New opportunities and challenges to engineer disease resistance in cassava, a staple food of African small-holder farmers

The storage root crop cassava (Manihot esculenta) is the most important source of dietary calories in the tropics after rice and maize [1]. It plays a central role in food and economic security for small-holder farmers and holds unrealized potential as a cash and commodity crop. In Africa, cassava production is constrained by three major diseases: cassava mosaic disease (CMD), cassava brown streak disease (CBSD), and cassava bacterial blight (CBB) [2]. Strong inbreeding depression and heterozygosity present challenges to conventional cassava breeding programs. While important advances are being made with genotype-by-sequencing technologies and marker-assisted breeding, these techniques alone will not unlock the crop's full potential. Historically, cassava has received less research investment than other major crops. Over the last decade, however, multiple genomic platforms and diverse germplasm collections have been developed, along with effective genetic transformation protocols. As a result, numerous reports of genetically engineered cassava have now been published (see Chavarriaga-Aguirre and colleagues) [3]. Here, we briefly summarize previous efforts with an emphasis on technologies relevant to small-holder farmers in Africa. In addition, we discuss new opportunities for using genome-editing technologies to create disease-resistant cassava plants. We conclude by highlighting additional challenges that must be overcome prior to delivering biotechnology-improved cassava varieties to small-holder farmers in Africa.