such as Rhizoctonia solani, Botrytis cinerea, Sclerotinia sclerotiorum, were significantly inhibited by the essential oils of A. conyzoides (Kong et al., 2001; 2002a). The quantity and variety of allelochemical produced by A. conyzoides varies depending on its growth stages and habitats and so do their growth inhibitory effects on the pathogens (Kong et al., 2002a, 2004a). A. conyzoides produces different volatiles in larger quantities when infected with Erysiphe cichoracearum (Kong et al., 2002a).

Table 1. Inhibitory effects of essential oil from the A. congzoides collected from different growth stages and habitats on fungal pathogens.

The essential oil collected Pathogens from R. solani B. cinerea

S. sclerotiorum

Growth stages 4-leaf

Pre-flowering

Peak-flowering

Mature

Habitats

Cultivated field

Under citrus canopy

Roadside

Control

50% Carbendazin

100c

Test concentrations of the essential oil were 100 ^g/ml. All data are inhibitory percentage of spore germination of fungal pathogens tested and mean of 3 replicates with standard error. Data in a column not followed by the same letter are significantly different, p=0.05, ANVOA with Ducan's multiple range test.

2.3. Allelochemicals and pathogen management in the A. conyzoides intercropped citrus orchard

Besides the volatiles, A. conyzoides can biosynthesize and release non-volatile allelochemicals into the soil, thus, inhibiting the growth of other plants and microorganisms in soils. Polymethoxyflavones, ageratochromene and its analogues are rare in natural products but they have been found in A. conyzoides (Adesogan and Okunade, 1979; Gonzalez et al., 1991; Horie, et al 1993; Okunade, 2002). These compounds have obvious anti-microbial activity and have been used in managed ecosystem.

In South China A. conyzoides is often intercropped in citrus orchards as an understory plant that quickly becomes dominant in citrus orchards. In addition, intercropping A. conyzoides makes the citrus orchard ecosystem more favorable for predatory mites (Amblyseius spp.). These mites are effective natural enemies of the citrus red mite (Panonychus citri). Further investigations showed that the pathogenic fungi Phytophthora citrophthora, Pythium aphanidermatum and Fusarium solani were isolated from both the A. conyzoides intercropped and non-intercropped citrus orchards soils. However, populations of these fungi were lower in the A. conyzoides intercropped citrus orchard than in the non-intercropped citrus orchard (Figure 1), indicating that intercropping with A. conyzoides in citrus orchards markedly decreased the population of soil pathogenic fungi. It may have resulted from the phytotoxins in the A. conyzoides intercropped citrus orchard soil (Kong et al., 2004c).

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