Developing dual resistant cassava to the two major viral diseases

Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are the two important biotic constraints affecting cassava production in sub-Saharan Africa (SSA). Deployment of cassava varieties dually resistant to both diseases is the most effective and realistic way of reducing losses to African farmers. Crosses were carried out between the Tanzanian local cassava variety Namikonga (CBSD resistant/CMD susceptible) with an introduced cassava germplasm AR37-80 (CBSD susceptible/CMD resistant) from South America to develop dually resistant F1 progenies and they were evaluated for 2 seasons at Naliendele in Southern Tanzania which is a CMD and CBSD hotspot area. CMD-resistant progenies had low foliar severities (≤ 1.8 on a five-point scale) similar to CMD resistant parent. CBSD resistant progenies had minimal foliar severity (≤2.0) and root necrosis (≤1.2) similar to the CBSD resistant parent while CBSD tolerant progenies had severe foliar severity of up to 3.3 but minimal root severity (≤ 1.2). Traits with minimal environmental influence also had high heritability (≥0.65) and high selection accuracy (≥0.70) and they included CMD foliar symptoms, CBSD foliar symptoms at 6 MAP, root necrosis, root necrosis incidence, root weight, root number per plant, and harvest index. Correlation analysis showed that the presence of diseases reduces usable roots, root weight, root number per plant, and harvest index. Dual resistance can improve yield as observed in the progenies, Namar 050 and Namar 371 which had high root weights of 27.5 t/ha and 28.2 t/ha with high genetic gains of 56.1% and 58.5%, respectively. Dual resistant progenies identified were Namar 050, Namar 100, Namar 130, Namar 200, Namar 334, Namar 371, and Namar 479 as they had minimal CMD and CBSD symptoms severity (≤ 2.0) and could be used for breeding cassava varieties with superior characteristics.