Effective isolation distance for prevention of cassava virus spread in farmer-preferred cultivars in Uganda

Abstract

The two viral diseases, cassava brown streak disease (CBSD) and cassava mosaic disease (CMD), are a major constraint to cassava production in Uganda. The Ministry of Agriculture, Animal Industry and Fisheries (MAAIF) recommended the 50 m isolation distance for prevention of virus infections in crops. However, the minimum isolation distance for cassava has not been verified for effectiveness of control of CBSD and CMD. This study assessed the effective isolation distance for management of viral diseases in farmer-preferred cassava cultivars. A total of 150 cassava farmer fields (74 in Bukedea district and 76 in Kumi district) from eastern Uganda; a major cassava producing area were assessed for farmer-preferred cultivars. Data were collected through observations and interviews using a questionnaire. For CBSD/CMD incidence and severity, 30 cassava plants of different genotypes were assessed randomly along transects of each field. Farmer-preferred virus free cassava cultivars (NASE 03, NASE 14 and NAROCASS 1) from farmers’ fields in eastern Uganda were used as field sourced (FS) planting materials for subsequent testing of effective isolation distance for management of CBSD/CMD. Tissue culture (TC) material of the same cultivars were sourced from the National Crops Resources Research Institute, and Makerere University Agricultural Research Institute, Kabanyolo (MUARIK) tissue culture laboratories. Both FS and TC materials were tested at isolation distances of 50, 100, 150 and 250 m for virus prevention at MUARIK. The experiment was laid out in a randomised complete block design and was run for 12 months after planting (MAP). Results showed that farmers from Bukedea and Kumi districts preferred NASE 03 mostly, with 70.3% and 64.6%, respectively followed by NASE 14 with 17.5% and 25%, respectively. CBSD and CMD were prevalent in both districts. In corroboration, the most preferred cultivar, NASE 03 was found most susceptible to CBSD and CMD in both Bukedea and Kumi districts with CBSD incidence of 62.2% and 52.7% and CMD incidence of 56.9% and 34.3% respectively. Mean CBSD/CMD prevalence significantly varied (P<0.05) among isolation distances in both FS and TC plants, and the 250 m isolation distance was the most effective in reducing disease prevalence. Across cultivars and planting material category at 12 MAP, the 50 m isolation distance had the highest mean foliar incidence for CBSD (29.2%) and CMD (16.1%); while mean severity was 1.4 for CBSD and 1.2 for CMD. At 250 m, all FS and TC plants had CBSD/CMD severity of 1.0 and 0% incidence. These results show that the recommended 50 m isolation distance by MAAIF cannot significantly prevent the spread of cassava viruses and the 250 m isolation distance can be instrumental in clean seed multiplication and production of farmer-preferred susceptible cassava cultivars thereby contributing to the management of CBSD/CMD in Uganda