Allelochemicals Biologica... - Name

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CHUIHUA KONG
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).
P. citrophthora P. aphanidermatum F. solani
Figure 1. Population of major pathogenic fungi in the A. conyzoides intercropped and nonintercropped
citrus orchards soils. Population density was mean individual in per gram soil.