40 Records out of 22207 Records

Understanding resistance in inter-specific rice cultivars to the parasitic witchweed Striga

Author: Cissoko, Mamadou

Awarding University: University of Sheffield, England

Level : PhD

Year: 2012

Holding Libraries: DA. H1c 62-11517 ; Institute of Commonwealth Studies Library ;

Subject Terms: Striga ; Rice ; Oryza sativa ; Oryza glaberrima ; Striga hermonthica ; Striga asiatica ; Genetics ; Weeds ;

Abstract:

Both cultivated rice species, Oryza sativa (L.) and Oryza glaberrima (Steud.), are grown in Africa. To take advantage of superior traits from each species, AfricaRice Center and partners developed inter-specific rice cultivars called NERICA (NEw RICe for Africa) for rain-fed upland ecosystems. NERICA rice cultivars showed different susceptibilities to both S. hermonthica and S. asiatica species under controlled environment conditions. Some cultivars showed good broad-spectrum resistance against several Striga ecotypes and species whilst others showed intermediate resistance or were very susceptible. In addition, some cultivars showed resistance to a particular ecotype of Striga but were susceptible to others. The phenotype of a resistant interaction was often characterized by necrosis at the host parasite interface and an inability of the parasite to penetrate the host root endodermis. In general, the most resistant NERICA cultivars grew better than the very susceptible cultivars although even a small number of parasites caused a reduction in above ground host biomass. There was however, genetic variation for tolerance to Striga (the ability to grow and yield well in the presence of Striga) amongst the NERICA cultivars. The NERICA cultivars were also grown in field trials at Kyela in Tanzania (under S. asiatica infestation) and at Mbita Point in Kenya (under S. hermonthica infestation) in 2010 and 2011 to determine the impact of environment on the expression of resistance. The resistance of the NERICA cultivars against S. hermonthica and S. asiatica, in the field, was broadly similar to that observed in the laboratory although there were some exceptions. These results allow us to recommend particular cultivars for Striga-infested regions but they also illustrate the necessity of understanding the genetic basis of resistance to different ecotypes of Striga for breeding of durable resistance (and pyramiding of appropriate resistance genes) in host cultivars adapted to different rice agro-ecosystems in sub-Saharan Africa. Sixty four lines of an inter-specific CSSL population and the parent cultivars O. glaberrima MG12 and O. sativa Caiapo were phenotyped for resistance to S. hermonthica. MG12 showed good resistance to S. hermonthica whilst Caiapo was very susceptible. The CSSLs showed a range of susceptibility to the parasite, however, only two CSSLs showed the same strong resistance phenotype as MG12. Graphical genotyping and a Quantitative Trait Loci (QTL) analysis revealed a large QTL on chromosome 12 (designated STR12.1) which explained at least 80 % of the variation for resistance in the population and suggests that resistance to S. hermonthica (in MG12) is due to one (or a few genes) of major effect. This finding opens the way for the identification of candidate Striga resistance genes (through fine mapping approaches) and their transfer to farmer-preferred cultivars via marker assisted breeding.

Field ecology of western flower thrips, Frankliniella occidentalis in french bean agroecosystems in Kenya

Author: Ounya, Johnson Nyasani

Awarding University: Leibniz Universitat Hannover, Germany

Level : PhD

Year: 2012

Holding Libraries: Kenya Agricultural Research Institute Headquarters Library ;

Subject Terms: Western Flower Thrips ; Beans ; Phaseolus vulgaris ; Frankliniella occidentalis ; Ecology ; Integrated pest management ; Cucurbita pepo ; Megalurothrips sjostedti ; Thrips ; Weeds ; Galinsoga parviflora ;

Abstract:

