Evaluation Of Cassava (Manihot Esculenta Crantz) Germplasm For Resistance To Cassava Brown Streak Disease And Virus Elimination Using In Vitro Techniques

ABSTRACT

Cassava brown streak disease (CBSD) is an important disease causing losses of up to 70% in the most susceptible cultivars. Its effects include: reduced root quality due to pitting, root constriction and necrosis, and reduced number and weight of tuberous roots. To contribute to CBSD management, a study was conducted to: (i) Screen East African cassava landraces and F1 populations for CBSD resistance; (ii) Analyse symptom expression and virus accumulation in CBSV graft inoculated cassava varieties (iii) Eliminate virus from infected cassava using in vitro techniques. Two F1 populations were developed: Namikonga × AR37-80 and Pwani × AR37-80. Landraces and F1 population were screened for CBSD resistance in CBSD hotspot areas in Kenya and Tanzania. A partially balanced lattice design was used for the studies done in Tanzania, while a randomized complete block design (RCBD) was used for the study done in Kenya. Results showed that genetic makeup accounted for the largest variation observed (22.8 – 78.2%), followed closely by genotype by environment interaction (11.7 – 46.8%) in CBSD foliar symptoms, root necrosis, root necrosis incidence and usable roots. Similar observations were made on evaluated root traits including: root weight, number of roots per plant, dry matter content and harvest index, for genotype (22.5 – 84.2%) and genotype by environment interraction (10.5 – 44.7%). The results showed that although these traits are largely under genetic control, they can also be affected by environment to varying degree. The study identified new sources of CBSD resistance/tolerance among the landraces and F1 populations. In Tanzania, 28 resistant landraces were identified including: Chimaje, Chipanda, and Supa B. Additionaly, 27 tolerant landraces were identified including: Kikwada, Mbuyu, and Mreteta. In Kenyan, only tolerant landraces were identified including: Weite, Manchoberi, and Merry-go-round. Among the F1 populations, progenies categorized as resistant included: NAMAR050 NAMAR130, and NAMAR371 while those categorised as tolerant were PAR024, PAR057, NAMAR116 and NAMAR441. Different responses to CBSD inoculation were observed with Kaleso and Nase 1 showing the least symptoms and virus accumulation. Apart from Kaleso, Nase 1 was identified as a good CBSD progenitor in breeding CBSD resistant varieties. Some of the resistant landraces had high yield and could be used directly for cultivation and in cassava breeding programs for transfer of resistance to farmer preffered varieties. Thermotherapy at (35°C) for 2 weeks combined with subculturing into regeneration media can be used for production of virus free cassava since this treatment had the highest survival rate (77%) and success in virus cleaning (91%). The findings of this study will be important in future CBSD management.