|
Control of the
witchweed Striga hermonthica
by intercropping with Desmodium spp.
Z.R. Khan,
A. Hassanali, W. Overholt, T.M. Khamis,
J.A. Pickett & L.J. Wadhams
It was discovered through field work in Kenya that
inhibition of witchweed, Striga
hermonthica, a in maize-Desmodium
uncinatum intercrop was significantly greater than that observed
with other legumes, e.g. sun hemp, soybean and cowpea. The objective of
this study was, therefore, to investigate the mechanisms by which D. uncinatum inhibits the parasitic plant.
This could be by increasing available nitrogen, by offering shade
or by an allelopathic effect, all of which are known to give some
control of these parasitic weeds. A field trial was established to
determine the role of shading, nitrogen and allelopathy in Striga
suppression by D. uncinatum.
The treatments were maize intercropped with D.
uncinatum without/with 120 kg nitrogen per ha, maize monocrop
without/with 120 kg nitrogen per ha, and maize monocrop with artificial
ground shading made of maize straw with and without 120 kg nitrogen per
ha. Emergence and attachment of Striga
to maize was measured and maize plant height and grain yield were
recorded. To investigate the possibility of an allelochemical mechanism,
effluent from D. uncinatum roots was used to irrigate maize planted with
approximately 3,000 Striga seeds.
D. uncinatum and maize were
planted in different sets of pots in previously autoclaved soil.
Distilled water dripped into D.
uncinatum containing pots which were placed above the pots
containing maize. Comparison was made between maize plants irrigated by
root exudates of D. uncinatum
against those irrigated with water passing through pots with autoclaved
soil but without D. uncinatum.
Emergence and attachment of Striga
to maize was monitored in all treatments. In field trials, although soil
shading and addition of nitrogen fertiliser showed some benefits against
Striga infestation, a clear
allelopathic mechanism was also observed.
This was confirmed by a dramatic reduction in Striga
infestation when eluate from D.
uncinatum roots was introduced into pots of maize growing in soil
seeded with high levels of Striga.
Growth of the parasitic weed was almost completely
suppressed, whereas extensive infestation occurred with control eluate.
The allelopathic mechanism was found to involve inhibition of
development of haustoria of Striga. Work is ongoing to identify the compounds, released from the
D. uncinatum roots, involved
in suppression of the parasite. The sophisticated mode of action
demonstrated here, when fully elucidated, may give more exploitable
leads which are needed not only in subsistence agriculture but also to
answer future world demands in agricultural production and in developing
new approaches for molecular biology in Striga.
|
