Cassava brown steak disease (CBSD), caused by Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), is the most important biotic constraint to cassava production in East and Central Africa. Concerted efforts are required to prevent further spread into West Africa as well as to reduce losses in areas already affected. The study reported here was part of a five-country (Kenya, Malawi, Mozambique, Tanzania and Uganda) programme that aimed to identify superior cassava cultivars resistant to CBSD and to disseminate them widely in the region. Seventeen tissue-cultured and virus-tested cultivars were evaluated in Tanzania across nine sites with diverse CBSD inoculum conditions. Experiments were planted using an alpha-lattice design and assessments were made of surrounding inoculum pressure, CBSD foliar and root incidence and root yield at harvest. There were large differences in CBSD infection between sites, with greatest spread recorded from the north-western Lake (Victoria) zone. Differences were driven by Bemisia tabaci whitefly vector abundance and CBSD inoculum pressure. Both CBSV and UCBSV were almost equally represented in cassava fields surrounding experimental plots, although CBSV predominated in the north-west whilst UCBSV was more frequent in coastal and southern sites. However, the incidence of CBSV was much greater than that of UCBSV in initially virus-free experimental plots, suggesting that CBSV is more virulent. Cultivars could be categorised into three groups based on the degree of CBSD symptom expression in shoots and roots. The seven cultivars (F10_30R2, Eyope, Mkumba, Mkuranga1, Narocass1, Nase3 and Orera) in the most resistant category each had shoot and root incidences of less than 20%. Fresh root yield differed between sites and cultivars, but there was no genotype by environment interaction for this trait, probably attributable to the large fertility and soil moisture differences between sites. Susceptible cultivars and the local check performed well in the absence of CBSD pressure, highlighting the importance of exploiting quality and yield traits of local landraces in breeding programmes. Overall, our results emphasized the importance of applying a balanced strategy for CBSD management. This should use both improved and local germplasm resources to generate high yielding cultivars for specific end-user traits, and combine the deployment of improved cultivars with phytosanitary control measures including the use of healthy planting material and planting during periods of reduced CBSD infection.

木薯褐条病（CBSD）由木薯褐条病毒（CBSV）和乌干达木薯褐条病毒（UCBSV）引起，是东非和中非木薯生产最重要的生物限制。需要共同努力，防止进一步蔓延到西非，并减少已受影响地区的损失。这里报道的这项研究是五个国家（肯尼亚、马拉维、莫桑比克、坦桑尼亚和乌干达）计划的一部分，该计划旨在鉴定对 CBSD 具有抗性的优质木薯品种，并在该地区广泛传播。在坦桑尼亚的 9 个地点，具有不同 CBSD 接种条件的 17 个组织培养和病毒测试品种进行了评估。实验采用 α 格子设计进行种植，并对周围接种物压力、CBSD 叶和根发生率以及收获时的根产量进行评估。不同地点的 CBSD 感染存在很大差异，其中西北湖区（维多利亚）的传播范围最大。差异是由烟粉虱载体丰度和 CBSD 接种压力造成的。 CBSV 和 UCBSV 在实验地周围的木薯田中几乎相同，但 CBSV 在西北部占主导地位，而 UCBSV 在沿海和南部地区更为常见。然而，在最初无病毒的实验小区中，CBSV 的发病率远高于 UCBSV，这表明 CBSV 的毒性更强。根据芽和根中 CBSD 症状表达的程度，栽培品种可分为三组。 抗性最强的七个品种（ F10_30R2 ​​、 Eyope 、 Mkumba 、 Mkuranga1 、 Narocas1 、 Nase3和Orera ）的芽和根发病率均低于 20%。不同地点和品种之间的鲜根产量有所不同，但该性状没有因环境相互作用而产生基因型，这可能是由于不同地点之间肥力和土壤湿度差异较大。在没有 CBSD 压力的情况下，易感品种和当地检查表现良好，凸显了在育种计划中利用当地地方品种的质量和产量性状的重要性。总体而言，我们的结果强调了对 CBSD 管理应用平衡策略的重要性。应利用改良品种和当地种质资源来培育具有特定最终用户性状的高产品种，并将改良品种的部署与植物检疫控制措施结合起来，包括使用健康的种植材料和在 CBSD 感染减少期间种植。