| dc.description.abstract | The thesis describes a study of groundnut
rosette disease with particular reference to the
aphid vector, its alternative hosts and control
with the systemic insecticide,menazon.
The vector was shown to enter the growing crop
within 10 days of germination. The number of
plants infested by aphids and the actual number of
aphids on plants was found to remain low until 32
days from germination. The main increase in aphid
population occurred in the period 40 to 50 days
after germination, which also coincides with the
period of maximum flower production and pod setting
in most cultivars of groundnuts at Serere, Uganda.
Aphids were found to favour feeding on areas of
rapid cell proliferation, and to occur in large
numbers on young gynophores.
Little effect on the actual number of plants
infested by aphids per unit area was detected by
either varying the plant population,or including
yellow coloured or reflecting panels in treatment
plots. There was ample evidence from counts
carried out on a number of trials that the actual
number of aphids per unit area was lower on plots
with high plant populations and that colony size was
significantly larger at low plant populations.
There was some evidence that this effect was
due to microclimate in that aphid populations were
very low on alternative hosts growing in among
pasture in dense stand, while isolated plants of the
same species often carried large aphid populations.
A similar condition was observed with volunteer
groundnuts which did not carry aphid populations
over the dry season in pasture, but often had large
aphid populations when growing on bare fallowed
land.
Hitherto unreported alternative hosts of the
vector in Uganda were discovered and the euphorbiaceous
weeds, Euphorbia hirta L. and E. prostrata
Act., recognised as important sources of dry season
carryover, particularly in certain specialised
habitats. Gliricidia sepium Jacq. and the pasture
legumes Centrosema pubescens Benth. and Macroptilum
atropurpureum (Moc and Sesse) Urb. also were shown to
be potential sources of vector throughout the year.
None of the alternative hosts tested was shown to be
a source of the virus. The vector was not found on
Stylosanthes guyanensis Aubl. (Syn S. gracilis
H.B.K.). Whereas a large proportion of groundnut
volunteers showed virus symptoms, very few plants
apparently carried a resident aphid population even
in the dry season, when growing as a weed in sown
pastures or in areas of natural regeneration.
Volunteer plants on bare fallowed land were shown to
be severe potential sources of the vector and the
virus.
The number of plants per unit area showing
virus symptoms during growth was found to be higher
at low plant populations. The percentage of plants
showing rosette symptoms was greater at low plant
populations in all trials. No clear relationship
could be established between the number of alate
aphids or number of infested plants per plot and the
number of plants infected by rosette virus.
Spraying with the systemic aphicide, menazon,
reduced and maintained the aphid population at a
very low level in both large and small field trials
It also resulted in a significant reduction in the
number of plants showing rosette symptoms during
growth and at harvest and in increased yields. The
number of pods per plant was significantly increased
by spraying.
Increased yields of groundnuts were also
obtained by applying menazon to plots with high
plant populations compared with the equivalent unsprayed
plots. The actual number of plants
showing rosette symptoms on both sprayed and
unsprayed plots at these plant populations often
differed little.
There was a high correlation between plant
population and yield on both sprayed and unsprayed
plots. There was also a marked negative correlation
on unsprayed plots between the percentage of plants
showing rosette symptoms and yield. On sprayed
plots, with a very much lower percentage incidence
no direct relationship could be established between
symptom expression and yield.
The correlation between "peg" symptom expression
and yield was considerably closer in all trials than
that established between foliage symptoms and yield.
In view of the observations made on aphid populations
on the fruiting areas of groundnut plants it is
suggested that the classical field rosette "peg"
symptoms result from a combination of both virus
infection and direct aphid damage. Plants in field
trials, particularly under conditions of low plant
population, were able to produce a recovery crop
despite exhibiting severe rosette foliage symptoms.
The spray regime adopted utilizing 4.2 oz.
active ingredient menazon per acre, applied 4 times
at 10 day intervals commencing 10 days from emergence
of the crop,was found to be the most consistent
and successful over a range of climatic and
edaphic conditions. Yield increments of up to
600 Ibs. dry groundnuts in shell were obtained at a
range of plant spacings under rain grown conditions,
while increments of well over 900 Ibs. were obtained
with irrigation. Reducing the number of sprays
from four to two, even with increased dosage of
chemical gave less successful control of rosette
symptom expression. However, halving the dosage
rate}while maintaining the number of applications
at four at Serere)gave results equivalent to those
obtained with the full dosage and four sprays.
The standard spray regime gave significant
improvements in both shelling percentage and quality
of kernels produced in most trials | en |