Western flower thrips (WFT), [Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae)], is one of the most important pests of French beans (Phaseolus vulgaris L.) in Kenya. Control of WFT is difficult because it has a wide host range, high reproduction rate, cryptic feeding habit, and ability to pupate in soil. Development of sustainable integrated pest management (lPM) strategies against the WFT requires a sound understanding of its field ecology in terms of colonisation pattern, seasonal abundance, and feeding and oviposition behaviour. An understanding of seasonal abundance of WFT is important in predicting when and where economically damaging populations may occur, understanding how crop damage occurs, planning efficient sampling protocols, and in developing effective management programmes that are area specific. Information on feeding and oviposition preference of WFT is a key research need for formulation of IPM strategies based on manipulation of cropping systems. However, the above named aspects have not been studied in details within French bean fields in Kenya. Therefore, the objectives of this study were to determine (1) seasonal abundance ofWFT and its natural enemies in French bean agroecosystems in Kenya, (2) the effect of intercrops on thrips species composition and population density on French beans, and (3) feeding and oviposition preference of WFT for crops and weeds encountered in French bean fields in Kenya. To determine the seasonal abundance of WFT and its natural enemies in field-grown French beans in Kenya, Field studies were conducted in two major French bean agroecological zones in Kenya from January 2009 to December 2009. French beans were sampled every two weeks for WFT and natural enemies. Colonisation of French beans with WFT in both small and large scale farms in high and mid altitude zones started at 2- and 3-leaf stage, respectively. There was an increase in the number of WFT from budding stage to podding/flowering stage. A decline in population density of WFT was at crop senescence. Two natural enemies of thrips, [Orius spp. (Hemiptera: Anthocoridae) and Ceranisus menes (Walker) (Hymenoptera: Eulophidae)], were recorded on French beans in all agroecological zones and their population grew in tandem with the population of WFT. Temperature and relative humidity were weakly correlated with the population density of WFT, while rainfall had a negative effect on the population density of WFT. Overall, the population density of WFT was least in the first growing season (January - April) which was in the long rains season. Higher population densities of WFT on French beans in all farm sizes and agroecological zones were recorded in the third growing season (September - December) which was in the short rains season. Results from this study suggest that seasonal abundance of WFT in the two agroecological zones is influenced by rainfall (depending on amount), phenological stage of French beans and surrounding host plants (where infestations on French beans arise from). To study thrips species composition and thrips population density on French beans planted as a sole crop and as an intercrop with either sunflower, Irish potato, or baby com, in different combinations field experiments were conducted in two seasons. French beans hosted four thrips species, Megalurothrips sjostedti (Trybom), Frankliniella schultzei (Trybom),F. occidentalis (Pergande), and Hydatothrips adolfifriderici (Kamy) in order of decreasing abundance. The main thrips species on Irish potato and sunflower was F. schultzei. Baby com hosted only Frankliniella williamsi (Hood) and Thrips pusillus (Bagnall). A mono crop of French bean hosted more thrips than a French bean intercrop mix. Plots with French bean alone had about 1.4 times higher yields compared to intercropped plots of French bean with sunflower and French bean with baby com. However, the percentage of pods that could get rejected on the market due to thrips d

Management options addressing soil fertility decline and weed infestation in smallholder maize production systems of Western?Kenkya

Author: Ngome, Ajebesone Francis

Awarding University: University of Bonn, Germany

Level : PhD

Year: 2011

Holding Libraries: ;

Subject Terms: Soil fertility ; Weeds ; Maize ; Western Kenya ; Management ;

Abstract:

ABSTRACT NOT AVAILABLE

Use of legumes and lime to improve soil fertility and control sheep sorrel (Rumex acetosella L.) weed in potatoes (Solanum tuberosum L.) in Timboroa, Kenya

Author: Kiiya, Wycliffe Wabwille

Awarding University: Egerton University, Kenya

Level : PhD

Year: 2009

Holding Libraries: Kenya Agricultural Research Institute Headquarters Library ;

Subject Terms: Lime/Soil fertility/Legumes/Sheep sorrel USE Rumex acetosella/Rumex acetosella/Solanum tuberosum/Lupins/Lupinus alba/Purple vetch/Vicia benghalensis/Pisum sativum/Weeds/ ;

Abstract:

