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Incidence of root-knot nematodes
on weeds in South Africa
R
oot-knot nematodes (
Meloidogyne
spp.) are the most widespread and
damaging of the plant-parasitic
nematodes worldwide and they
survive and even thrive on weeds.
They are well-known for their widespread
distribution and extremely polyphagous
nature with more than 3 000 plant hosts,
including crops, ornamentals, medicinal
and aromatic plants as well as weeds. They
are favoured by warm temperatures that
are prevalent in the tropical and subtropi-
cal regions.
However, some species are able to adapt
to local climatic conditions and may be
found in temperate climates. The most
commonly occurring root-knot nematode
species in South Africa are
M. incognita
and
M. javanica
. The aboveground symptoms
of root-knot nematode infection are often
mistaken for nutrient deficiency symptoms.
It is a fact that agricultural fields will inevi-
tably have weeds and most producers and
agricultural scientists view weeds as an in-
dependent problem, since weeds compete
for water, nutrients, light and space, which
reduce crop growth and yield.
Although competition is the most impor-
tant effect weeds have on crop production,
weeds are part of the ecology of a field
and can have other less obvious effects,
such as serving as a reservoir for insects,
diseases and nematodes.
The importance of weeds
as nematode hosts
The host status and degree of nematode
reproduction on weeds is a major concern
in developing and implementing inte-
grated nematode management program-
mes because weeds are present during
crop growth and afterwards with the
fallow period.
Weed control is often conducted relative
to weed populations and threshold levels
established to manage weed/crop compe-
tition. Nematode reproduction on weeds
may seem to be a simple problem to solve,
with the solution simply being to control
the weeds, particularly during the off-crop-
ping season. However, this could lead to
increased grower cost, greater soil erosion
potential, less nutrient recycling and lower
soil organic matter levels.
Some options to reduce these negative as-
pects would include selectively eliminating
major weed hosts with herbicides (mostly
broadleaf weeds), encouraging the growth
of non-host weeds (mostly grassy weeds),
or planting cover crops that suppress weed
populations.
It is essential to highlight that without a
weed management programme in place for
both in-season and off-season periods, the
benefits of crop rotation for nematode man-
agement won’t work or func-
tion due to weed hosts of
plant-parasitic nematodes.
61
May 2015
ON FARM LEVEL
Integrated pest control
Root-knot nematodes / Weeds / Hosts
NANCY NTIDI,
ARC-Grain Crops Institute
SCIENTIFIC NAME
FAMILY NAME
ROOT-KNOT
NEMATODES
Amaranthus deflexus
Amaranthaceae
Mj; Mi
Amaranthus hybridus
Amaranthaceae
Mj; Mi; Mh
Bidens bipinnata
Asteraceae
Mj; Mi; Mh
Chamaesyce inaequilatera
Euphorbiaceae
M.sp.
Chenopodium carinatum
Chenopodiaceae
Mi
Chenopodium murale
Chenopodiaceae
Mi
Chloris virgate
Poaceae
Mi; Mh
Commelina benghalensis
Commelinaceae
Mi; Mj; Mh
Cynodon dactylon
Poaceae
Mi; Mj
Cyperus esculentus
Cyperaceae
Mi; M.sp.
Cyperus rotundus
Cyperaceae
Mi
Datura ferox
Scrophulariaceae M.sp.
Datura stramonium
Scrophulariaceae Mi
Eleusine corocana
Poaceae
Mi; Mj
Flaveria bidentis
Asteraceae
Mj
Hibiscus cannabinus
Malvaceae
Mi; Mj; Mh
Hibiscus trionum
Malvaceae
Mi; Mj
Ipomoea purpurea
Convolvulaceae
Mi; Mj
Nicandra physaloides
Solanaceae
Mi
Tagetes minuta
Asteraceae
Mi; Mj
Mi =
M. incognita
; Mj =
M. javanica
; Mh=
M. hapla
; M. sp. = Species of root-knot
nematode not identified
TABLE 1: LIST OF FEW COMMON WEEDS AND THEIR SCIENTIFIC AND
FAMILY NAMES OCCURRING AS HOSTS OF ROOT-KNOT NEMATODES IN
SOUTH AFRICA.
1: Weeds occurring between the crop plants.
2: A galled
Hibiscus
spp. root due to infection by root-knot nema-
tode infection.
1
2