asiatica (L.) Kuntze][STRLU][CDFA list: A][Federal Noxious Weed] Photographs
Lour., Striga zangebarica Klotzsch.,
Striga pusilla Hochst., Striga coccinea Benth.,
Striga spanogheana Miq., Striga parvula Miq., Stiga
DESCRIPTION:Noxious annual semiparasite of
tropical and subtropical annual grasses. Plants are capable of
parasitizing numerous grass species. In the U.S., witchweed
is primarily a parasitize of corn, sorghum, and weedy grasses,
especially crabgrass (Digitaria spp.). A heavy infestation
can severely damage crops and is difficult to control. Symptoms
in host plants include stunting, chlorosis, and wilting. Witchweed
is native to semi-arid and tropical grassland regions of Africa
and Asia, but can also flourish in temperate regions outside its
natural range. Populations are highly variable, and flower color
varies regionally, from red, orange, or yellow in Africa to pink,
white, yellow, or purple in Asia. In the U.S., infestations currently
exist only on agricultural lands near the eastern border between
North and South Carolina. Federal and state quarantine and eradication
programs have greatly reduced the area of these infestations over
a period of many years, but with great economic loss. Plants in
the Carolinas have red flowers and were probably introduced from
Africa. Although witchweed is not established in California
at publication time, it is included on the state noxious weed
list since early detection of newly introduced plants is most
likely to result in successful eradication efforts. Infestations
are widespread in Africa, Asia, Australia, Indonesia, Philippines,
Madagascar, and New Guinea.
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above ground, but white succulent shoots can be found attached
to host roots.
above ground contains chlorophyll and is bright-green,
typically sparsely covered with coarse, short, white, bulbous-based
hairs. Stems erect, stiff, branched, to 30 cm tall, rarely to
45, +/- square in cross section in the upper portions of plants.
Leaves nearly opposite, narrowly lanceolate to linear,
about 1-3 cm long, with successive leaf pairs perpendicular to
one another (decussate). Underground stems round with scale-like
leaves, white, turn bluish when exposed to air.
and UNDERGROUND STRUCTURES:Succulent, round, white, lack root hairs, attach
to host roots.
Sessile, axillary. Corolla two-lipped, tube bent, to 10
mm long, 6-9 mm wide, bright to brick red, occasionally
yellow or white, with outer surfaces sparsely covered with tiny
glandular hairs. Calyx (sepals as a unit) tubular, with 10
ribs, 5 lobes. Bracts below calyx 2, linear-lanceolate, ~
5 mm long. Flowers self-pollinate before opening
when sticky pollen balls cling to the elongating style.
and SEEDS:Capsules ovoid, 5-sided, with a narrow wing
at each corner. Style 1, persistent, often with clinging pollen
masses. Capsules can contain up to ~ 1400 seeds (~ 550 average).
Seeds brown, oval, dust-like, ~ 0.2 mm long. Seed surfaces
striated, overlayed with a reticulate pattern (visible with magnification).
with agricultural lands, especially those with light soils and/or
low nitrogen fertility.
time, there are no known infestations in California.
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root parasite. Reproduces by seed.
Seeds disperse with wind, water, soil movement, human activities,
and by clinging to the feet, fur, or feathers of animals, farm
machinery, tools, shoes, and clothing. Seeds require an afterripening
period of 6 weeks under warm conditions to 40 weeks under freezing
conditions. Dormant seeds survive freezing [to 15º
C (5º F)] for at least 49 days and can remain viable under
field conditions for up to 14 years or more. Germination is complex
and requires about a 1-3 week "conditioning" period
at a suitable temperature regime [20-40º C (68-104º
F), optimum 35º C (95º F)] under moist conditions, followed
by a chemical signal from a nearby root of a host plant. Proximity
of host root to seed must be within a few millimeters. Under these
conditions, seeds germinate within 24 hours. After 3 weeks of
conditioning without a chemical signal, germination ability of
seeds decrease, and some seed may pass into a secondary dormancy.
Light exposure or wet soils inhibit germination. Irregular or
light rainfall appears to promote seed germination and plant vigor.
High soil nitrogen reduces damage to host plants. Corn is usually
parasitized 2-3 weeks after planting, and witchweed shoots
emerge about 3-8 weeks later. Flowers develop about 3 weeks after
emergence. Viable seed is produced within 2 weeks of flowering.
A minimum of ~ 60 days is required from seed germination to seed
FAVORING/DISCOURAGING SURVIVAL:Much damage to crops can occur before witchweed emerges. Light infestations
can usually be controlled by hand pulling before seed is produced.
For heavier infestations, an integrated management plan is required.
Options include: growing trap-crops (those that stimulate witchweed
seed germination but do not host the parasite) such as cotton
or catch-crops (susceptible crops that are harvested before witchweed
seed is produced) for 3 or more years; allowing land to lay fallow
for several years; injecting the soil with ethylene (a germination
stimulant); enhancing soil nitrogen fertility; growing the most
tolerant cereal varieties; utilizing herbicides known to prevent
witchweed emergence or seed production.
SIMILAR SPECIES:Witchweed is unlikely to be confused
with any other agricultural weed.
Prevention and Control: Witchweed is not currently established
in California. However, it has been designated as a class A noxious weed to
ensure eradication efforts for any future infestations. Witchweed primarily
parasitizes corn, sorghum, and other weedy grasses, especially Digitaria species.
Witchweed may cause extensive economic losses in affected crops. Seed may survive
in the soil for up to ten years. Eradication activities include, surveys for
infestations, quarantine of infested areas, and subsequent control methods.
Survey and quarantine should detect and eliminate any vectors of witchweed spread.
Infested agricultural equipment may be the main avenue of spread and any equipment
working in infested areas should be completely cleaned of all plant residues
and soil. Control methods include maintaining excellent weedy grass control
in every crop or fallow period to prevent witchweed survival and reproduction
on noncrop species. Ethylene gas may be used to stimulate witchweed seed germination
when no available hosts are present. Soil fumigation may also be effective for
killing the witchweed seed bank. Planting non-susceptible trap crops may also
allow witchweed germination ad subsequent mortality if no susceptible hosts
are available. Witchweed eradication has been highly successful in North and
South Carolina with over 99% of the infestations eradicated.