Abstract:

Sweetpotato, Ipomoea batatas (L.) Lam. Family Convulvulaceae, is an important staple
food, feed and raw material for both industrial and non-industrial products. Production in
Africa is estimated at over 13.4 million hectares and is second only to China in the world.
Sweetpotato production is however constrained by the presence of biotic and abiotic
stresses. Cylas puncticollis (Boheman) and Cylas brunneus (Fabricius) are the main field
pests of sweetpotato in Africa causing yield losses estimated at between 60-100% during
the dry season. The management of these pests therefore, calls for the development of
interventions that can be replicated and used across Sub-Saharan Africa and elsewhere in
the world. Host plant resistance is one of the valid options especially at small-scale
resource poor farmer level, where an affordable and effective weevil management is
urgently needed. Previous research indicated variation in levels of resistance among
Ugandan varieties to Cylas spp. It was hypothesised previously that bio-chemical content
of sweetpotato may influence this variation and provide insight into the mechanisms of
resistance. The amount and kinds of these bio-chemicals in sweetpotato varieties with
differences in susceptibility to Cylas spp. and their relationships to weevil resistance was
unknown. The objectives of this study therefore were to: evaluate levels of susceptibility
in farmer-selected sweetpotato varieties (improved and landraces) and sweetpotato
breeding lines in Uganda to C. brunneus and C. puncticollis in field trials; determine and
validate the modes of resistance to C. brunneus and C. puncticollis observed in the field
through laboratory feeding and oviposition bioassays; identify and quantify plant
chemicals in sweetpotato varieties that may confer resistance to C. brunneus and C.
puncticollis; and determine the effect of selected sweetpotato phytochemicals on C.
brunneus and C. puncticollis feeding, oviposition and development.
Field evaluation of 136 Ugandan sweetpotato varieties was done over two seasons and at
two sites (Namulonge and Serere) in Uganda. Using Principal Component Analysis
(PCA) of percentage root damage data and external and internal stem base damage
rankings among others, sweetpotato varieties that were consistently less damaged across
sites and seasons were identified. These were further evaluated in the laboratory using
oviposition and feeding bio-assays and analysed for biochemical content and composition
using Liquid Chromatography-Mass spectrometry (LCMS) and High Performance Liquid
Chromatography (HPLC). Root surface, latex and whole root content of the varieties
were analysed. Artificially synthesized root compounds were incorporated into weevil
diets and their effect on the development of 1st instar larvae of both species investigated.
Additionally the artificially synthesized compounds were smeared on root surfaces to
investigate their effect on adult female weevil biology.
Laboratory experiments confirmed resistance of seven resistant varieties including New
Kawogo, Anamoyito, Dimbuka2, Orurengo 2, Kyebagambire, ARA228 and APA356.
These varieties showed protracted development time and reduced adult weevil eclosion
compared to the susceptible control varieties (NASPOT 1 and Tanzania). From biochemical
analyses, hydroxy cinnamic acids including hexadecyl p-coumaric acid,
hexadecyl caffeic acid and octadecyl caffeic acids were identified in sweetpotato latex
flesh and surfaces of sweetpotato varieties. The compounds such as hexadecyl caffeic
acids and hexadecyl-p-coumaric acids were found to occur in significantly higher
amounts in the latex, root surfaces and flesh of the roots of resistant varieties such as
New Kawogo compared to Tanzania and NASPOT 1 varieties. There were significantly
(P < 0.05 df= 12) more emergent C. puncticollis adults from susceptible varieties
compared to New Kawogo in incubation bio-assays and significant reduction (P < 0.05)
in the mean oviposition and feeding by C. brunneus and C. puncticollis on this variety
compared to susceptible NASPOT 1. Field resistant varieties when screened in the
laboratory gave prolonged adult weevil development time for both Cylas spp. and low
susceptibility indices while the reverse was true on susceptible varieties. Diet
incorporated bio-assays with hexadecylcaffeic acid and hexadecyl-p-coumaric acids
significantly (P < 0.05) reduced larval survival weights and mean survival to pupation in
the treated periderms compared to controls; and this response was dose dependent for
both compounds on both weevil spp.
This study clearly showed that weevil resistance in sweetpotato must be more than
simply escape, but is quantifiable and manageable and thus capable of being used in the
development of resistant varieties through breeding. It recommends that the recent
advances in identifying the sweetpotato genome and success in transforming sweetpotato
and incorporating weevil resistant Bt toxins by other researchers involved in transgenic
research, could be complemented with the identification of quantitative trait loci
responsible for variations in phytochemical composition and thus develop ecologically
more stable and sustainable resistant varieties, for sweetpotato resistance to Cylas
species in Uganda and elsewhere in the world.