Abstract:

A comprehensive survey of the incidence of ACMD was carried out,
between 1990 and 1992, in all the 32 important cassava growing
administrative districts of Uganda. The incidence varied between
districts, counties within districts and also between fields
within a county. Overall incidence in the country as a whole was
57%. Incidence in individual fields ranged from 0% in districts
in the wetter and more humid Lake Victoria (southern) region to
100% in districts in the drier short grassland savannah regions
in the east, north and west. The differences in incidence are
related to the main agroecological regions of the country and
were associated with differences in climate and abundance of
whitefly vectors.
Infection of local selections (Ebwanateraka, Senjonjo, Bao,
Bukalasa 11) and improved genotypes (TMS 30786, TMS 30337, TMS
60140, TMS 60142, TMS 30395 and TMS 30572) was studied during
the 1990-1992 seasons in four locations in different
agroecological regions and in on-farm trials in northern Luwero.
There were significant differences in ACMD incidence and adult
whitefly populations between locations and between genotypes in
location. The local varieties and TMS 30786 were quickly
infected and final incidence reached 90%-100% in some locations.
In contrast, TMS 30395, TMS 30572 and TMS 60142 sustained very
little infection and symptoms tended to become inconspicuous
even disappear during the later stages of growth,
varieties also harboured low whitefly populations.
ACMV was multifactorial and due to components that
of infection,
expression, and probably to resistance to the vector,
between loctions were asociated with differences in whitefly
populations.
Spatial spread was studied in four locations (Namulonge, Migyera,
Isimba and Kitgum) subject to different infection pressures.
Spread was more rapid at Kitgum, Migyera and Isimba than at
Namulonge where final incidence was less than 5% in each of the
seasons. Clear edge effects and environmental gradients
extending from upwind edges of the fields were evident in most
trials. Dispersal gradients were also evident in some sub-plots
with infected sources at some locations but overall, it was
concluded that the small individual infected sources within
fields had little effects in enhancing spread. Most spread in
all locations was attributed to viruliferous whiteflies moving
from sources outside the field.
Yield loss in plants infected as cuttings and compensation within
stands of infected and uninfected plants was studied in plots
planted with varying proportions of infected cuttings of three
local varieties (Ebwanateraka, Bao and Bukalasa 11). Losses
varied within varieties (21-85%), but mean loss averaged over all
varieties and seasons was 43%. Healthy plants surrounding
infected neighbours compensated for up to 26-42% of the loss in
yield of the infected ones depending on variety. Overall annual
yield loss due to ACMV in Uganda was estimated as 23%, equivalent
to 1.1 million metric tonnes, or Ug Shs 216 billion or US $ 180
million worth of cassava. This would suffice to supply the
annual energy requirements of approximately 2 million people (15%
of the population) in Uganda. Possible control methods of ACMD
for the different regions of Uganda are discussed in relation to
findings from this study and from elsewhere in Africa.