Abstract:

Field experiments were conducted at Namulonge
Agricultural and Animal Production Research Institute
(NAARI) during the 1994 short rains (SR) and the 1995 long
rains (LR), and at Kalengyere Research Station (KRS) during
the 1995LR. The studies evaluated the ef f iciency of a
systemic insecticide (dimethoate) in preventing aphid
infestation and infection of aphid-borne viruses in potato
(Solanum tuberosum L.). A screen house experiment was also
conducted at NAARI in the 1995LR to establish the onset of
mature plant resistance to virus infection in potato in order
to limit insecticide spray application to critical growth
periods of potato plants.
The response of aphids to dimethoate spray treatment on
Kalengyere and Namulonge and, between aphid species infesting
potato, particularly at Namulonge. This affected the ability
of dimethoate to prevent aphid infestation and, consequently,
the spread of aphid-borne viruses in potato.
At Namulonge, dimethoate suppressed all potato infesting
aphids except Myzus persicae (Sulz.). This led to heavy M.
persicae infestation on sprayed than unsprayed potatoes. In
contrast weekly dimethoate sprays effectively controlled aphid
colonisation at Kalengyere, and population development in
sprayed potatoes. Unsprayed potatoes at Kalengyere were
heavily infested by aphids, with rapid aphid population growth
occurring between the fifth and the sixth weeks after plant
emergence. A comparison between Namulonge and Kalengyere
showed that there was higher aphid pressure at Kalengyere than
seven Solanum potato varieties varied significantly between
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at Namulonge and, a higher aphid pressure at Namulonge during
the long rains than the short rains.
The commonest potato aphid species caught in aphid traps
were not the most abundant aphid species from potato plants.
In addition, correlations between total trap catches and aphid
infestation indices from plants did not reveal significant and
consistent correlations within seasons, locations and
insecticide treatments. However, pooled data revealed that
percentage of potato plants infested by aphid was
significantly correlated with the total number of alate aphids
trapped. Aphid collections from plants at Namulonge and
Kalengyere showed that aptarae aphids adequately determined
the proportion of potato plants infested by aphids. At
Kalengyere however, potato infestation by aphids was basically
a function of alate aphids on both sprayed and unsprayed
potatoes.
Seed potato tuber indexing for latent infection with
three aphid-borne potato viruses and one mechanically
transmitted virus before planting indicated that Kalengyere
Research Station has potential of providing seed potato free
from aphid-borne viruses. However, seed from three of the
seven varieties were infected with a mechanically transmitted
virus.
Attempt to prevent the spread of aphid-borne viruses in
field grown potato using weekly spray of dimethoate spray at
both NAARI and Kalengyere gave contrasting results. At NAARI,
all the potato varieties used in the study were infected with
potato leaf roll virus (PLRV) in the first season of their
introduction from Kalengyere to Namulonge, irrespective of the
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insecticide treatment. All the varieties were, however, free
from potato virus Y (PVY) and potato virus S (PVS) infection.
At Kalengyere, the total effectiveness of dimethoate
against all potato infesting aphid species and, the absence of
aphid-borne viruses in field grown potatoes, probably indicate
the ability of dimethoate to control aphids and aphid-borne
viruses in Solanum potato at this site. It was also observed
that whereas higher aphid pressure at Kalengyere
than at Namulonge there was a greater virus pressure at
Namulonge than at Kalengyere.
Virus inoculation of screen house grown potato plants at
ten-day intervals after plant emergence using potato leaf roll
virus infected aphids showed that potato plants inoculated
after flowering may produce seed tubers which
aphid-borne viruses.
It was concluded from these studies that use aphicides in
field grown potato, differences of various potato cultivars in
susceptibility to both aphid and virus attack, and the on-set
of mature plant resistance can reasonably reduce infection and
spread of aphid-borne viruses in Solanum potato.