Witchweed [Striga asiatica (L.) Kuntze][STRLU][CDFA list: A][Federal Noxious Weed] Photographs

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SYNONYMS:Striga lutea Lour., Striga zangebarica Klotzsch., Striga pusilla Hochst., Striga coccinea Benth., Striga spanogheana Miq., Striga parvula Miq., Stiga hirsuta Benth.

GENERAL 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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SEEDLINGS:Not visible above ground, but white succulent shoots can be found attached to host roots.

MATURE PLANT:Foliage 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.

ROOTS and UNDERGROUND STRUCTURES:Succulent, round, white, lack root hairs, attach to host roots.

FLOWERS:Summer-Fall. 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.

FRUITS 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).

HABITAT:Primarily associated with agricultural lands, especially those with light soils and/or low nitrogen fertility.

DISTRIBUTION:At publication time, there are no known infestations in California.

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PROPAGATION/PHENOLOGY:Obligate 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 production.

MANAGEMENT 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.

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Werth, C.R., Riopel, J.L. and Gillespie, N.W. 1984. Genetic uniformity in an introduced population of Witchweed (Striga asiatica) in the United States. Weed Science 32: 645-648.

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