By Ismail Rabbi, IITA
Header Image: Symptom of cassava mosaic disease (CMD) on young plants. Left row is a susceptible variety (score level 5) while the row on the right is a highly resistant clone (score 1). Photo provided by Ismail Rabbi.
Cassava mosaic disease, caused by the cassava mosaic virus, is the most important disease of cassava in Africa and is now threatening cassava production in Asia. Now, diagnostic molecular markers developed under the Next Generation Cassava Breeding Project can be used to rapidly screen for and mobilize the virus resistance trait into the Southeast Asian cassava varieties and other regions of the world where the disease could potentially spread.
Cassava cultivation in Southeast Asia started in early 1900s, and today it has become one of the major crops in many countries in this region. The first outbreak of the disease in Cambodia and Southern Vietnam was reported in 2015 (Wang et al). According to the Global Cassava Partnership for 21st Century (GCP21) the emergence of this devastating disease threatens the 55 million tons of annual cassava production in the region, and it called for an urgent action to curb its spread.
African cassava breeding programs have successfully developed disease-resistant varieties thanks to years of research and breeding to mitigate the challenge of the disease in the continent. Recent breakthroughs in genetic analyses have uncovered the major genes responsible for the strong resistance against the virus disease in Africa cassava genepool. Diagnostic molecular markers that tag these genes have been developed under the Next Generation Cassava Breeding Project in partnership with the High-Throughput Genotyping Project (HTPG).
In a pilot initiative, Chalermpol Phumichai, a researcher from Kasetsart University, Thailand, crossed a CMD resistant African cassava variety called TME3 into Huay Bong 80 from SE Asia. Genotyping of the progenies from these crosses clearly showed six out of 94 carried the markers for the resistance gene.
Using DNA markers in the absence of the pathogen will allow breeders in Asia to carry out pre-emptive breeding and potentially save cassava production in the region from the onslaught of this devastating virus.
While adoption of these markers is still in its early stages, there has been a lot of interest and request from various breeding programs around the world. The Brazilian Agricultural Research Corporation (Embrapa) has already started to use them to carry out pre-emptive breeding for resistance against the virus disease in Brazil. They submitted nearly 1000 accessions developed from crosses that involve sources of CMD resistance. Field testing of seeds from the marker-assisted selection populations will be carried out at IITA-Ibadan. Hopefully more breeding programs will start using them in the near future, especially in Asian countries such as Vietnam where CMD is emerging as a threat to cassava production.
References: Wang, H.-L., X.-Y. Cui, X.-W. Wang, S.-S. Liu, Z.-H. Zhang, and X. Zhou. 2015. First Report of Sri Lankan cassava mosaic virus Infecting Cassava in Cambodia. Plant Dis.: PDIS-10-15-1228-PDN. Available at http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-10-15-1228-PDN.