Mapping Sierra Leone’s cassava reveals regional pest and disease patterns

A nationwide field survey has mapped the distribution of cassava traits, pests and disease across 276 smallholder farms in Sierra Leone, revealing sharply regional differences in whitefly and grasshopper abundance while cassava mosaic disease remained widespread. The January 2022 survey, published in Discover Agriculture in 2026, assessed agronomic characteristics and biotic pressures in all five regions of the country. Researchers found that green apical leaves, light-green leaves, green leaf veins and seven-lobed leaves dominated the sampled cassava populations, providing a phenotypic baseline for germplasm conservation, targeted collection and future crop improvement.

Each farm was georeferenced, and 30 cassava plants were randomly selected along two diagonals using harmonized Central and West African Virus Epidemiology protocols. Investigators recorded qualitative traits such as apical leaf color, leaf color, vein color, petiole color and foliar-scar prominence, as well as the number of leaf lobes. They also counted whiteflies and grasshoppers and scored cassava mosaic disease symptoms on a five-point scale. Geographic Information System tools were then used to map trait distributions, while analysis of variance and Pearson correlation tested differences among regions, farm sizes and plant ages.

The most frequently observed phenotypes were green apical leaves, present in 65.3% of the sampled material; light-green leaves, at 64.3%; green leaf veins, at 55.6%; and prominent foliar scars, at 53.5%. Seven-lobed leaves were also the dominant form, accounting for 62.6%, compared with 27.6% for five-lobed leaves. Three-lobed plants occurred mainly in the southern region, whereas nine-lobed types were uncommon but present nationally. Purple apical leaves were concentrated largely in the south, while all-red leaf veins appeared only in the east, indicating geographically uneven representation of less common phenotypes.

Insect abundance was not uniform. Whitefly counts were highest in the western region, where the mean was 7.00 per plant, and lowest in the east at 3.03; the regional difference was statistically significant, and the western mean differed from every other region. Grasshoppers showed a different pattern, reaching their highest numbers in the eastern and southern regions and their lowest levels in the west. Counts generally increased with farm size: the reported mean was 13.4 grasshoppers per plant on farms of 4–5 hectares, 12.2 on medium farms of 3–4 hectares and 7.0 on small farms of 0–1 hectare. Both insect groups were more abundant among plants aged four to nine months than among plants aged one to three months.

Cassava mosaic disease, or CMD, presented a different spatial signal. Mean severity scores ranged from 2.33 to 3.40 across the five regions, but the differences were not statistically significant, with an F value of 2.86 and a P value of 0.070. Symptom severity was instead significantly associated with farm size. Small farms averaging 0–1 hectare had the highest mean score, 4.29, followed by medium farms of 2–3 hectares at 3.8 and large farms of 4–5 hectares at 2.94. The observational survey therefore identified an association between smaller field size and greater disease expression, rather than demonstrating that farm size itself caused the difference.

Contaminated planting material was the dominant route of CMD infection. Researchers classified 99.2% of infected plants as showing the cutting-borne pattern, in which symptoms appeared on older, lower leaves and were distributed through the canopy, compared with 0.8% classified as whitefly-borne. Severe symptoms occurred in the western, northern, northwestern and eastern regions, while mildly affected and apparently tolerant morphotypes were recorded in the south and northwest. The researchers linked the unusually low infection recorded in parts of the south to the cultivation of tolerant cassava types, while also noting that other healthy types were present in the northwest.

Correlation analysis identified visible leaf and petiole characteristics that co-occurred with different levels of pest abundance and disease severity. Whitefly abundance had moderate positive associations with light-green leaves, light-green apical leaves, purplish-green apical leaves, dark-green leaves and seven- or nine-lobed leaves, with coefficients ranging from 0.44 to 0.50. It was negatively associated with purple petioles, yellowish-green petioles and three-lobed leaves, with coefficients from −0.34 to −0.47. These relationships do not prove that a particular color or leaf shape causes insect preference, but they may help researchers prioritize traits for further testing, particularly because the study population included a wide range of farmer-recognized cassava types.

Grasshopper abundance was moderately associated with green and greenish-red petioles, green leaf veins and light-green leaves, with correlation coefficients of 0.55 to 0.60, and negatively associated with purple leaf, apical and petiole traits, ranging from −0.39 to −0.45. CMD severity showed a similar pattern: it was positively associated with light-green leaves and apical leaves, dark-green leaves and greenish-red, yellowish-green and green petioles, with coefficients of 0.58 to 0.65, but negatively associated with purple leaf, apical and petiole traits. The authors proposed that pigmentation and canopy structure might be related to insect feeding or pathogen defense, although controlled trials would be needed to test those mechanisms directly.

The results offer a phenotypic guide rather than a finished set of breeding solutions. Traits spread widely across Sierra Leone may represent useful candidates for conservation and evaluation, while distinctive regional combinations could help researchers direct germplasm collection and parental selection. The authors also emphasize that farmers’ naming practices, preferences, exchange of stem cuttings and retention of locally valued types probably shape the observed distributions as strongly as environmental conditions do. The study was limited to one cassava survey conducted in a single month, relied on phenotypic rather than genomic measurements, and did not directly establish resistance, yield effects or causal relationships. Its datasets are available from the corresponding author on reasonable request.

Subject of Research: Spatial patterns of cassava traits, insect pests and mosaic disease across Sierra Leone

Article Title: Spatial distribution of phenotypic and biotic traits in cassava fields across Sierra Leone

Article References: Amara, V., Samura, A. E., Norman, P. E., Steven, S. J., Tiendrebeogo, F., Eni, A. O., & Pita, J. S. (2026). Spatial distribution of phenotypic and biotic traits in cassava fields across Sierra Leone. Discover Agriculture, 4(1), Article 324. https://doi.org/10.1007/s44279-026-00798-8

Image Credits: AI Generated

DOI: 10.1007/s44279-026-00798-8

Keywords: cassava, Sierra Leone, crop diversity, GIS mapping, cassava mosaic disease, whiteflies, grasshoppers, smallholder farms, phenotypic variation, plant breeding, Spatial, distribution