| Parasitic
weeds |
Namibian
crop pests
#
63, 64, 65 and 73
|
| Striga spp. |
Order: Scrophulariales |
Family: Scrophulariaceae |
A total of five Striga species are
recorded for Namibia but the following three are the most common pests:
S. hermonthica, S. asiatica
and S. gesnerioides. A
fourth parasitic species (not a Striga species) was photographed
recently in northern Namibia.
No.
63 Striga hermonthica
Striga hermonthica normally parasitise
sorghum plants. In some cases it was reported on pearl millet as well.
S.
hermonthica is usually larger than the other Striga species. The flowers
are pale red, but vary considerably from red or pink to white.
Photo from Eefa village,
Oshana region |
|
No.
64 Striga asiatica ( = Striga lutea)
Striga asiatica parasitise maize
and sorghum as well as wild grasses. In north central Namibia it is mainly
found on sorghum. S. asiatica is usually smaller and more compact
than S. hermonthica. The flowers are bright red, but other colours
such as blue and brick red have also been recorded.
Photo from Eefa village, Oshana
region
|
 |
No.
65 Striga gesnerioides
Striga gesnerioides has a variety
of hosts, mainly legumes. It was expected to parasitise cowpea in northern
Namibia but this is being disputed at present. No cases are known from
outside West Africa of S. gesnerioides on cowpea (C. Riches, May
2000, pers.comm). The few specimens we have found Namibia so far have only
parasitised weeds. Please report if you find
any specimens of Striga gesnerioides on cowpea in Namibia.
It is normally a small species of
up to 15 cm high but taller specimens have also been recorded. The flowers
are pink. Leaves are small and scale-like and the plant has a strange,
bushy appearance. The plant is usually pale green, but darker plants have
been found growing on weeds in Namibia.
Photo from Ruacana beach, Kunene
region.
More photos
|
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No. 73
Alectra sp.
This species has not been identified
yet, but is most likely an Alectra vogelii. Farmers reported that
cowpea plants were also attacked by a parasitic weed with yellow
flowers.
The plant was eventually picked and photographed by researchers. Here,
a mature cowpea plant is shown to the left with 4 stems of the parasite
closely attached to the right.
Photo from Mahenene Research Station,
Omusati region, photograph by Thomas Sappe & Klaus Fleissner.
More
photos
|
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Common names:
|
Witchweed (English); onime
(Oshiwambo); oludhigo (Oshindonga) |
Pest status:
|
There are not sufficient data to
evaluate the pest status of parasitic weeds in northern Namibia.
At present there is only evidence, that all three Striga species are widespread
but not common. The existence of local names for parasitic weeds, however,
indicates that it may be important to some farmers.
The pest status is complex because
the forms of parasitic weeds that are found on one species cannot germinate
on another host plant. Careful observations and records are therefore necessary
to clarify which crops are parasitised by which species. |
Distribution:
|
During the 1997/98 season, many
fields were checked for pests by extension staff in north central Namibia.
The investigations included all aspects of pests, diseases and weeds, so
they were not detailed.
Striga species were first noticed
half way through the season, in a few fields in Omusati and Oshana regions.
In most cases it was S. hermonthica, in one case S. hermonthica
and S. asiatica together and a few reports were made about S.
gesnerioides on cowpea.
|
Biology:
|
The tiny brown seeds of Striga can
stay dormant in the soil for many years, but they cannot germinate until
the right host plant is grown in the field. In order to germinate, the
seed must be in close contact with roots of young host plants, such as
for instance sorghum, millet or cowpea.
The roots of the host plant release
chemical substances that trigger the germination of Striga seed. The Striga
then develops a specialised structure (haustoria) that links the Striga
to the host plant. More haustoria develop, until the Striga is closely
anchored to the host plant and can suck nutrients and energy from the host.
Germination of Striga typically happens
10 days after the first rains, while host plants are developing. The attack
is underground, and the main damage has already occurred, before the Striga
can be seen above ground. Later on, Striga develops its own leaves and
is less dependent on the host plant.
Striga flowers a few weeks after
germination and each plant may produce as many as 20 000 seeds. These seeds
need about six months to break their dormancy and then stay viable in the
soil, waiting for the next host crop to be planted.
|
Control:
|
Control of Striga is difficult and
time consuming. At present, chemical control is not recommended in Namibia,
as the chemicals are expensive, handling of them is very difficult and
no research results are available to support chemical treatment in Namibia.
Farmers are advised to improve soil
fertility where Striga is a problem. Soil fertility has an effect on Striga
infestation, the more fertile soils are less infested with Striga. Use
of manure and/or small amounts of fertilizer may reduce Striga infestation,
when combined with weeding of Striga plants before seed setting.
The option of using crop rotation
is limited in north central Namibia. Very few crops can grow in the semi-arid
climate, so most farmers can only rotate sorghum with pearl millet.
Hand weeding of the infested areas
before Striga set seeds is the most important control method at present.
Striga should be weeded out as soon as any flowering is observed, as the
development of seeds takes only a few weeks. It may be necessary to weed
the area twice in a season.
ICRISAT, IITA and other research
organisations have identified varieties of sorghum and cowpea that are
resistant or tolerant to Striga / Alectra infestation, but these are not
yet adapted to Namibian conditions.
Note: For a
complete description of the control options please refer to the excellent
description in:
Parasitic
Weeds of the World.
Biology and Control. C. Parker and C.R.
Riches. 1993. CAB international. ISBN 0 85198 873 3
The book is available in the Agriculture and Environment library,
Ongwediva |
Other notes:
|
Please report any occurrence of
parasitic weeds. Records should include date; farmer’s name; village; region;
name of the host plant; name of collector as well as a description of the
plant features, including common names and the dialect used. Dig up a few
samples of the entire plant for identification and dry the plants well
before sending them. (The description of samples is needed because Striga
like many other parasitic plants change colour when they are dried). |
Acknowledgements
This page could not have been written
without the kind help of staff from the National herbarium of Namibia.
The text was compiled from a FSRE Training Manual: "Crop Pests in Northern
Namibia: Pearl millet, Sorghum, Maize, Cowpea and Cucurbits." 1999.
ISBN: 0-86976-483-7.
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