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Lost Crops of Africa: Volume I, Grains
the seed will germinate. Both chemical types are extremely active. The stimulants, for instance, can be diluted 10,000-fold or more and still cause striga seed to germinate.8
If compounds like these can be synthesized, mimicked, or economically extracted from plant roots, they could be (at least in humanitarian terms) among the most valuable of all organic chemicals. For example, it may be possible to produce striga-suicide sprays, perhaps even in the regions that require the most help. This approach has been exploited by Robert Eplee of the U.S. Department of Agriculture to dramatically reduce striga attachment in greenhouse tests.
Also, another striga signal has been identified. This compound (2,6-dimethoxybenzoquinone) "tells" the germinating striga seedling to form the organ (haustorium) that pierces the victim's root. This, too, may offer a way to overcome striga. For instance, an antagonist chemical might blunt striga's underground weapon. If the pest can find no host, it never develops a growing shoot (apical meristem), it never becomes photosynthetic, and it dies.9
Recently, scientists have found that nature is ahead of them. At least one strain of sorghum can already foil striga by producing water-soluble compounds that are striga inhibitors. This sorghum, SRN-39, both resists the parasite and has desirable agronomic characteristics and good-quality grain. Its striga resistance appears to be simply inherited (only one or two genes). Crosses with other cultivars have already been made and promising progeny obtained. Moreover, an assay has been developed to screen breeding material for this resistant characteristic. These results suggest that sorghum breeders may soon be able to breed for striga resistance rapidly and efficiently.10 Similar progress has been achieved in maize.
It has also been found that some leguminous plants—Crotolaria species are examples—excrete their own striga-stimulating signals but do not serve as hosts. Although the striga germinates, it immediately dies. Thus, plants like these could be employed to deplete the striga seed bank in the soil. They may prove extremely valuable species for fallow crops or alley crops. Crotolaria species (rattleboxes) are le-
Information from L. Butler.
New results suggest that striga uses a "chemical radar" approach to host detection. The striga itself releases enzymes that remove the stimulants from the root's surface. This is a novel, and very effective, means of detecting the presence of a potential host. Disruption of this enzymatic function is also being effectively exploited by the U.S. Department of Agriculture.
In fact, about 10 years ago a series of SAR (Striga asiatica resistant) varieties that have a very high level of resistance to the white-flowered asiatica were developed in India. More recently, in southern Africa, five SAR lines have been found to have reasonable resistance to the red-flowered asiatica found in Africa. As with SRN-39. inheritance is simple. (Information from L. House.)