Early storage root bulking index and agronomic traits associated with early bulking in cassava

One of the attempts by farmers in counteracting the devastating effects of cassava brown streak disease (CBSD) on yield and quality of cassava is early harvesting. However, most varieties grown by farmers are often late bulking which increases the disease severity while on the other hand early harvesting results in significant yield losses. Farmers, therefore, need early storage root bulking cassava varieties in order to reduce the time to harvest leading to a faster rate of return to investment, while at the same time avoiding devastating effects of CBSD on yield and quality of cassava. The study was, therefore, conducted to identify high-yielding and early storage root bulking cassava genotypes as well as traits associated with early storage root bulking and estimate yield loss if any due to early harvesting. The overall aim was to generate information that would guide future improvement programmes for high-yielding and early-bulking cassava varieties in Malawi and other countries facing similar challenges. Trials were implemented using a square lattice design with three replications at two locations for two growing seasons with three harvest intervals (6, 9 and 12 months after planting, MAP). High yields were obtained of up to 9.5 t/ha at 6 and 17.8 t/ha at 9 MAP. Furthermore, the study revealed that yields obtained at 9 MAP were higher than those obtained at 12 MAP for some genotypes which suggests that such genotypes would be considered as early storage root bulking. Simple correlation analysis identified harvest index, storage root number, storage root diameter and storage root length as the selection criteria to achieve high fresh storage root yield (t/ha) and dry mass yield (t/ha). Path coefficient analysis allocated harvest index and shoot mass as the major selection criteria in improving fresh storage yield and dry mass yield. The study suggests that both source and sink capacities were important for determining early yield. Therefore, these two traits are the key determinants of early storage root bulking and should be used when selecting early-bulking cultivars and indirectly selecting for storage root number, storage root diameter and storage root length.