Browsing by Author "Olanya, O. M."

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    Epidemiology and population dynamics of Phytophthora Infestants in Sub-Saharan Africa: Progress and constraints
    (African Crop Science Journal, 2001) Olanya, O. M.; Adipala, E.; Hakiza, J. J.; Ojiambo, P.; Mujalazi, J. M.; Forbes, G.
    Global estimates of losses attributed to plant diseases are approximated at 24.8 million dollars, of this amount 3.4 million dollars has been recorded for potato. Of the potato diseases, late blight (Phytophthora infestans) is the most significant constraint in tropical Africa. Variation in losses of potato caused by late blight have been documented in several countries, and has shown that yield losses can range from 30 to 75% on susceptible varieties. In terms of disease cycle, the sources of primary inoculum have not been adequately investigated, however, the continuos cropping of potato and tomato ensures inoculum presence year-around in tropical Africa. Data on the low incidence of tuber blight and the lack of evidence for potato seed-borne infection suggests that tuber blight is not a significant source of primary inoculum in the tropics. Population studies of P. infestans in Sub-Saharan Africa (SSA) have been conducted primarily on isolates from Uganda, Kenya and S. Africa. Mating type tests with A1 tester isolates coupled with DNA analysis revealed that the fungal isolates from Uganda, Kenya and S. Afirca are of A1 mating type (US 1 clonal lineage). Variation and lack of consistency in oospore production (10 % selfing, 24 % mating, & 15 % non-oospore producers) have been detected among the isolates from Uganda and Kenya. Similarly, variability in metalaxyl sensitivity, has been detected among these isolates. Fungicide and variety reaction studies conducted in Uganda, Kenya and Ethiopia suggests that significant late blight control can be achieved when the protectant fungicide, Dithane (a.i mancozeb) is applied on a scheduled basis. On-farm research also indicates that three timely applications of a protectant or a protectant fungicide alternated with systemic fungicide can be effective for late blight management. Results of in-vitro tuber blight development and host-specificity studies imply that isolates from potato are more virulent than isolates from tomato. Studies are underway to quantify general resistance of potato varieties as well as to monitor the significance of fungal population deviations in the region. Decision support systems are in the process of being developed to optimize fungicide application and variety resistance for late blight management
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    Impact of Fungicide Application and Late Blight Development on Potato Growth Parameters and Yield in the Tropical Highlands of Kenya and Uganda
    (African Crop Science Journal, 2001) Ojiambo, P. S.; Namanda, S.; Olanya, O. M.; El-Bedewy, R.; Hakiza, J. J.; Adipala, E.; Forbes, G.
    The effects of fungicide application and late blight development on potato varieties with different levels of resistance to late blight were quantified in Kenya and Uganda during the 1999 cropping seasons. Experiments were laid out in a randomised complete block design with three replications in three sites, Loreto and Kabete in Kenya and Kalengyere Research Station in Uganda. At each experimental site, plots consisted of 4 m rows with plants spaced at 0.75m x 0.3 m. Treatments consisted of three potato varieties and four application intervals of Dithane M-45 arranged in a factorial combination. Areas under disease progress curves (AUDPCs) was significantly lower in the sprayed plots than in the unsprayed plots. Disease levels were lower in plots with 7 days application interval and most severe in plots of 21 days spray interval. Yields were higher in the 7 days intervals and lower in the 14, 21-days intervals and in the unsprayed plots. Fresh tuber weights and dry tuber weights were higher in the sprayed plots and lower in the unsprayed plots, respectively. At Kabete, Kerr's Pink had the highest AUDPC value of 2139.3 and disease was least on Asante with an AUDPC value of 409.4. The corresponding AUDPC values for the control plots were 671.5, 945.6 and 2367.8 for Asante, Tigoni and Kerr's Pink, respectively. At Loreto, disease was also most severe on Kerr's Pink with an AUDPC value of 2076.3 and least severe on Tigoni that had an AUDPC value of 444.8. The corresponding AUDPC values for the control plots were 840.4, 1000.2 and 2740.0, for Tigoni, Asante and Kerr's Pink, respectively. The highest tuber yield of 41.3 t ha-1 recorded at Kabete for Asante was significantly higher than yields of Kerr's Pink (22.0 t ha-1) but was not different from the yields of Tigoni (39.9 t ha-1). At Loreto, Tigoni had the highest yields of 34.6 t ha-1 that was significantly different from that of Kerr's Pink (6.2 t ha-1) in the sprayed plots. At Kalengyere, disease levels were significantly (P = 0.05) lower in the sprayed plots than in the control plots and disease severity was least on variety Rutuku (AUDPC value of 17.9) in 7 day spray interval. Tuber yields were significantly (P = 0.05) higher in the sprayed plots and highest tuber yield was recorded on variety Kabale (39.5 t ha-1) in plots with 7 day spraying interval.
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    Relationships of Fungicide Application to Late-Blight Development and Potato Growth Parameters in the Tropical Highlands of Uganda and Kenya
    (International Potato Center, 2001) Olanya, O. M.; El-Bedewy, R.; Ojiambo, P. S.; Hakiza, J. J.
    The impact of fungicide applications on late blight development, potato growth parameters, and yield was quantified at field sites in Kenya and Uganda during the 1999 and 2000 cropping seasons. In Kenya, three potato varieties were evaluated at two sites, at altitudes of 1800 m and 2200 m. In Uganda, three varieties were evaluated at one site at an altitude of 2400 m. Leaf, stem, root, and tuber biomass were sampled and quantified during the cropping season. Dithane M-45 fungicide was applied to the experimental plots at the onset of disease at intervals of 7, 14, and 21 days. Final tuber yields differed significantly by as much as 15% between fungicide-treated and untreated controls across the three sites. The severity of late blight differed among sites and between years as a result of variations in environmental conditions. Fungicide application intervals significantly affected disease progress, and the area under disease progress curve (AUDPC) was significantly lower in the fungicide-treated plots than in the control (unsprayed) plots. Tuber biomass accumulation was also significantly affected by fungicide applications. Varieties Tigoni and Rutuku have relatively higher levels of polygenic resistance to late blight and showed low levels of disease severity