Low soil pH and infertility exacerbated by continuous cultivation without adequate replenishment of mined nutrients, coupled with total harvest of crop residues as livestock feeds, have led to the spread of weeds in the North Rift, Kenya. This has resulted in low potato (Solanum tuberosum L.) yield averaging 7t as opposed to the potential of 30 t ha-1 or more. A focused farm survey, four field experiments and a greenhouse study were undertaken at Timboroa from long rains 2002 to short rains 2004 to determine the influence of household and farm characteristics on soil fertility, weed types and their distribution, to screen and identify suitable legumes for growing in the region for use as green manures / smother crops in the control of weeds, with special emphasis of sheep sorrel (Rumex acetosella L.) weed. The effectiveness of lime applied at 0 or 6 t ha-1 alone or combined with CAN applied on potatoes at three rates (0, 60 or 120 kg N ha-1) in controlling sheep sorrel weed was also investigated. Phosphorus was applied at potato planting stage as TSP at a uniform rate of 90 kg P ha-1. The survey and soil analysis confirmed that soils in the region were acidic with pH (H20) of 4.3 and were deficient in P, Ca and Mg. The survey also showed that inadequate quantities of farmyard manure was used on crops because few livestock were kept. In the screening experiment, lupine (Lupinus albus L.) and purple vetch (Vicia benghalensis L.) gave significantly (p<0.05) higher ground cover and biomass than other legumes and were identified as the best bets for the region. Soil incorporation of lupine and garden pea (Pisum sativum L.) significantly (p<0.05) reduced sheep sorrel weed biomass by 18.0% and 9.0%, respectively while application of 120 kg N ha-1 together with incorporation of lupine reduced sheep sorrel weed density equivalent to hand weeding twice. Potato yields increased with N application only up to 60 kg N ha-1 but continued increasing in limed plots suggesting availability of more nutrients such as P and Ca with reduced soil acidity. The lack of significant potato yield increase beyond 60 kg N ha-1 when only N was applied was attributed to more potato vegetative growth at the expense of tuber production. Liming alone significantly (p<0.05) reduced sheep sorrel weed density and biomass resulting in increased potato yields. The study established that wild radish (Rhaphanus raphanistrum L.) weed density and biomass increased with liming, suggesting that the weed should not be allowed to form seed if lime is to be adopted a strategy for controlling sheep sorrel weed.

Crop-to-wild geneflow : environmental risk assessment for the release of genetically modified sorghum in Kenya

Author: Mutegi, Evans

Awarding University: University of the Free State, South Africa

Level : PhD

Year: 2009

Holding Libraries: Kenya Agricultural Research Institute Headquarters Library ;

Subject Terms: Sorghum bicolor ; Genetics ; Weeds ;

Abstract:

The important socio-economic position enjoyed by sorghum has made it a target for genetic modification to enhance productivity and/or nutritional quality. However, there are growing environmental concerns that through geneflow, transgenes might escape from genetically modified (GM) sorghum to its sexually compatible wild and/or weedy relatives, with possible negative effects. Characterising the levels and dynamics of gene flow in the wild-weedy-domesticate complex of sorghum in traditional agroecosystems of Africa, including Kenya, is of interest to biosafety regulators. This study used approaches based on population genetics theory to (i) characterise the genetic structure of the wild-weedy-domesticate complex of S. bicolor at a country scale in Kenya and (ii) to estimate the extent and direction of geneflow between cultivated and wild-weedy populations at a local scale in the country. The structure and dynamics of diversity was first estimated and compared at country scale by genotyping 329 cultivated and 110 wild sorghum individuals using 24 microsatellite markers. Subsequently, the magnitude and direction of geneflow between the two congeners was estimated at a local scale by analysing 483 individuals comprising of 15 wild-weedy and 12 cultivated sorghum populations using 10 microsatellite markers. Overall, cultivated sorghum harboured lower diversity than its wild counterpart. Levels of genetic diversity in cultivated and wild sorghum differed significantly among regions, with most of the diversity being partitioned more within than among the crop's growing regions. There were generally low levels of differentiation within and between cultivated and wild sorghum at country scale, but the extent of crop-to-wild genetic proximity varied among sorghum growing regions. These findings may reflect important historical geneflow between cultivated sorghum and its progenitor, with the level of crop-to-wild genetic exchange varying among regions. At local scale, the extent of gene flow was approximately nine times higher from cultivated sorghum to its wild-weedy relatives than vice versa. Additionally, the extent of crop-to-wild geneflow varied significantly among farms. Overall, this study suggests that deployment of GM sorghum in Kenya's agroecosystems will most likely lead to movement of trans genes into sympatric populations of its wild-weedy relatives. Keywords: autocorrelation, diversity, genetic structure, introgression, phylogeography, Sorghum bicolor, spatial analysis, wild-weedy-domesticate complex

