| dc.description.abstract | Aphids are insects of the Family Aphididae in the Order Homoptera. Their importance as pests
is due to their ability to carry plant viruses, the effect of their sucking damage on the growth of
plants and their rapid rate of reproduction. Some of the factors influencing aphids as vectors of
plant viral diseases were investigated in the laboratory and field using kale and cabbage plants
which are in the family Cruciferae and genus Brassica.
A laboratory experiment was carried out to determine the effect of aphid species and host
plant age on virus transmission. Three aphid species, Brevicoryne brassicae, Myzus persicae
and Lipaphis erysimi were used to transmit two viruses Cauliflower mosaic caulimovirus,
CaMV and Turnip mosaic Potyvirus, TuMV to kale plants that were three, seven and eleven
weeks old. No significant difference in the efficiency of transmission of CaMV and TuMV
was observed between the aphid species. The plant age did not influence virus transmission.
A laboratory experiment to determine the persistence of transmission of CaMV and TuMV by
the 3 aphid species was conducted by exposing kale plants at one-true-leaf stage to an
individual viruliferous aphid of each species (Brevicoryne brassicae, Myzus persicae and
Lipaphis erysimi). Each aphid was moved Jo six different plants, in series, lasting 40 minutes
on each plant in order to transmit the virus. The r-esults showed that M persicae and L.
erysimi than B. brassicae transmitted TuMV longer and that CaMV was transmitted longer by
L. erysimi than B. brassicae and M persicae. These differences were however not significant.
A field experiment to determine the effect of aphid lJ..opulation dynamics on virus incidence
was conducted during two growing seasons from November 2000 to March 2001 and from
June to October 2001 using cabbage, variety Gloria FI, at two sites namely, Kenya
Agricultural Research Institute - Thika and the University of Nairobi, Upper Kabetc campus.
Treatments were applied both in the nursery and after transplanting the seedlings in the field.
Treatments in the nursery were fleece cages, straw mulch and untreated control. The fleece
cover offered a physical barrier to aphids and ensured that the crop could only be infested after
transplanting, while straw mulch was meant to repel the aphids. All the seedlings were
transplanted onto untreated field plots in season 1 while in season 2 the seedlings from straw
mulched nursery were transplanted into untreated plots and also in straw mulched plots. Aphid
infestation and virus infection of the plots were then monitored.
The aphid scores in the straw mulched plots were significantly lower than the non-mulched
nurseries (p=O.028) while no difference was observed for virus infection in the nurseries. The
aphid infestation and virus incidence in non-mulched field plots from fleece covered and straw
mulched nurseries were significantly lower than from control field plots one week after
transplanting and later no differences were observed. Aphid infestation and virus incidence at
week one after transplanting was not different between the straw mulched and non-mulched
plots from straw mulched nurseries but later the mulched plots had significantly lower aphid
infestation and virus incidence. The virus .incidence was positively correlated with the number
of alate aphids, which was in turn correlated to aphid population and rainfall. The aphid
population was positively correlated to.the rainfall. There was appositive correlation between
relative humidity and population of M. persicae while that of B, brassicae was negative.
Negative correlation between weight of cabbage heads and aphid population and virus
incidence of the plots were observed | en |
