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June 13, 2010

Striga-resistant varieties to boost sorghum yields

by Steven Tendo

In April, scientists in eastern and central Africa embarked on identifying sources of resistance to Striga, a parasitic weed. Supported by the Association for Strengthening Agricultural Research in Eastern and Central Africa (Asareca), the researchers from Sudan, Kenya, Eritrea and the International Crop Research Institute (ICRISAT) are using biotechnological tools in locating and identifying Quantitative Trait Loci (QTL) that gives resistance to Striga.

QTL is a statistical method that links two types of information phenotypic data (trait measurements) and genotypic data (usually molecular markers)—in an attempt to explain the genetic basis of variation in complex traits.

According to Dr Charles Mugoya, the programme manager of the Agro biodiversity and biotechnology programme of Asareca, knowing the location and identification of QTLs for Striga resistance is a useful tool in aiding marker assisted breeding/selection (MAB or MAS) of Sorghum for Striga resistance.

MAS is an indirect selection process where a trait of interest is chosen not based on the trait itself but on a marker (morphological, biochemical or one based on DNA/RNA variation) linked to it. So far, QTLs underlying different resistance phenotypes have been identified and the scientists are now backcrossing populations to generate striga resistance QTLs into farmer preferred sorghum varieties.

Mugoya said Striga hermonthica, also locally known as the witchweed, is a major constraint to sorghum production in particular and cereal production in general, especially in more marginal areas like semi-arid regions, where continuous cropping as a result of population pressure, has led to widespread soil infertility.

The weed is genetically diverse and several factors contribute to its diversity. These include a high turnover of several generations of witch weed populations leading to high genetic diversity; hybridisation; broad geographic distributions; long distance dispersal and locally adapted host races.

“Owing to its great potential genetic diversity, efforts to control it, through conventional breeding to generate striga resistant varieties or agronomic practices to reduce the striga seed bank in the soil, have been ineffective and striga continues to be a menace, with reported cases of up to 100 per cent sorghum yield loss in the region,” he noted.

Sorghum is ranked second, after maize as the most important cereal crop in East Africa and in the Asareca region. According to statistics from the Food and Agricultural Organisation (FAO), sorghum is the fifth most important cereal crop worldwide and together with maize and pearl millet, it forms the most important dry land cereal crop for the semi-arid tropics particularly in Africa.

It is grown in at least 86 countries, on an area of 47 million hectares, with annual grain production of 69 million tonnes and average productivity of 1.45 tonnes per hectare. Sorghum yields in Africa however range between 500-800 kg per hectare compared with yield levels of up to 7,000kg (7 tonnes) per hectare in the developed world.

The bulk of African sorghum production is centred around the savannah areas of east, west and central Africa, where it forms a major component of the daily menu for millions of people, either as porridge or as traditional beer. Uganda is using sorghum for the beer industry. In addition, in many developing countries, sorghum stover is used to feed cattle.

Striga infested areas in Africa is estimated at 21 million ha. In many parts of East Africa, people lose half of their crop production to Striga and total yield losses occurs in infested farmer’s fields especially during drought periods. The change in farming systems from shifting cultivation to more permanent cropping, concomitant with loss in soil fertility, and frequent cultivation of susceptible host plants, are main factors responsible for increased Striga infestation.

The project promises to increase sorghum productivity in order to address food insecurity and poverty in East and Central African semi-arid zones and boost yields by at least by 20 per cent.

Widespread cultivation of striga resistant varieties is expected to reduce labour demand, since weeding to control striga seeds will not be necessary. In addition, control of striga by the application of herbicides will become unnecessary, thus saving the environment.

Daily Monitor

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