Response of selected Latin American cassava genotypes to cassava brown streak disease in Uganda

Abstract

Cassava brown streak disease (CBSD) is a big threat to food security in the predominantly cassava growing regions in Africa. The disease causes total crop loss in susceptible varieties. A more sustainable method to manage CBSD would be to use host plant resistance. Although most African cassava germplasm continue to succumb to the disease, novel sources of CBSD resistance were reported in the Latin American gene pool when germplasm collection at CIAT, Columbia, was screened by graft-inoculation technique in a glasshouse in Germany. This study undertook an evaluation of the Latin American cassava genotypes for resistance to CBSD and root quality in Uganda. Specifically, the study validated the resistance reported in the Latin American cassava genotypes in a hotspot environment and established variability for root quality attributes in Uganda. Two sets of experiments were established at Namulonge; one under natural field conditions, using the randomized complete block design (RCBD) with 3 replications. The second experiment was conducted in a screenhouse, where the plants were graft-inoculated with a virulent strain of Cassava brown streak virus (CBSV). Data was collected on CBSD severity and viral detection every three months from the time of planting. To understand variability for fresh root yield, dry matter, and starch contents, data was collected from field trials set at three sites at Abi, Serere, and Kabarole. Results from the screening experiment indicated the Latin American cassava genotypes did not show CBSD symptoms in the field, as opposed to the Ugandan genotypes that showed varying levels of CBSD symptoms. In the second experiment, after graft inoculation, highly significant differences (p ≤ 0.001) among genotypes were realized. Significant differences (p ≤ 0.001) were observed for both CBSD foliar and root necrosis. Overall, the mean CBSD severity of Ugandan clones was higher than in the Latin American genotypes, suggesting better resistance for CBSD in the background of Latin American clones. Interestingly, three genotypes, COL 40, COL 2182, and PER 556, neither exhibited CBSD symptoms nor tested positive for CBSV in the two experiments. Notably, all the Latin American cassava genotypes yielded lower than the top-check variety, NAROCASS 1. However, genotype COL 40 exhibited mean dry matter content (DMC) of 38% across the three study environments, which was higher than the mean DMC of the local check variety NAROCASS 1 (35%). Taken together, this study provides information about sources of CBSD resistance, and it is therefore essential to keep track of these clones as potential sources for subsequent CBSD resistance breeding. Consequently, the finding also emphasizes the suitability of genotypes such as COL 40 that combine disease resistance with desirable root quality attributes, as well as the need to incorporate graft inoculation techniques while screening advanced breeding materials for CBSD resistance.