Browsing by Author "Kawuki, R.S."

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    Effect of self-pollination with heat-treated pollen on parthenocarpy and homozygosity in cassava
    (African Crop Science Journal, 2020) Buttibwa, M.; Kawuki, R.S.; Baguma, J.K.; Nalela, P.; Eyokia, M.; Oshaba, B.; Ceballos, H.; Lentini, Z.; Baguma, Y.; Tugume, A.K.
    Cassava’s (Manihot esculenta Crantz) high heterozygosity complicates its genetic improvement via selective breeding. Double haploid (DH) technology can be used to improve the crop’s heterozygosity, thereby improving the capacity for genetic improvement. The objective of this study was to evaluate the effect of self-pollination using heated pollen on pollen tube penetration, fruit set, seed and haploid embryo development in cassava genotypes for the production of haploid cassava. Pollen from two cassava genotypes, NASE3 and NASE14, was heated at 40, 50 and 60 oC for 0.5, 1.0 and 2.0 hr each. The heated pollen was used in six rounds of self-pollinations. Pollen tube penetration was monitored by fluorescent microscopy, followed by early embryo rescue and ovule culture. Ploidy and zygosity were assessed using flow cytometry and single-nucleotide polymorphism analysis, respectively. Pollen germinated on the stigma, grew within the style through the nucellar beak, but did not reach the embryo sac, thus achieving no fertilisation in all the 5756 self-pollinated flowers. There was a reduction in pollen germination (in vitro and in vivo), pollen tube penetration and fruit set with increasing temperature. Heat-treated pollen stimulated division of the egg cell and induced development of parthenocarpic fruits. Up to 6 embryoids per ovule were observed and all regenerated plantlets were diploid, with up to 93.0% increased homozygosity. For the first time, plant regeneration from ovules, pollinated with fresh pollen at 14 days after pollination, was achieved indicating improved speed in plant regeneration. The data generated are important for the development of protocols for cassava DH plant production.
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    Limits of phytosanitation and host plant resistance towards the control of cassava viruses in UgandaLimits of phytosanitation and host plant resistance towards the control of cassava viruses in Uganda
    (African Journal of Rural Development, 2017) Kawuki, R.S.; Adiga, G.; Orone, J.; Alicai, T.; Edimu, M.; Omara, T.; Pariyo, A.; Esuma, W.; Omongo, C.; Bua, A.; Kanju, E.; Baguma, Y.
    Cassava (Manihot esculenta Crantz) and the viruses that infect it, notably cassava mosaic virus and cassava brown streak viruses, have a unique history of co-evolution and co-existence. While cassava originated in South America, both viruses and the diseases they cause have largely been limited to the East African region, where they have, and continue to be key yield-robbing stresses. For sustainable control, we assume that deployment of resistant varieties when carefully combined with phytosanitation will combat these viruses. We have thus generated empirical data and tested the limits, i.e., how long this strategy can last. This entailed the comparison of elite cassava varieties, one set of virus-indexed tissue culture plantlets, and the other set, re-cycled planting materials under farmer’s cyclic propagation for 6-23 years. Trials were established at diverse sites in Uganda. We observed that both officially-released and unofficially-released cassava varieties are common in farmer’s fields; these varieties have varying susceptibility levels to viruses. Worrisome was that some officially-released varieties like NASE 3 registered cassava mosaic disease (CMD) incidences of up to 71% (virus-indexed), which was not any different from its re-cycled counterparts. Other varieties like NASE 14 have maintained high levels of CMD resistance six years after official release. Predominant re-cycled cassava varieties notably TME 204, I92/0057, TME 14, and to a limited extent NASE 14, are key reservoirs for cassava brown streak disease (CBSD) associated viruses. These findings highlight the limits of phytosanitation, i.e., in areas like Kaberamaido associated with high CMD pressure, varieties NASE 1 and NASE 3 can not be recommended; on the contrary, these varieties can be deployed in Kalangala, where they can survive with phytosanitation. And for CBSD, the findings justify the urgent need for phytosanitation (community-led) and development of varieties with higher levels of resistance and/or tolerance, as no immune variety has so far been identified.
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    Responding to the soybean rust epidemic in sub-Saharan Africa: A review
    (African Crop Science Journal, 2003-04-16) Kawuki, R.S.; Adipala, E.; Lamo, J.; Tukamuhabwa, P.
    Soybean rust (Phakopsora pachyrhizi Sydow), a major threat to soybean production, is a new pathogen on the African continent, where it is increasingly threatening soybean production. The fungus is highly variable, and this complicates most disease management strategies. Most research on soybean rust, its epidemiology and management has been in the Orient, and only limited work has been done in Africa. However, experiences in the Orient will be important for soybean rust researchers in Africa, who are currently combating the epidemic. This approach will be necessary to design and implement rust management strategies in the African countries already affected, and those as yet unaffected. Because there is little information on distribution, epidemiology, and management of soybean rust from Africa, this paper provides an overview on 1) soybean rust epidemiology, disease assessments and management, as experienced in the endemic areas, 2) the distribution of soybean rust in Africa, 3) on-going research activities in the African countries most affected, and 4) discusses priority research activities. This review is intended to stimulate future research activities, leading to a better understanding of the pathogen, its biology and ecology, and management.
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    Soybean rust severity, rate of rust development, and tolerance as influenced by maturity period and season
    (Crop Protection, 2004-05-15) Kawuki, R.S.; Tukamuhabwa, P.; Adipala, E.
    Studies were conducted in Uganda for three consecutive seasons to evaluate soybean lines of different maturation periods against rust. All the lines were highly susceptible to rust with only two lines graded as moderately resistant: TGx 1835-10E (early maturing) and TGx 1838-5E (late maturing). These two lines were consistently associated with non-sporulating lesions. Within each maturation group, soybean lines differed significantly in rates of rust development (RRDs), rust severities at R6 growth stage, and yielding ability under rust stress. Most of the lines had RRDs higher than the local check. However, it was only the early maturing lines that yielded higher than the local check. Higher levels of rust tolerance were observed in the early maturing lines (yields >1000 kg/ha), and lowest in late maturing lines (yields<500 kg/ha). Most variation in yields was due to differences among soybean lines, except the late and medium maturing lines were it was due to seasonal variation. Most variation in RRDs was due to the seasons and not soybean lines, and rust severity increased with crop age.