Optimal planting window: evaluating the response of CBSD-tolerant and CBSD-susceptible cassava varieties in Tanzania

Cassava brown streak disease (CBSD) remains a major constraint to cassava production in East Africa, with planting date and site conditions strongly influencing yield outcomes. In Tanzania, farmers typically plant cassava twice a year: March–April (primary season) in the long rainy season and November–December (secondary season) during the short rainy season. To explore the benefit of extended planting dates, we evaluated root production of three cassava varieties: CBSD-tolerant Kiroba and CBSD-susceptible Albert and Mkombozi across extended planting windows (October–May) at two contrasting sites: Chambezi (coastal agroecology, high CBSD pressure) and Ukiriguru (inland mid-altitude agroecology (Lake Zone), low CBSD pressure). Using virus-free planting material, Kiroba was tested at both sites, Albert only at Chambezi, and Mkombozi only at Ukiriguru. Therefore, genotype × site comparisons are limited to Kiroba , and Albert and Mkombozi results were interpreted as site-specific responses. Taking all varieties together, average yields were 28.1 t/ha at Chambezi and 7.5 t/ha at Ukiriguru. At Chambezi, soil moisture was generally sufficient across all six planting dates, but CBSD incidence rendered most of Albert ’s October–December harvests unusable, while Kiroba remained unaffected. In contrast, yields at Ukiriguru were primarily determined by soil moisture, with effective root production restricted to October–December plantings. Bemisia tabaci abundance was similar at both sites, but disease progression correlated with vector presence only at Chambezi. These findings show that planting date is critical for CBSD-susceptible varieties under high disease pressure, whereas tolerant varieties are less constrained. In low-pressure zones, soil moisture availability is the dominant yield determining factor. Extending cassava planting windows can enhance productivity, but success depends on deploying CBSD-tolerant varieties, isolating susceptible farmer-preferred types from inoculum sources, and making use of clean planting material. The results further indicate that successful crop production can be achieved for plantings within a 6-month plant range coastal Tanzania, while the range in the Lake Zone is only 3 months. Results also highlight critical phases when adequate watering must be applied, guiding farmers to prioritize adequate watering during the establishment phase (first 4–6 weeks after planting), when cassava is most vulnerable to moisture stress. Overall, these insights provide practical strategies for improving cassava productivity and advancing sustainable crop cultivation.