Promising results in two-fisted approach to tackling Striga on maize
The ACCI’s pioneering efforts to combat Striga, with a two-pronged approach that combines breeding and biocontrol, have yielded more encouraging results. This time the target crop is maize.
Striga is a plant that feeds on other plants, a parasitic weed that wreaks havoc on cereal crops across Africa. Two species affect cereal crops — Striga hermonthica, found mainly in the north, and Striga asiatica in the south. A third species attacks cowpeas, Striga gesnerioides.
For the past few years, the ACCI has been pioneering the integration of breeding for Striga resistance and breeding for compatibility with a fungal biocontrol agent called Fusarium oxysporum f.sp. strigae (FOS). According to ACCI director, Professor Mark Laing, this project is a world first because no other study has focused on breeding for this combination of traits.
So far, the focus has been on sorghum, but PhD student Admire Shayanowako has now completed his research on breeding maize using this approach, with encouraging results.
Shayanowako, a keen birdwatcher who hails from Harare, Zimbabwe, began his research at the ACCI in 2016, with funding from South Africa’s National Research Foundation and the Third World Academy of Science (TWAS).
“We have developed a model whereby we selected maize genotypes that are resistant to Striga and compatible with the biocontrol agent. The two work in tandem,” says Shayanowako, explaining that resistance to Striga in maize is partial, so the addition of FOS helps to boost it.
To find maize that was resistant to Striga, landraces and open-pollinated varieties (OPVs) that have been used by farmers for generations in Southern Africa were collected.
“There had already been deliberate selection (by farmers) for Striga resistance,” says Shayanowako. “We also know that OPVs are genetically diverse, so somewhere in their genome, there should be resistance genes.
Important in this approach is that it is potentially an affordable solution for farmers. Striga has its greatest impact on crops grown in low fertility soils, mainly impacting on smallholder farmers who can’t afford to fertilise their crops.
“This is a poor man’s problem, and if we can discover OPVs maize varieties that have good Striga resistance, it would be a low-cost solution,” says Shayanowako. “With OPVs, the farmer grows his own seed, so (this system) is self-maintaining.”he second part of the arsenal, FOS, is a host-specific parasite of Striga. While it doesn’t harm maize, it does work against various stages of the parasite. The FOS is applied as a seed coat. When the seed germinates, the fungus grows into the roots of the host as a symbiotic infection. It then suppresses the germination of the parasite, as well as attacking the Striga plants if they attach to the host.
“One of the problems of Striga is that it produces thousands of seeds,” says Shayanowako. “One plant can produce 500 000 seeds that can stay in the soil for more than a decade. That alone is a challenge to control.”
FOS helps on this front by producing methionine that is converted to ethylene by microbes in the soil, causing suicidal germination of Striga seeds. It also damps down the numbers of Striga seeds that germinate.
The ACCI is using an ingenious way to store the FOS that enables it to be sent all over Africa. The biocontrol agent is grown on wooden toothpicks, using a microbial medium that makes colonisation possible.
“We put the toothpicks inside sterile straws that are heat sealed, and then they can then be transported to other countries,” says Shayanowako. “On reaching their destination, the toothpicks are used to inoculate a sterile rice or sorghum grain medium that is used as a carrier for the bio-product.”
In 2018, Shayanowako received a scholarship to attend training run by Montana State University in how to use this toothpick technology. He has since passed on his knowledge to fellow ACCI PhD students, Frederico Madabula, Armel Rouamba and Ati Nkosi, a masters student.
“We are all contributing to studies conducted in Ethiopia and Tanzania that involved working on Striga control on sorghum. Mine is the first on maize. In my study we went further and tried to develop markers for Striga resistance in maize.” This genotyping work was funded by BECA-ILRI in Kenya.
“The outcome of the study was that we identified some maize populations that showed partial resistance to S. asiatica and compatibility with FOS that were sub-tropical and adapted to the southern African region”.
“We got some maize lines from the International Institute of Tropical Agriculture (IITA) ,” he says. “One of their mandates is to develop resistance to Striga in maize, so they’ve been doing research on this. We used some of their maize lines as male parents to our sub-tropical lines, and then developed populations that we evaluated for resistance breeding.”
Two promising populations have been identified that he says should be advanced further using recurrent selection, a process to accumulate multiple genes for resistance.
Now that his PhD is complete, Shayanowako is working on post-doctoral research for the ACCI, extending the breeding plus FOS compatibility work to legume parasites. “There’s a growing concern about the impact of a related parasitic weed, called Alectra vogelii, which attacks legumes, especially Bambara groundnut and cowpeas, within the region and around Africa. We are testing the potential of using FOS to control these parasite,” he says.
He’s also helping to advance sorghum lines that have been bred in Tanzania by a fellow ACCI graduate, Dr Emmanuel Mrema, for release as cultivars. These lines have also been identified for Striga resistance and compatibility with FOS. I
“In South Africa we are testing them for stability with the hope that they can be released as sorghum varieties here and in Tanzania,” says Shayanowako.