Cassava Brown Streak Disease (CBSD) is a viral disease that rots cassava roots and renders them useless. Before the twenty-first century it was largely restricted to coastal Eastern Africa. However, in the early 2000s, new outbreaks were reported from mid-altitude areas (>1000 m above sea level) of south-central Uganda, western Kenya, and northwestern Tanzania, precipitated by huge increases in the populations of the vector, whitefly, Bemisia tabaci. CBSD has subsequently been shown to be spreading as a pandemic throughoutthe major cassava-growing regions of East and Central Africa and threatens to spread further westwards into Central and West Africa.
Research by IITA has shown that the most effective and convenient approach, particularly for resource-poor farmers, to reducing losses from CBSD is the use of host-plant resistance or the deployment of less-susceptible cultivars. Historically, much of the breeding work to combat CBSD has focused on tolerance since complete resistance to infection is rare.
In 2015, IITA continued efforts to control, contain, and even push back CBSD on this front. In Tanzania, four IITA-developed varieties tolerant of CBSD and resistant to Cassava Mosaic Disease (CMD, another widespread disease of the crop) were officially released for use by farmers in the country. These were: KBH 2002/363 (Chereko), KBH 2002/066 (Kipusa), KBH 2006/026 (Mkuranga 1), and KBH 2002/482 (Kizimbani).
In addition, IITA is in the advanced stages of evaluating more than 30 highly promising breeding lines in Tanzania. Four of these (UKG 2009/0052, UKG 2009/0128, UKG 2009/0164, and UKG 2009/0181) have performed very well under on-farm conditions and have been proposed for a one-year evaluation under National Performance Trials (NPT)—a final step towards full official release. Once they are released, these varieties will be the first to have dual resistance/tolerance for CBSD and CMD for the Lake Zone of Tanzania, an area where CBSD is so devastating that many farmers have totally abandoned cassava production.
In Uganda, two IITA-developed varieties have been officially released during the year: TZ 130 (NARO-CASS 1) and MM 2006/0130 (NAROCASS 2). Their release is a milestone since these are the first that offer dual resistance/tolerance for CMD and CBSD for the mid-altitude areas of the Great Lakes region.However, IITA continues to pursue the objective of developing a variety that is truly resistant to CBSD. By definition, a truly resistant variety should not be readily infected, even when exposed to large amounts of vector-borne inoculum. If and when infected, such a variety should develop inconspicuous symptoms without adverse effects on growth and yield. It should also support low virus (if any) content and thus be a poor source of infection. Developing a truly CBSD-resistant variety will entail using different modes and new sources of resistance.
To this end, IITA breeders have started to look for and identify such sources of resistance by introducing germplasm from IITA’s Genetic Resources Center in Ibadan, Nigeria. Nine Nigerian cultivars were introduced by tissue culture into Tanzania, where they were evaluated for CBSD resistance in the field for three seasons at Chambezi, a known disease hotspot. Initial findings have shown that two cultivars —TMS-IBA961089A and TMS-IBA000388—had either a significantly higher marketable yield of fresh roots or else performed as well as Kiroba, the improved control variety. Furthermore, the two cultivars showed no quantifiable virus concentrations. Due to their outstanding performance, TMS-IBA961089A and TMS-IBA000388 have been earmarked for on-farm evaluation across several sites after which they will be included in NPTs just before official release in Tanzania. If they consistently perform well, these cultivars will be used as new sources of resistance to generate new varieties in future that are truly resistant to CBSD.