Dissertations

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Browsing Dissertations by Subject "Central Uganda"

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    Genetic diversity and resistance to cassava brown streak disease in central Uganda
    (Kyambogo University [unpublished work], 2019-11) Ogwal, Sam
    Cassava (Manihot esculenta Crantz), a common food security crop in many sub-Saharan African countries is being threatened by cassava brown streak disease (CBSD) which causes up to 100% yield loss in susceptible varieties. Cassava brown streak disease is caused by two distinct Ipomoviruses namely Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV Cassava brown streak disease causes symptoms characterised by chlorotic patches on leaves, dark brown "streaks" on stems, cracked constricted corky, yellow-brown necrotic root tubers of cassava. However, variability of symptoms exists within cassava genotypes and CBSV strains depending on the environmental conditions. Currently in central Uganda, there is limited information on CBSV genetic and symptom diversity, as well as sources of resistance to CBSV. Yet development of CBSD control strategies requires knowledge of CBSV genetic diversity, virus-host interaction and availability of resistant cassava germplasm to CBSD. The objective of this study was therefore to 1) assess genetic diversity and distribution of cassava brown streak disease causal viruses in central Uganda. 21 unravel the symptom diversity of CBSD causal viruses on selected cassava genotypes. 3) identify potential sources of resistance to cassava brown streak disease in central Uganda. Symptomatic leaf samples collected from the four districts in central Uganda were used for total RNA extraction using CT AB protocol. The purified samples were subjected to PCR amplification of specific DNA sequence by in vitro DNA synthesis done as prescribed by Mullis and Faloona (1988). PCR products were separated using electrophoresis and the sequencing of PCR products was done in Netherland at Macrogen laboratory and the sequenced products were edited using Edit Seq in DNA Star, aligned using clustal logarithm MEGA 6 software and coat protein nucleotide sequence data analysed using AMOVA. The results revealed that there was variation in nucleotide sequence and the coat protein among and within CBSV strains with isolates clustering into two groups. Additionally, CBSV was the dominant strain, followed by UCBSV and last mixed infection among 18 out of 20 symptomatic leaves sampled. Symptoms diversity and sources of resistance were identified by inoculating disease free cassava grown in screen house with CBSV strains singly and in combinations using chip bud method of grafting. Symptom diversity was identified based on the dominant symptoms from symptoms appearance at 1 MAI to 3 MAI. For resistance, incidence and severity was assessed. Later severity scores were used to compute area under disease progress curve (AUDPC). Findings showed that there was significant difference (P<0.001) in reaction of cassava genotypes to CBSV viral strains. In addition, significant variety x viral strains interactions (P<0.001) was observed. The highest CBSD incidence and severity was recorded from Kwatampola and the lowest in NASE14. Overall, NASE 14 and NAROCASS were resistant while Aladu, NASE12, Njule, Bam_unanika and Bukalasa were moderately susceptible. Symptom diversity and severity was scored visually from symptom appearance and continued at monthly intervals for three months using a scale of 1 - 5. Findings showed Variation in symptoms of CBSD existed on leaves and stems of cassava genotypes inoculated with CBSV, UCBSV and CBSV+UCBSV strains. Infact, the symptoms varied from chlorotic spots on leaf tips to mild chlorosis, mottling of secondary to primary veins and mild stem streaks. It was concluded that CBSV and UCBSV were dominant strains of CBSV. Additionally genotype NASE 14 was most resistant to CBSD. Accordingly CBSD symptoms varied from chlorotic spots to mild stem streaks. This study has an implication on the management of CBSD in Uganda. However another study to assess the relationship of CBSV viral load, CBSD symptom severity and resistance of cassava genotypes needs to be conducted.
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    Transmission and diversity of pineapple mealybug wilt disease viruses in Central Uganda
    (Kyambogo University(Unpublished work), 2020-07) Mawa, Everrest Michael
    Pineapple mealybug wilt disease (PMWD) is one of the latest breakouts of diseases attacking pineapple in Uganda. Although, first reported around 2009, the etiology and epidemiology is not fully elucidated. Pineapple mealybug wilt disease was reportedly devastating in the districts of Mukono and Kayunga with incidence ranging from 15 to 100%, respectively. The causal viruses of PMWD belong to the genus Ampelovirus. However, information on the transmission and diversity of the causal viruses of pineapple mealybug wilt disease in Uganda is limited and scanty. Therefore, this study was undertaken to (1) identify and characterise the diversity of viruses associated with PMWD, (2) determine mealybug action threshold for transmission of PMWD and (3) identify sources of resistance to PMWD. Forty four (44) symptomatic pineapple plant leaves were collected from the districts of Masaka, Luwero, Kayunga and Mukono in central Uganda for laboratory analysis. Molecular analysis using polymerase chain reaction (PCR) was done to identify and assess the diversity of pineapple mealybug wilt disease causal viruses. Total plant RNA was extracted using Cetyltrimethyl ammonium bromide (CTAB), the heat-shock protein 70 homolog (Hsp 70h) gene of causal virus was amplified by RT-PCR using specific primer pair 225/226 for PMWaV-1 and 223/224 for PMWaV-2, respectively. Polymerase chain reaction products were separated by electrophoresis and the resulting amplicons were sequenced using Sanger method. The action threshold for transmission of PMWD was assessed using pineapple variety Smooth cayenne inoculated with five levels of viruliferous mealybugs 0, 1, 5, 10 and 15, respectively. The experiment was arranged in a completely randomised design (CRD) with eight replications. Screening for resistance to PMWD was done on five pineapple varieties (Smooth cayenne, Red Spanish, MD2, Sassilimu and Victoria) inoculated with 15 viruliferous mealybugs. Data was subjected to AMOVA and ANOVA for genetic diversity and action threshold and resistance, respectively. Results showed that only PMWaV-1 was associated with pineapple mealybug wilt disease in central Uganda. Accordingly, high PMWD transmission was recorded in 15 mealybugs. There was significant difference (P˂0.05) in resistance among the pineapple varieties. Red Spanish was highly resistant while Sassilimu, MD2 and Smooth cayenne was moderately resistance and Victoria highly susceptible. The results of this study have therefore shown that only PMWaV-1 was associated with PMWD in central Uganda. The mealybug action threshold for the transmission of PMWD is 15. The study also revealed that the sources of resistance to PMWD are variable although Red Spanish was highly resistant. The implications of these findings therefore is that an effective management strategy against PMWD can be instituted based on the identified and characterized causal virus and action threshold of mealybug as well as the sources of resistance identified. However, there is need for additional studies to unravel genetic diversity using more samples as well as the molecular basis for resistance of pineapple varieties to PMWD.