Hope for farmers in Africa as scientists make breakthrough against dangerous Striga

In any other setting, its lilac blooms would pass as a beautiful flower. But to Hassan, a sorghum grower in Sudan’s North Kordofan state, and his compatriots in East and Central Africa, the plant’s appearance is often the harbinger of a lost harvest.

It is not uncommon to find farmers who have lost between 70 and 100 per cent of the expected yield from their cereal crop to the parasitic Striga weed, nicknamed witches weed because of its devastating impact on livelihoods.

With an estimated 21 million hectares of African farmland infested, Striga has become a major threat to food security on the continent.

Relief is in sight however, after four varieties of Striga resistant sorghum were recently approved for growing.

Under development for almost eight years, the new varieties have roots that block the parasite from attaching to their host, eventually causing the weed’s death through water and nutrient starvation.

Released by the Agricultural Research Corporation of Sudan (ARC) in Khartoum on June 19, the varieties were jointly developed by the Association for Strengthening Agricultural Research in Eastern and Central Africa (Asareca) and ARC.

Prof Abdalla H Mohamed, who specialises in sorghum breeding and genetics at ARC, describes sorghum as the crop of hope because of its good adaptation to marginal, hot and drought prone areas.

He says with the new varieties, the crop has outstanding potential to meet the growing global demand for food at a time of uncertainties posed by climate change.

However, Striga, because of its capacity to destroy the yield of staple crops in the semi-arid tropics, is the biggest limiting biotic factor in the production of sorghum in Sub-Saharan Africa, the professor says.

The importance of Striga resistant varieties is more pronounced in countries like Sudan, where sorghum accounts for 73 per cent of the crop area.

“In Sudan, the food stored is sorghum. It is a cash crop and feed for livestock,” Prof Abdalla says.

“It provides a safety margin for both humans and livestock. It is important in Sudan because of the large crop area it occupies, and is also a staple in countries like Eritrea and Ethiopia. In Tanzania it is an important crop in the alcoholic beverages industry, while it is also an important feed crop for chicken and livestock in Kenya, Uganda and elsewhere in the region,” he adds.

Abdalla says as climate change ravages the global economy, farms will shrink, and because of sorghum’s outstanding potential to withstand drought and high temperatures, it will likely remain one of the few sources of nourishment.

Baselines studies conducted by ARC show that depending on infestation levels, without intervention, yields can be wiped out with only Striga flowers being visible. With the resistant varieties, yields can recover by as much 84 per cent.

Striga resistant sorghum has mechanical barriers — thickening of the root walls which make it impossible for the parasite to penetrate. As a result, while the weed can still germinate, it will not be able to penetrate the root, leading to withering.

Damage to crops

Until the recent breakthroughs, Striga had been an intractable problem because of its unique coping mechanisms.

A single plant can produce up to 50,000 seeds that lie dormant in the soil for up to 20 years. 

When cereals are planted and the seeds sense a root, they are stimulated to germinate, attaching to the host, sucking out nutrients and water, which the weed uses to synthesize its food as it chokes its host.

Striga has roots, but they are for anchoring in the soil and can not suck water or nutrients. It uses a special hooking mechanism — the pistorium — to attach to the host’s root. Sorghum mono-cropping, practised widely by farmers in the hot tropics, exacerbates infestation.

The resistant varieties, named Asareca 1-4 after the organisation’s role in their development, while initially specific to Sudan, will be tried out in other sorghum growing countries in the region.

Dr Charles Mugoya, the programme manager for Agro-biodiversity and Biotechnology (Agrobio) and Clet Wandui Masiga, a conservation biologist and geneticist for the programme, conducted the molecular biology research during the new strains’ development, while Prof Abdalla and ARC carried out the conventional breeding and field trials.

“This is regional pride for Africa, because it shows that Africa and its scientists can solve the problems of Africa. This is the first time in Africa that we have a variety released through Marker Assisted Breeding,” Dr Mugoya told the pre-release workshop.

