Strategies to stop Striga
Strike three: Hi-tech alternatives

credit: Dr Dennis Friesen, CIMMYT

Perhaps the greatest concern with Striga is not just how many crop species it already affects but for its potential to widen its host-range. "Striga is evolving and that is frightening", says Dr Jonathan Gressel, Professor of Plant Sciences at The Weizmann Institute of Science in Israel. For example, Striga never used to affect teff in Ethiopia, but now it does. Genetic modification could offer an answer, he says. Herbicides can be used to kill parasitic weeds provided they do not also kill the crop. But sometimes this resistance is possible through plant breeding and does not require genetic manipulation.

credit: Dr Dennis Friesen, CIMMYT

The Mexico CGIAR research centre (known by its Spanish acronym CIMMYT) together with The Weizmann Institute of Science with funding from the Rockefeller Foundation, have developed a low-dose imazapyr (a systemic ALS-inhibiting herbicide) seed coating applied to its adapted IR-maize germplasm. Small quantities of imazapyr applied to the seed before planting prevents the phytotoxic effect of Striga on the maize, which occurs even before the emergence of the Striga from the soil [for a more detailed explanation see Wiping out witchweed].

"Farmers often like interplanting their maize for example with cowpea," says Dr Gressel. "And with the seed coating this is still possible because the herbicide dissipates in the root of the maize and is therefore no threat to the crop alongside it." The seeds are $4 to $5 more expensive per hectare than other hybrid maize seed, but the potential gain from using it is huge, says Dr Gressel. "By putting the herbicide on the seed, they don't need to spray the whole field. This uses 10 to 20 times less herbicide than spraying - just half a milligram per seed. In 50 field trials [in Kenya], the average difference was a three-fold yield increase."

Commercialisation of the seed is in hand - CIMMYT is undertaking trials elsewhere in sub-Saharan Africa, the Kenyan government is undertaking its own field trials, and BASF (manufacturers of the imazapyr herbicide) are also involved. Although the seed coating/IR-maize combination offers one option in the control of Striga, the threat of the weed's evolution means that we need a flow of new answers from research says Dr Gressel. "No solutions are forever."