| dc.description.abstract | Postharvest deterioration limits the production, marketing and
utilization of sweetpotato (Ipomoea batatas L.). This study was conducted to
identify the fungi associated with postharvest deterioration of sweetpotato
roots in Kenya and to study how the deterioration is influenced by certain
preharvest and postharvest cultural practices, varietal differences and some
physical factors.
A baseline field survey was conducted in the mam sweetpotato
growing areas of Western, Nyanza and Central Provinces of Kenya. A short
checklist was used to obtain information on sweetpotato management practices
from farmers, traders and the local Ministry of Agriculture (MOA) field staff.
Diseased sweetpotato root samples were also collected and used to isolate the
causative fungi.
Preharvest experiments, each followed by postharvest laboratory
evaluations, were set up to determine the effect of preharvest factors on
postharvest pathological deterioration of sweetpotato roots. The factors
investigated were vine removal at zero, one and two weeks before harvesting,
time of harvesting at 16, 22 and 28 weeks after planting, cultivar effect using
the cultivars, Yanshu 1, KSP 20, KEMB 10, KEMB 36 and soil pH using pH
levels 4.6, 5.8, and 6.1. Disease development was evaluated on healthy
sweetpotato roots which were artificially inoculated using circular agar plugs
from a two-day old Potato Dextrose Agar (PDA) culture of the most virulent
single-spore isolate of each test pathogen.
Experiments were also conducted to determine the effect of postharvest
washing and solar curing on postharvest deterioration of sweetpotato roots
during prolonged storage (100 days) at room temperature (15.2oC-26.7oC) and
relative humidity (31.4-81.7%) conditions. The effect of storage temperature
on pathological deterioration was also studied using the following
temperatures: 12°C, 16°C, 20°C, 24°C, 28°C, 32°C and 36°C.
Six pathogenic fungi, Botryodiplodia theobromae, Rhizopus oryzae,
Rhizopus stolonifer, Fusarium oxysporum, Macrophomina phaseolina and
Ceratocystis fimbriata, and three saprophytic fungi, Aspergillus niger, Mucor
circinelloides and Penicillium spp., were identified on naturally infected roots.
The pathogens Rhizopus stolonifer, Rhiiopus oryzae and Botryodiplodia
theobromae were selected and used for inoculating healthy sweetpotato roots in
all subsequent experiments throughout the study.
Vine removal before harvesting, and especially at two weeks, and
delayed harvesting at 28 weeks after planting significantly (p<0.05) enhanced
postharvest pathological deterioration of sweetpotato roots, while early
harvesting reduced deterioration. Cultivar differences in root susceptibility to
postharvest pathological deterioration were significant (p<0.05) with culti var
KEMB 36 showing high disease resistance and cultivar KEMB 10 high disease
susceptibility compared with the other cultivars. The different soil pH levels
did not significantly (p<0.05) influence postharvest pathological deterioration
of sweetpotato roots.
Postharvest washing did not significantly (p<0.05) influence
deterioration of sweetpotato roots, but solar curing significantly (p<0.05)
reduced percent loss of marketable roots during prolonged storage at room
temperature and relative humidity. Low temperature at 12-16oC significantly
(p<0.05) suppressed infection while temperature at 24-36oC significantly
(p<0.05) enhanced postharvest pathological deterioration.
The results showed that vine removal before harvesting and delaying
harvesting predisposed sweetpotato roots to pathological deterioration. They
also showed that differences in susceptibility to postharvest pathological
deterioration occur in sweetpotato cultivars. In addition, it was showed that the
influence of soil pH on root deterioration was not significant (p<0.05). Solar
curing had potential in extending the storage life of sweetpotato roots, but the
effect of washing such roots was not significant (p<0.05). The storage
temperature influenced postharvest pathological deterioration of sweetpotato
roots and the temperature range 24-36oC was ideal for root infection, while
infection was suppressed at 12-16oC.
Postharvest pathological deterioration of sweetpotato roots result from
naturally occurring fungal infections, and this study has also shown that
preharvest vine removal and delayed harvesting are some of the cultural
practices that could predispose sweetpotato roots to infection. Cultivar
genotype, curing of roots after harvest and the regulation of temperature during
storage also playa significant role in the control of postharvest pathological
deterioration of sweetpotato roots. Integrated strategies aimed at reducing
postharvest fungal infections and subsequent losses in sweetpotatoes in Kenya
are recommended | en |