He added that while it was true that the continent faced many challenges and constraints in developing agriculture, the successful development of the new strains demonstrated that “we also have opportunities to come together as a group of African countries to address them without foreign help.

“Asareca believes that in Africa there are African scientists who can address some of the challenges that Africa is facing. Therefore Asareca has made itself available to provide technical and financial assistance to make sure that scientists in Africa produce solutions to some of the challenges the continent faces,” he said.

The new varieties combine the strong resistance to Striga displayed by the N13 strain of sorghum, and the superior yields of farmer preferred varieties across Sudan.

The varieties released in Sudan are already under test in Kenya and Uganda. They are showing very high promise in Uganda, even before adaptability tests, and Dr Mugoya hopes this success will be applied to other crops.

“It is a big step forward in combating this witches weed. We hope this resistance will be durable, and will not be broken,” said Abdel Moneim El Ahmadi, the director for research and development at the Arab Sudanese Seed Company, one the stakeholders that will play a key role in breeding the new varieties.

With a high multiplication factor whereby a single acre can plant 200 acres, Abdel is confident that the new varieties will proliferate very fast once they is acceptable to farmers.

“The good thing is that it covers all the popular varieties. These are the most widely distributed varieties, so if you add resistance to them and the other good characteristics, I would expect them to spread very fast,” he said.

Relief for farmers

Twenty million acres are under sorghum in Sudan. And while there is preference for wheat flour in urban areas, in rural areas where it doubles up as animal feed, sorghum is preferred.

The crop is well adapted to the environment in Sudan and varieties have been developed for the major ecological zones, with fast maturing varieties for low rainfall areas, and slower ones for the wetter parts.

Hassan, a farmer from North Kordofan was ecstatic when the improved seeds were released. “The new varieties will solve some very critical problems for us.

With Striga gone, birds are the only problem now, and once we have the seed we shall grow it immediately,” he said on the sidelines of the launch.

Hassan plants seven hectares of sorghum every year. When the harvest is good, he gets between six and 12 100kg bags per hectare planted with the old varieties.

According to Adil Omer Salih Abdel Rahim, acting director at ARC, just as sorghum is the most widely planted crop in Sudan, Striga is the biggest impediment to achieving high yields.

“Striga is the most serious pest for sorghum production, not just in Sudan, but in many other countries. Sorghum is also the most important cereal in Sudan, and this achievement will contribute very much to the improvement of sorghum production in Sudan.

“What we have done is to introduce the genes for resistance into farmer-preferred varieties and the released varieties have the same level of resistance as the parent N13 variety, yet unlike it, they are very high yielders,” Adil explains.

ARC has a programme for seed multiplication, and hopes to provide the seeds of all released cultivars to the national seed multiplication company.

Between two and three tonnes of the new seed stock is targeted annually, and it is hoped that within three years resistant varieties will have proliferated.

According to Mr Masiga, some features of Striga explain its economic importance and persistence: it parasitises staple food crops that are critical for poor smallholders; and it emerges only after it has parasitised its host, as a result much of the damage is done while the weed is invisible.

The weed also emerges after farmers have finished weeding, so removal before flowering requires extra labour.

Since a single Striga plant can produce more than 50,000 dust-like seeds, this creates huge seed banks that take a long time to eradicate.

Combined with the fact that Striga seeds can remain viable in the soil for up to 20 years and they are easily spread by wind, animals, shoes, tools, and water run-off, the weed is a formidable adversary.

Across Africa it is estimated that up to 21 million hectares of farmland are infested with Striga. In the 1980s, the weed threatened 56 per cent of Africa’s grain production area.

Currently some 3,074,000 hectares of sorghum crop in Eastern and Central Africa are infested with Striga, representing a 32 per cent infestation rate  and a 22 per cent loss in yields.

The equivalent monetary value associated with these losses is $623 million annually.