Striga
Striga asiatica
witchweed
Asiatic witchweed
slender, not fleshy;
secondary roots present;
haustoria multiple, small, globular.
erect, sometimes fleshy, hispid, puberulent, or glabrous.
drying green, simple or branched medially, obtusely square, not fleshy, antrorsely scabrous-hispid.
cauline, opposite, subopposite, or alternate;
petiole absent;
blade not fleshy, not leathery, margins entire.
basal blade opposite, lanceolate scales, mid-stem blade alternate, ascending or spreading, linear or narrowly elliptic, 20–50 x 2–4 mm, surfaces scabrous-hispid.
terminal, racemes or spikes;
bracts present.
racemes, lax;
flowers alternate;
bracts linear, 20–35 x 1–2 mm, longer than calyx, surfaces hispid.
present or absent;
bracteoles present.
sepals 5(–8), calyx radially or bilaterally symmetric, tubular, lobes lanceolate or subulate;
petals 5, corolla red, brownish red, or purple, rarely white or yellow, bilabiate, salverform, abaxial lobes 3, adaxial 2;
stamens 4, didynamous, filaments glabrous;
staminode 0;
ovary 2-locular, placentation axile;
stigma capitate.
sepals 5(–8), 7–8 mm;
calyx: tube 5 mm, ribs 10, hispid, teeth 5 and equal or 6–8 and subequal, lanceolate or sublanceolate, 2–3 mm;
corolla red, rarely yellow, throat yellow, glandular-pubescent, tube bent, expanded distally, 15 mm, abaxial lobes 12 mm wide, adaxial 5 mm wide;
style 7–10 mm, rolled in when dry, sparsely hairy or glabrous.
dehiscence loculicidal.
oblong, 5–7 x 2–3 mm.
400–600, brown or black, ovoid, wings absent.
= 24, 40 (Nigeria).
Striga
Striga asiatica
Species ca. 40 (2 in the flora).
Striga produces leaves of different sizes; typical proximal leaves are scalelike, and mid-stem leaves are larger. Striga is distinguished from its close relative Buchnera by its bilabiate corolla with an abruptly bent tube, one pollen sac, and glabrous filaments. Buchnera has a bilabiate corolla with a straight or slightly curved tube, two pollen sacs, and pilose filaments. Striga has been divided into three sections based on the number of ribs on the calyx tube (R. Wettstein 1891–1893): sect. Pentapleurae Wettstein with five, sect. Polypleurae Wettstein with ten, and sect. Tetrasepalum Engler with 15.
Thirty-four species and subspecies of witchweeds occur in Africa; 22 are endemic (K. I. Mohamed et al. 2001). All Striga species parasitize hosts in the Poaceae except S. gesnerioides, which grows on hosts in Acanthaceae, Convolvulaceae, Euphorbiaceae, Fabaceae, and Solanaceae. Striga asiatica, S. aspera Bentham, S. forbesii Bentham, S. gesnerioides, and S. hermonthica (Delile) Bentham are of economic importance. Crops most affected by Striga include Digitaria exilis (fonio), Oryza subspp. (upland rice), Pennisetum glaucum (bulrush millet), Sorghum vulgare (sorghum), and Zea mays (maize). Striga gesnerioides is a serious pest on Vigna unguiculata (cowpea, Fabaceae) and a minor pest on other dicot crops. All species of witchweed are listed as noxious weeds by the United States Department of Agriculture and 11 state governments. New infestations of quarantine pests in the United States, such as witchweeds, should be reported to the State Plant Health Director in the appropriate state (http://www.aphis.usda.gov/services/report_pest_disease/report_pest_disease.shtml).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Striga asiatica was discovered in southern North Carolina and adjacent South Carolina in 1956 (H. R. Garriss and J. C. Wells 1956). At one time, 38 counties in the Carolinas were infested (R. E. Eplee 1981). Through a comprehensive control program and quarantine measures, the original infested area has been reduced by 99%, with only about 2135 acres remaining in 2009 in five counties (Bladen, Cumberland, Pender, Robeson, and Sampson) in North Carolina and no acres infested in South Carolina (R. Iverson et al. 2011). Additional information on S. asiatica, maps of infected areas, and the eradication program in the Carolinas can be found in Iverson et al. The authors treat S. asiatica in the narrow sense; reports of S. asiatica from southern Asia require further study. They may refer to S. lutea Loureiro or S. hirsuta Bentham, species included by some (http://www.cabi.org/isc/datasheet/51786) in S. asiatica.
Use of the name Striga lutea by C. J. Saldanha (1963) and C. E. Smith (1966) for plants in the flora area is a misapplication. Saldanha rejected S. asiatica, believing that its Linnaean basionym could not be lectotypified; see F. N. Hepper (1974) for a counterargument and lectotypification.
Studies on climatic requirements and potential for spread indicate that Striga asiatica could invade new areas as a result of global warming (D. T. Patterson et al. 1982; K. I. Mohamed et al. 2006, 2007).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Calyx ribs 10; mid-stem leaf blades ascending or spreading, linear or narrowly elliptic, 20–50 mm; bracts linear, longer than calyces; corollas red, rarely yellow, with yellow throats; parasitic on Poaceae. | S. asiatica |
1. Calyx ribs 5; mid-stem leaf blades appressed, lanceolate, 3–7 mm; bracts lanceolate, shorter than calyces; corollas brownish red or purple, rarely white; parasitic on dicots. | S. gesnerioides |
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