Host and virus effects on reversion in cassava affected by cassava brown streak disease

The phenomenon of virus-infected plants naturally recovering health is known as reversion', and is a type of resistance mechanism exploited in some crop plants for disease control. Various parameters were investigated that affect reversion from cassava brown streak disease (CBSD) in three cassava varieties (Albert, Kaleso and Kiroba) that differ in levels of resistance to the disease. Cassava plants were inoculated by grafting with two virus species (Ugandan cassava brown streak virus, UCBSV and Cassava brown streak virus, CBSV) that cause CBSD, and the plants grown from them were subsequently assessed for reversion. The rate of reversion depended on the cassava variety, virus species, and the length and position of the stem cuttings used. A significantly high proportion of progenies were virus-free (reverted) for the resistant variety Kaleso (641% for UCBSV and 549% of CBSV), compared to the tolerant variety Kiroba (567 and 455%) and the susceptible control Albert (389 and 351%). The highest number of virus-free plants was generated from short 10cm long cuttings (e.g. 601% for Kaleso for CBSV) compared to 20cm long stem cuttings (e.g. 214% for Albert). Cuttings taken from upper stems of diseased plants produced most virus-free progenies compared to middle and lower parts. More than 50% virus-free plants were obtained in the resistant and tolerant varieties. This is a highly valuable finding and could be exploited for developing strategies to control the current CBSD epidemic in eastern and central Africa.