| dc.description.abstract | The Overall Objective of this study was to determine the incidence and distribution of
Potato viruses, relationship to aphid vector population and effect of these virus diseases on yield loss and seed quality of different potato varieties in Kenya. The study was carried out in 3 phases, which were a field survey, on-farm monitoring and an on-station experiment on aphids and viruses.
A survey of potato viruses and their aphid vectors was carried out in three major potato
growing areas, namely Tigoni in Kiambu district, Njabini in Nyandarua district and Molo in Nakuru district. Sixty seed farms (20 per district) were surveyed. Viral diseases were most prevalent in the Tigoni area (47.1%) followed by Njabini (25.6%) and Molo (25%). The most encountered single potato viral infection was potato virus S (PVS), (79.1 %), followed by potato leaf roll virus (PLRV), (34.9%), potato virus Y (PVY), (8.5%) and potato virus X (PVX), (7.0%). The aphid populations, taken from 90 leaves per farm (30 plants, 3 leaves per plant, from top, middle and bottom positions) were highest in Tigoni area (average 73 per farm), followed by Njabini area (26 per farm) and lowest in Molo area (11 per farm). The most prevalent aphid species was Aphis gossypii (61.8%), followed by Macrosiphum euphorbiae (20.5%) then Myzus persicae (14.9%) and the least was Aulacorthum solani (2.8%). Bottom positioned leaves had the highest aphid populations (42.5%), followed by middle leaves (37.4%) and lastly top leaves (20%). There were statistically significant differences (P<0.05) in aphid populations at the different survey areas and also in the populations of the different aphid species, but there were no
significant differences in aphid populations at the different leaf positions.
On-farm monitoring of the aphids and viruses for 2 seasons at 4 farms in the Tigoni area
revealed a similar trend in aphid species and potato virus occurrence. Farmer management
practices affected the number of aphids and the virus incidence on the potato crop. Farms where uncertified seed was used had, significantly higher virus incidence than farms where certified seed was used. Farms where aphid control measures were practiced had significantly lower aphid populations than farms where no aphid control measures were practiced (P<O.05). On-station field experiments done at Tigoni and Kabete stations also revealed that there were significantly lower aphid populations in plots where spraying with aphid control chemicals was done than in plots where there was no spraying for aphid control. Plots planted with certified seed potato had significantly lower virus incidence than plots planted with uncertified seed.
There was a significant reduction in yields with increase in virus incidence at both sites. There was a significant decrease in potato yields with increase in aphid populations at Kabete but there was no significant relationship between aphid populations and yields at Tigoni. There was no significant relationship between the aphid populations and virus incidence at both Kabete and Tigoni, but it was clear that, as aphid populations increased, the virus incidence also increased at both sites. This trend suggests that, with time there is a possibility of a significant increase in virus incidence with increase in aphid populations. The results of these studies show that, high aphid populations result in high virus levels leading to serious yield losses. Seed production in low altitude areas is under threat due to high aphids and virus levels. Using uncertified seed from markets also results in low yields due to viruses. It is therefore recommended that fanners should be taught proper control measures for aphids, together with other potato virus management practices such as use of certified seed and rouging virus symptomatic plants. This is necessary to reduce the virus disease spread in order to maintain good yields in seed and ware potato production systems in Kenya. | en |
