S. asiatica is not a conspicuous weed;it had spread to infest almost 200,000 hectares before being noticed in the USA. There is nothing about the shoot system of S. asiatica to suggest that it is a parasite. The height of the weed is variable, but rarely exceeds 30-40 cm, while some forms may be no more than a few centimetres high. Most other morphological characters are also variable. In vigorous plants there may be many branches, while small individuals or ecotypes may be unbranched. Length of the normal-looking green leaves may vary from 1 to 5 cm but leaf shape is generally narrowly lanceolate. Stem and leaves are sparsely covered in scabrid hairs.
Where infestation is suspected, from previous history or from unexpected wilting symptoms, uprooting the crop can reveal the small white seedlings of S. asiatica on the roots, but the attachments are very fragile and gently washing the roots out of the soil will ensure a better chance of finding them.
A technique for detecting the seeds of S. asiatica as contaminants of crop seed is described by Berner et al. (1994). This involves sampling the bottom of sacks, elutriation of samples in turbulent flowing water and collection of seeds and other particles on a 90 µm mesh sieve. Striga seeds are then separated from heavier particles by suspension in a solution of potassium carbonate of specific gravity 1.4 in a separating column. Sound seeds collected at the interface are then transferred to a 60 µm mesh for counting.
Related invasive species
- Striga asiatica
Related Farm Practice
- Indicators
- Host plants
- Soil
- Ecotypes
- Hosts
- Drought
S. asiatica is a hemiparasitic plant, native to Africa and Asia. In common with most other parasitic weeds, it is not especially invasive in natural vegetation, but is much feared in crop land where infestations can build up to ruinous levels, especially with repeated growing of susceptible cereal crops. For this reason it is included in almost all lists of noxious, prohibited plant species. It has recently been reported in Queensland, Australia. There is also evidence for its continuing spread and intensification within a number of countries in Africa in particular in rice in Tanzania and maize in Malawi. A study by Mohamed et al. (2006) suggests that on the basis of climatic data, there are many territories into which Striga species, including S. asiatica, could be introduced and thrive. Global warming could further increase this potential.
The symptoms of attack by S. asiatica may be apparent some time before the weed emerges, hence, the common name 'witchweed'. At an early stage, these symptoms are indistinguishable from those caused by drought, i.e. wilting and curling of the leaves, but they are strong indicators of S. asiatica if they occur when the soil is still moist. The infected plant may also show stunting from quite an early stage and pronounced scorching of the leaf borders and finally of the whole leaf area may occur at a later stage, hence, the common name 'fireweed' and equivalents in other languages.
As shoot growth is reduced, root growth is increased in plants infected by S. asiatica (Patterson, 1990).
S. asiatica is an obligate parasite and cannot develop without a suitable host plant. Apart from the major crops including sorghum, maize, pearl millet, finger millet, Panicum millets and rice, a wide range of wild hosts are parasitised. For the USA, these are listed by Nelson (1958), the most common being the weed Digitaria sanguinalis. There are many other wild hosts in Africa and Asia. Those recorded up to 1956 are listed in McGrath et al. (1957). Some further host species almost certainly include Andropogon gayanus, Axonopus compressus, Chrysopogon acicularis [ C. aciculatus ], Digitaria smutsii [ D. eriantha ], Eleusine indica, Elionurus elegans, Eragrostis malayana [ Eragrostis montana ], Ischaemum indicum [ Polytrias indica ], I. timorense, Microchloa indica, Rottboellia cochinchinensis, Sporobolus festivus and Stenotaphrum dimidiatum (synonym S. secundatum). Further hosts are listed by Cochrane and Press (1997). A number of broad-leaved hosts are recorded in McGrath et al. (1957) but occurrence on anything other than Poaceae is quite rare and some records may be suspect. Due to the very fragile connection between host and parasite it is often difficult, in a mixed grass community, to identify the host with any certainty.
There are distinct ecotypes of S. asiatica with different host specificity and each form may have quite limited host range. Botanga et al. (2002) describe a range of forms in Benin, including red- and yellow-flowered types with differing virulence on maize and sorghum but do not ascribe sub-specific names to these forms. Hence, there are many regions in Africa and Asia where the species occurs but is restricted to wild hosts and does not affect crops.