RNA interference as a resistance mechanism against parasitic weeds

Author: Runo, Steven Maina

Awarding University: Kenyatta University, Kenya

Level : PhD

Year: 2008

Holding Libraries: Kenyatta University Moi Library ;

Subject Terms: Cuscuta pentagona/Ribonucleic acid/Parasites/Weeds/Genes ;

Abstract:

The parasitic plant dodder (Cuscuta pentagona) establishes a continuous vascular system through which water and nutrients are drawn. To discover the route of mRNA transportation, insitu RT-PCR technique was used to locally amplifiy host transcript within parasitic tissue. The stability of host mRNA molecules was also checked by monitoring specific transcripts along the growing dodder thread. Four mRNAs, B and C subunits of PPi-DEPENDENT PHOSPHOFR UCTOKINASE (LePFP), the small subunit of Ribulose-1,5- bisphosphate carboxylase/oxygenase (RubisCO), and GIBBERELLIC ACID INSENSITIVE (LeGAI), were found to move from host to dodder. LePFP mRNA was localized to the dodder parenchyma cells and to the phloem. LePFP transcripts were found in the growing dodder stem up to 30 cm from the tomato-dodder connection. Long distance RNA movement from host plants into parasites presents an opportunity to control parasitic plants by transforming the host with RNA interference (RNAi) vectors that target the parasite's essential genes. RNAi plants against Striga and dodder respectively have been developed using KNOX] (KNOTTEDI-like homeobox) genes required for meristem maintenance and proper leaf patterning as targets for RNAi. Transgenic tobacco plants (Nicotiana tabacum) were exposed to dodder 4 weeks after transplanting to pots. Two transgenic lines showed irregular infection appearance. Dodder formed haustoria in a single spot along the plant stem, and grew from that spot extensively without forming haustoria in other spots. In addition, six weeks old Nicotiana benthamiana plants were infiltrated with Agrabacterium harboring dodder KNOX RNAi construct. As a control, N. benthamiana plants were infiltrated with Agrabacterium harboring the GREEN FLUORECSCENT PROTEIN (GFP) reporter gene construct. After 3 days, when the GFP plants showed expression in leaves, the plants were exposed to dodder. Dodder infection was evaluated after about 3 weeks. Plants that were infiltrated with Agrobacterium harboring dodder KNOX RNAi construct showed partial resistance to dodder with a phenotype similar to the one observed on the stably transformed lines while GFP plants showed normal infection. Although genetic engineering offers the prospects of improving the maize germplasm for resistance against Striga, maize is recalcitrant to transformation and time consuming. A protocol to rapidly establish transformed hairy roots in maize using Agrobacterium rhizogenes is described. Two tropical maize lines (CML 216 and Katumani) and one temperate line (A188) were used in the generation of transformed maize roots. Using a pTF102 binary and pMDC plasmid containing the GUS and GFP reporter genes respectively, transformation frequencies could be estimated. Transformation frequencies were found to be maize genotype specific. Transformation frequencies of 88% were obtained from maize lines- CML 216, 57% and 47 % from A188 and Katumani respectively using the A. rhizogenes strain K599. The transformed roots exhibited a hairy root phenotype that is the hallmark of A. rhizogenes mediated transformation. Multiplex PCR analyses on roots that showed GUS activity confirmed that all transformed roots were positive for the expected GUS fragment. The methodology developed is suitable for application to functional genomic projects for genes whose phenotypes manifest in the roots representing a significant advantage over existing transformation protocols which rely on expensive and time consuming methods.

Ex-ante economic evaluation of the herbicide coated maize in the control of striga in Western Kenya.

