Potato plays an important role in food security in Kenya but yields are low (<10 t/ha), and this is partly attributed to the lack of healthy planting material. This study is the first wide-scale survey to determine the occurrence and distribution of common potato pests and diseases in Kenyan seed (certified and quality declared) and ware crops. Potato crops growing on 101 farms in 21 districts were examined. Approximately 36% of plants in farmers' fields sampled both during the long rains (main potato-growing season) and short rains seasons displayed virus-like disease symptoms. Six viruses (potato leafroll virus (PLRV), Potato virus A (PVA), potato virus M (PVM), potato virus S (PVS), potato virus X (PVX), and potato virus Y (PVY)) were detected using double antibody sandwich enzyme-linked immunosorbent assay in potato samples. Sequencing of polymerase chain reaction products from PVY-infected plants revealed the presence of recombinant strains of PVY (NTN and Wilga). Four aphid species, Macrosiphum euphorbiae, Aphis gossypii, Myzus persicae, and Aphis fabae, colonized potato in all districts, occurring in greater numbers west of the Great Rift Valley than to the east. There was a positive correlation between virus incidence and aphid numbers in the long rains (main) potato-growing season. PLRV, PVM, PVS, PVX, and PVY were detected in solanaceous weeds. Ralstonia solanacearum was Potato Research (2013) 56:325-342

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Information on the population fluctuation of African white rice stem borer (Maliarpha separatella Ragonot), which is a major rice pest in Kenya, is not known. Availability of such information can assist in the development of an integrated management strategy for the pest. Therefore, a study was conducted at Mwea irrigation scheme in Central Kenya, to investigate the influence of irrigation water provision and cropping systems on population of M. separatella. Farmer fields in different parts of the scheme were sampled every fortnight. The farms represented three irrigation water provision schedules and three cropping systems regimes. The irrigation water provision schedules were, System of Rice Intensification (SRI), flood irrigated (conventional method) and sporadic irrigation. The cropping systems were main season crop (conventional method), double crop and ratoon. Water provision schedules were treated as main plots, while cropping systems were the sub plots. Results showed that the highest percentage of white heads (13.66) occurred in areas, where rice was double-cropped and where, there was sporadic water provision (9.70). There were no significant differences in the number of white heads (empty panicles) between the on-season (6.37) and ratoon (4.25) cropping system. There were also no differences between flood (4.42) and System for rice intensification (5.48) methods of irrigation. This study demonstrates that irrigation methods and cropping systems greatly influence fluctuation of M. separatella infestation and suggests that in case of double cropping it will be necessary to control the stem borer and efforts should be made to ensure that farmers synchronize planting dates.

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Aphid-transmitted viruses probably cause greater economic loss in potato production than all other insect-related damage. Some 40 virus species are known to infect potato, and of these 13 are aphid transmitted. Monitoring of aphid populations in potato fields is therefore essential to determine areas with low aphid occurrence suitable for seed potato production and the right time for haulm destruction. While plenty of information on such areas is available in traditional seed-potato-producing countries of America and Europe, there is none available for Kenya. The current study determined the best locations for seed potato multiplication by monitoring aphids (from May 2009 to March 2010) in major potato-producing areas of Kenya. Two methods, yellow water traps (YWTs) and leaf-aphid counts, were utilized. Ten aphid species, Aphis gossypii (Glover), Aphis fabae (Scopoli), Aulacorthum solani (Kaltenbach), Acyrthosiphon pisum (Koch), Brevicoryne brassicae L., Cavariella aegopodii (Scopoli), Macrosiphum euphorbiae (Thomas), Myzus persicae (Sulzer), Myzus ascalonicus (Doncaster) and Rhopalosiphum maidis (Fitch) were caught in YWTs while four aphid species, A. fabae, A. gossypii, M. persicae and M. euphorbiae were also found colonizing potato leaves. The populations of the aphid species varied significantly, R. maidis had the greatest numbers, followed by B. brassicae, A. gossypii, M. euphorbiae, M. persicae and A. fabae in this order while A. solani was the least abundant. The populations of A. fabae, A. solani, C. aegopodii and M. ascalonicus did not vary between seasons and sites but A. gossypii, A. pisum, B. brassicae, M. euphorbiae, M. persicae and R. maidis populations varied significantly between the five sites, and between the three seasons. Although aphid numbers recorded at all sites were above the threshold for virus transmission, not all the species caught were efficient virus vectors. In conclusion, Njabini consistently had low aphid numbers and may be the site most suitable for seed potato multiplication.

The African white rice stem borer (Maliarpha separatella Ragonot) is a major pest of rice in Kenya. To understand and develop appropriate management packages, its population dynamics were studied at Mwea irrigation scheme in Central Kenya. This was for two wet and two dry periods. Farmer fields located in different parts of the scheme and outside the scheme were sampled every fortnight. Farms sampled represented five water provision schedules (System of Rice intensification (SRI), rain fed, flood irrigated, sporadic irrigation, and fallow period). Five planting regimes (on season, off season double cropping, ratoon, and late planting) and three management styles (controlled by National Irrigation Board (N.I.B), not controlled by N.I.B and out-growers) were studied. During each sampling, a 1mx1m quadrant was used randomly and pest counted on all the plants within the quadrant. Results showed that the number of M. separatella varied significantly (p<0.05) in the scheme. Pest densities were highest in off season planted rice (13.1). High numbers were also found in Non N.I.B controlled fields with sporadic irrigation (8.1) and the lowest in the N.I.B, fallow (2.5) and this was significant (p<0.05). There were no significant differences in pest infestations on the normal season crop (0.3) and the ratoon crop (0.6) p<0.05 and SRI and conventional irrigated fields (p<0.05). From the results it is recommended that off season planting of rice be discouraged and that efforts are made to ensure that farmers synchronize planting dates and other cultural practices for the crop, within the scheme and in rice farms outside the scheme.