Piecemeal versus one-time harvesting of sweet potato in north-eastern Uganda with special reference to pest damage

Abstract

In north-eastern Uganda, the sweet potato crop of small subsistence farmers is severely affected by many pests, including (rough) sweet potato weevils, nematodes and millipedes. Field experiments with sweet potato (Ipomoea batatas (L.) Lam.) were conducted at Arapai Station in Soroti District, north-eastern Uganda in three consecutive seasons to study the differences between the indigenous practice of harvesting piecemeal in combination with storage ‘in-ground on plants’ and one-time harvesting after crop senescence, with special reference to damage caused by sweet potato weevils (Cylas spp.), rough sweet potato weevils (Blosyrus spp.), millipedes (Diplopoda) and nematodes. The area has two rainy seasons per calendar year, the first one with long, reliable rains and the second one with short, unreliable rains. Severe sweet potato weevil damage in the vines was responsible for the mortality of 46% of the plants in Experiment 1, which was carried out during the first rainy season. Starting 3 months after planting (MAP), sizable storage roots could be harvested, although their number and weight declined after 4 MAP with piecemeal harvesting. The highest storage-root yield (17.8 Mg ha1) was found in Experiment 2 (second rainy season) at the final harvest. The yield of storage roots stored ‘in-ground on plants’ during the prolonged dry season (Experiment 3) was very low compared with the yields of Experiment 1 (first rainy season) and Experiment 2 (second rainy season). Sweet potato weevil damage of the storage roots was significantly less with piecemeal harvesting than with one-time harvesting, and piecemeal harvesting also increased the quality of the storage roots for human consumption and commercial purposes. However, with piecemeal harvesting the rough sweet potato weevil (Blosyrus spp.) caused more storage root damage than with one-time harvesting. No statistically significant differences between the two types of harvesting were found for damage caused by nematodes or millipedes. It was concluded that piecemeal harvesting of sweet potato storage roots contributes to the control of sweet potato weevil in both vines and storage roots and hence improves the quality of the harvested roots. As rainfall distribution affects the population dynamics of this weevil this method can only be used during a limited period of the year.