Whitefly (Bemisia tabaci) resistance among cassava genotypes in Uganda.

Abstract

The whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) is a major pest and vector of viruses in cassava both globally and especially in sub-Saharan Africa (SSA) where the pest poses the greatest threat to cassava production. For long, research efforts have focused on management of the two viral diseases transmitted by Bemisia tabaci: cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) and have largely ignored the whitefly vector that is driving the spread of these diseases. This study, therefore, aimed to: (i) evaluate cassava genotypes for resistance to whitefly infestation and damage, (ii) determine the population dynamics of whiteflies and viral diseases on selected cassava genotypes with whitefly resistance, and (iii) identify loci associated with phenotypic traits for whitefly resistance. Evaluation of resistance to B. tabaci was carried out in four locations including: Namulonge (Lake Victoria Crescent), Kasese (West Medium-High Farmlands), Ngetta (North-Eastern Savannah Grasslands) and Serere (Kioga Plains) during 2015 and 2016 growing seasons in Uganda. Whitefly nymph abundance and feeding damage were assessed on 50 test genotypes from 3 to 6 months after planting (MAP). In the 2015 season, the highest broad sense heritability estimates were 39% (4 MAP) and 53% (5 MAP) for whitefly nymph abundance and feeding damage, respectively. In the 2016 growing season, lower broad sense heritability estimates of 23% (3 MAP) and 41% (4 MAP) were recorded for whitefly nymph abundance and feeding damage, respectively. Analysis of variance of whitefly nymph abundance showed significant (P<0.05) location × genotype × season interactions at 3, 4, 5 and 6 MAP. There were also significant (P<0.05) location × genotype × season interactions at 3 and 4 MAPs for whitefly feeding damage. Ten genotypes, including: UG120202, UG120174, NASE13, UG120160, UG120286, UG120293, UG130075, CSI-142, CS1-144 and UG130085 showed good levels of resistance to whitefly infestation and feeding damage. These genotypes could serve as parental materials in breeding for whitefly and viral disease resistance. Determination of Bemisia tabaci population dynamics on resistant cassava genotypes to whitefly in the four study locations (Namulonge, Kasese, Ngetta and Serere) was studied from July to December 2015 (Season 1) and April to September 2016 (Season 2). Data were collected on adult whitefly populations and viral disease (CMD and CBSD) incidence monthly for six months. Mean parameter values were calculated, and analysis of variance performed using the ‘F’ (variance ratio) test. Growing season, location, crop age and cassava genotype significantly (P<0.001) influenced adult whitefly populations and disease (CMD and CBSD) incidence in both 2015 and 2016. CMD incidence was generally low in both 2015 (12.18%) and 2016 (6.39%). Moderate (47.44% ± 0.7821) and high (72.04% ± 0.6916) incidences of CBSD were, however, recorded in 2015 and 2016. Incidence of CMD and CBSD increased with crop age (from 3 MAP) across locations in both 2015 and 2016. Whitefly population increased steadily from low (10) in 2015 at 1 MAP to the highest (130) recorded mean population at 4 MAP, followed by a sharp decline to the lowest (5) numbers in older (6 MAP) cassava plants in 2015. Similar trends were recorded in 2016, where the whitefly population peaked at 3 MAP from 10 to 90 to the highest peak population range of 50 to 150, followed by a sharp decline to the lowest population of 10 at 6 MAP. The study demonstrated significant resistance to both whitefly infestation and CMD infection among the cassava genotypes. This information is vital for cassava breeders and multipliers of planting materials to guide distribution to farmers to control the spread of CMD and ensure sustainable food production and security in SSA. Genetic factors associated with resistance to Bemisia tabaci in cassava genotypes were studied using a genotyping-by-sequencing platform to characterise a panel of 700 genotypes and 32,762 single nucleotide polymorphisms (SNPs) distributed across the cassava genome. A genomic region on chromosome 4 was identified with three (3) SNPs significantly associated with whitefly resistance. A survey of the cassava genome sequence v 6.1 positioned the three SNPs in the vicinity of the Manes.04G056800.1 locus, which is a known gene that plays important roles in plant defence responses to CMD and CBSD. Manes.04G056300.1 and Manes.04G055500 also revealed two genes associated with unknown functions, which would have remained undetected using a candidate gene approach constrained by annotation for disease resistance. This study provides insights into the practical application of GWAS for dissecting the genetic basis of whitefly resistance in cassava. These findings offer practical grounds for development of whitefly resistance in cassava using marker-assisted breeding.