The purple witchweed Striga hermonthica threatens the lives of over 100 million people in Africa and infest about 40% of arable land in the savanna region, causing an estimated annual loss of $ 7 to 13 billion (www.icipe.org). It is almost certainly responsible for more crop loss in Africa than any other individual weed species. Over 5 million ha of crops - mainly sorghum, millets and maize - are affected in six countries of West Africa alone, possibly 10 million ha in Africa as a whole. One plant of S. hermonthica per host plant is estimated to cause approximately 5% loss of yield (Parker and Riches, 1993) and high infestations can cause total crop failure. Overall yield losses are estimated at 21% of all sorghum in northern Ghana, 10% of all cereals in Nigeria, 8% in Gambia and 6% in Benin. Other countries seriously affected include Cameroon, Cote d'Ivoire, Burkina Faso, Niger, Mali, Senegal, Togo, Sudan, Ethiopia, Kenya, Uganda and Tanzania.
The damaging effect of S. hermonthica on the host plant derives not only from the direct loss of water, minerals, nitrogen and carbohydrate, but from a disturbance of the host photosynthetic efficiency and a profound change in the root/shoot balance of the host, leading to stimulation of the root system and stunting of the shoot. Young Striga seedlings are completely parasitic on the host while they are below the soil level and, at this stage, cause maximum damage to the host.
As S. hermonthica occurs mainly under conditions of low fertility. It is also associated with farming systems in Africa in which farmers have few resources and very few options in terms of control measures.
S. hermonthica causes characteristic yellowish blotches in the foliage about 1 cm long by 0.5 cm wide. In later stages whole leaves may wilt, become chlorotic and die. Stems are shortened, though leaf number may not be reduced. Inflorescence development is delayed or prevented. Root systems, at least in early stages, may be stimulated, and haustoria 1-2 mm across appear like nodules.
The natural host range of S. hermonthica is normally limited to Gramineae (Poaceae), but weak attachment to groundnut, cowpea, lablab and soyabean was obtained in pots and there have been unconfirmed reports of infestation of groundnut and sesamum fields in West Africa. Apart from the wild hosts listed above, S. hermonthica is occasionally observed on crowfoot grass (Dactyloctenium aegyptium), Panicum walense, goose grass (Eleusine indica), rice grass paspalum (Paspalum scrobiculatum), Pennisetum violaceum and on Cynodon, Cymbopogon, Ophiuros and Brachiaria spp. Individual biotypes may have a narrower host range than the species. In particular, there are forms which attack sorghum but not pearl millet and vice versa.
Affected plant stages
Flowering stage, fruiting stage, pre-emergence, seedling stage and vegetative growing stage.
Affected plant parts
Leaves, stems and whole plant.
Leaves: abnormal patterns; yellowed or dead.
Stems: abnormal growth.
Whole plant: dwarfing; early senescence