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[KAMPALA] Scientists have identified two cassava varieties that are resistant to two deadly viral diseases of East African origin, opening up new ways to breed resistant versions of the crop.
They have done it by using genetic markers – genes or sequences of DNA with a known location on a chromosome that can be used to identify individuals or species.
According to the scientists, the cassava varieties — Namikonga and Albert — which are genetically related through a West African cassava variety TME117 are preferred by farmers in Tanzania and have been cultivated in disease hot spots within the country for decades.
“The identification of DNA markers that associate with the resistance to the most important cassava diseases establishes a base for marker-assisted breeding.”
Morag Ferguson, IITA
Cassava brown streak disease (CBSD) and cassava mosaic disease (CMD) devastate cassava production in Africa. Severe CBSD infections may cause yield losses of between 70 and 100 per cent and for CMD, the yield loss could be to 95 per cent, according to the International Institute of Tropical Agriculture (IITA).
CMD is spread throughout Africa, while CBSD previously known to be a low altitude disease confined to the East African coastal lowlands has been reported in some mid to high altitude areas along Lake Victoria and Central African region. Further spread towards West Africa is predicted.
“The identification of DNA markers that associate with the resistance to the most important cassava diseases establishes a base for marker-assisted breeding,” says Morag Ferguson, a co-author of the study published in the journal Theoretical and Applied Genetics on 13 July.
“The use of markers in breeding increases the efficiency and accuracy of breeding. It allows accurate selection of offspring from a cross with the desired combination of genes for dual CBSD/CMD resistance,” adds Ferguson, a molecular breeder at the IITA, Kenya.
The scientists from Kenya, South Africa, Tanzania and the United States crossed the two local cassava varieties — CBSD-resistant Namikonga and CMB-resistant Albert — through hand pollination, resulting in offspring that have genetic markers resistant to the two deadly diseases. Whereas Namikonga is vulnerable to CMB, Albert is susceptible to CBSD.
Ferguson explains that the team planted the offspring during the 2013 and 2014 seasons in two areas of Tanzania known to be disease hotspots, and assessed their potential in being resistant to CBSD and CMB.
In a related study published last month (29 August), a team of scientists including Ferguson found that the Namikonga variety has genes that limit the multiplication of the virus that causes CBSD, thus reducing disease progression.
According to the latest study in Research Reports, the findings offer opportunities to conduct research with the “ultimate aim of developing robust biomarkers for cassava breeders to develop durable resistance to CBSD”.
Ferguson tells SciDev.Net that the identified markers for the two diseases could boost breeding in cassava-growing countries such as Nigeria, which is the world’s largest producer.Commenting on the earlier study, Harriet Muyinza, a senior research officer at the National Agricultural Research Organisation, Uganda, tells SciDev.Net, “This is good news. These genes can also be shared across the region to improve local cassava breeds.
“This can be improved with other characteristics such as yield, taste and resistance to other biological stress factors.”
This piece was produced by SciDev.Net’s Sub-Saharan Africa English desk.
 Esther A. Masumba and others QTL associated with resistance to cassava brown streak and cassava mosaic diseases in a bi-parental cross of two Tanzanian farmer varieties, Namikonga and Albert (Theoretical and Applied Genetics, 13 July 2017)
 Teddy Amuge and others A time series transcriptome analysis of cassava (Manihot esculenta Crantz) varieties challenged with Ugandan cassava brown streak virus (Research Reports, 29 August 2017)