Striga, commonly known as witchweed, is a genus of parasitic plants that occur naturally in parts of Africa, Asia, and Australia. It is currently classified in the family Orobanchaceae, although older classifications place it in the Scrophulariaceae. Some species are serious pathogens of cereal crops, with the greatest effects being in savanna agriculture in Africa. It also causes considerable crop losses in other regions, including other tropical and subtropical crops in its native range and in the Americas. The generic name derives from Latin strī̆ga, "witch". Witchweeds are characterized by bright-green stems and leaves and small, brightly colored and attractive flowers. They are obligate hemiparasites of roots and require a living host for germination and initial development, though they can then survive on their own. The number of species is uncertain, but may exceed 40 by some counts.
Hosts and symptoms Although most species of Striga are not pathogens that affect agriculture, some species have devastating effects upon crops, particularly those planted by subsistence farmers. Crops most commonly affected are maize, sorghum, rice and sugarcane. Three species cause the most damage: Striga asiatica, S. gesnerioides, and S. hermonthica. Witchweed parasitizes maize, millet, sorghum, sugarcane, rice, legumes, and a range of weedy grasses. It is capable of significantly reducing yields, in some cases wiping out the entire crop. Host plant symptoms, such as stunting, wilting, and chlorosis, are similar to those seen from severe drought damage, nutrient deficiency, and vascular disease.
Lifecycle
Each plant is capable of producing between 90,000 and 500,000 seeds, which may remain viable in the soil for over 10 years. Most seeds produced are not viable. An annual plant, witchweed overwinters in the seed stage. Its seeds germinate in the presence of host root exudate, and develop haustoria which penetrate host root cells. Host root exudate contain strigolactones, signaling molecules that promote Striga seed germination. A bell-like swelling forms where the parasitic roots attach to the roots of the host. The pathogen develops underground, where it may spend the next four to seven weeks before emergence, when it rapidly flowers and produces seeds. Witchweed seeds spread readily via wind and water, and in soil via animal vectors. The chief means of dispersal, however, is through human activity, by means of machinery, tools, and clothing.
Haustoria development Once germination is stimulated, the Striga seed sends out an initial root to probe the soil for the host root. The initial root secretes an oxidizing enzyme that digests the host root surface, releasing quinones. If the quinone product is at the appropriate concentrations, a haustorium will develop from the initial root. The haustorium grows toward the host root until it makes contact with the root surface, establishing parasitic contact in relatively short order. Within 12 hours of initial haustorium growth, the haustorium recognizes the host root and begins rapid cell division and elongation. The haustorium forms a wedge shape and uses mechanical force and chemical digestion to penetrate the host root, pushing the host cells out of the way. Within 48–72 hours, the haustorium has penetrated the host root cortex. Finger-like structures on the haustorium, called oscula (from Latin osculum, "little mouth") penetrate the host xylem through pits in the membrane. The oscula then swell to secure their position within the xylem membrane. Striga sieve tubes develop along with the oscula. Shortly after the host xylem is penetrated, Striga sieve tubes develop and approach the host phloem within eight cells. This eight cell layer allows for nonspecific nutrient transport from the host to the Striga seedling. Within 24 hours after tapping the host xylem and phloem, the Striga cotyledons emerge from the seed.
Environment Temperatures ranging from 30 to 35 °C (86 to 95 °F) in a moist environment are ideal for germination. Witchweed will not develop in temperatures below 20 °C (68 °F). Agricultural soils with a light texture and low nitrogen levels tend to favor development. Still, witchweed has demonstrated a wide tolerance for soil types if soil temperatures are favorably high. Seeds have been shown to survive in frozen soil of temperatures as low as −15 °C (5 °F), attesting to their aptitude as overwintering structures. Soil temperature, air temperature, photoperiod, soil type, and soil nutrient and moisture levels do not greatly deter the development of witchweed. These findings, while limited to the Carolinas in the United States, seem to suggest that the pathogen could successfully infect the massive corn crops of the American Midwest.
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