Allelochemicals Biologica... - Name

ALLELOCHEMICALS : MANAGEMENT OF PLANT-PARASITIC NEMATODES 19
of extracts and diffusates resumed when solutions were removed indicating that
compounds found in solutions were nematostatic rather than nematicidal. More specific
information is needed on the range of activity and mechanisms involved in nematode
suppression with compounds found in marigold. Additional research in these areas
would increase the potential for successful incorporation of marigold or its active
allelopathic compounds into production systems for nematode control.
2.2. Sorghum-sudangrass (Sorghum bicolor X S. sudanense)
Sorghum has long been recognized for its allelopathic properties toward other plants
(Guenzi and McCalla, 1966) and more recently to be suppressive to nematodes (Kinlock
and Dunavin, 1993; Mojtahedi et al., 1993a). It was initially hypothesized that the
nematicidal compound in sorghum-sudangrass green manures was hydrogen cyanide
produced by hydrolysis of dhurrin in leaf tissue (Andewusi, 1990). However, Czarnota
et al. (2003) examined root exudate production and composition of seven genetically
diverse sorghum accessions, including two sorghum-sudangrass hybrids, and found
that although variation occurred in exudate constituents among accessions, the
predominant constituent in all exudates was the phenolic compound sorgoleone
(Czarnota et al., 2003).
Suppression of parasitic nematodes in the field with sorghum-sudangrass has
been inconsistent (MacGuidwin and Layne, 1995). It has been demonstrated that this
crop is not effective for reducing populations of lesion nematodes (Pratylenchus sp.),
an important plant-parasitic genus (MacGuidwin and Layne, 1995). Many studies
have confirmed that sorghum-sudangrass is effective in reducing field populations of
Meloidogyne spp., but that it cannot be recommended if stubby root nematode,
Paratrichodorus minor, is present and of concern due to the high reproductive rates
of P. minor on sorghum-sudangrass (McSorley and Gallaher, 1991; McSorley et al.,
1994a; McSorley and Dickson, 1995). The selective nature of parasitic nematode
control with this allelopathic crop serves as an example of why it is critical to know
what nematode species are present in a location when employing cover or rotation
crops for key nematode pests. Threshold levels for secondary parasitic nematodes
should be established and susceptibility of potential cover crop known when employing
these strategies.
2.3. Sesame (Sesamum indicum)
Sesame is an important seed and oil crop worldwide. In addition to being a poor host
for root-knot nematodes (Rodriguez-Kabana et al., 1988a; 1989), sesame is known to
produce several lignin compounds including sesamin and sesamolin which are
antioxidants that function as insecticides and insecticidal synergists and are
hypothesized to be the active allelopathic compounds in sesame (Bedigian and Harlan,
1986). Research has been conducted in the southern United States during the past 15
years to evaluate the potential for use of sesame as a profitable rotation crop for root-