Author: Ngare, Lucy Wangare

Awarding University: University of Nairobi, Kenya

Level : MSc

Year: 2005

Holding Libraries: University of Nairobi Upper Kabete Library ;

Subject Terms: Crops ; Maize ; Crop diseases ; Pest control ; Striga ; Herbicides ; Weeds ;

Abstract:

ABSTRACT NOT AVAILABLE

Effects of planting density and weeding regimes on forage and grain yield of maize

Author: Musembi, Benjamin Kivuva

Awarding University: University of Nairobi, Kenya

Level : MSc

Year: 2005

Holding Libraries: Kenya Agricultural Research Institute Muguga Library ;

Subject Terms: Maize ; Zea mays ; Weeds ; Planting ;

Abstract:

Maize is a major food and forage crop in Kenya and planting density and weeding regime influence yield. The effects of weeding regimes and maize planting density on maize forage, grain yield and quality were evaluated during the 200112 short rains and long rains of 2002 at Kenya Agricultural Research Insitute, Muguga in Central Kenya. Weeding regimes were weed free (WI), weedy (W2), herbicide (W3) and hand weeding twice (W4). Maize densities were 9 (DI) and 18 plants m-2 (02) intercropped with beans. Maize was thinned at 98 OAE at tasseling stage and assessed for forage yield and quality. Stover and edible weeds biomass yield and quality were also assessed. Soil moisture content down the profile, PAR interception, weed density, maize height and rate of tasseling was determined gradually over the season. Maize yield and bean biomass was also determined. The collected data was analysed using GENSTAT software and their means separated with LSD at P = 0.05. Percent interception of photosynthetically active radiation (PAR) was significantly higher in 02 than in DI before thinning time but was significantly higher in DI than in 02 after thinning in both seasons. Interception of PAR was significantly higher in W2 compared to WI, W3 and W4, which were similar in both experiments. Soil moisture content was significantly lower in W2 but similar in WI, W3 and W4. 02 had significantly lower soil moisture content than DI in season two through out the season. Thinnings biomass was higher where weeds were controlled and least in the weedy regime in both seasons. Thinnings biomass was significantly higher in 02 than D I in both seasons and D I had significantly higher maize grain yield than 02 in both seasons. Stover biomass was significantly higher in 0 I than 02 in season one but was similar in season two. Total forage biomass from 01 was same as in D2 in season one whereas in season two was significantly higher in D2 than D 1. Beans performed poorly due to low planting density and shading effects due to maize in both seasons. The tasseling rate was significantly lower in D2 than D 1 while W2 had significantly lowest tasseling rate as compared to WI, W3 and W 4, which had similar tasseling rate in both seasons. Maize plants were significantly short in W2 compared to WI, W3 and W4, which were similar. They were also shorter in D2 than in Dl in both seasons. Total weed biomass at 126 DAB was significantly higher in W4 than in W3 and in Dl than in D2 in both seasons. The cattle-edible weed biomass at the end of the two seasons was at least 55 % of the total. Thinnings had significantly higher digestibility (76 %) and crude protein (7 %) than stover. Two times hand weeding (W4) was two to three times more expensive than using herbicide (W3). Weeds competed for light and water leading to reduced thinnings, stover and grain yield but increased weed biomass, which was significantly lower than maize biomass in the weeded plots. High maize density increased intraspecific-competition (of maize in the hills) for water and light before thinning time (98 DAB) and significantly reduced plant height, tasseling rate and grain yield. Hand weeding is labour intensive and thus led to increased expenses than herbicide use. Planting maize at high density significantly increased forage quality and quantity, and overall light capture especially before thinning, but reduced weed biomass and grain yield.

An ecological study with special emphasis on invasive alien plants in Meru National Park, Kenya.

Author: Lusweti, Agnes Mukaba

Awarding University: Addis Ababa University, Ethiopia

Level : MSc

Year: 2004

Holding Libraries: National Museums of Kenya Library ;

Subject Terms: Ecology ; Flowers and plants ; Weeds ; Meru National Park, Kenya ;

Abstract:

In this study, the vegetation of Meru National Park, disturbances and Invasive Alien Plants were assessed. 383 species belonging to 65 families were recorded. Using classification and clustering techniques, at 0 9 dissimilarity ratio, four stable vegetation communities were delimited as; Acacia and ('omhretz~m wooded grasslands, Riverine vegetation, and Acacia-Commiphora bushland. After the rain season, a fifth community type, the Ground water forest,segregated from the C'ombretz~m wooded grassland community. Environmental characteristicsand soil chemical and physical factors were evaluated to explain vegetation variation.ANOVA, (P<0.05) indicated that all soil properties assessed were significantly different for all vegetation communities in both seasons Using Duncan's multiple range tests, it was established that TOC % and soil texture were the main segregating factors. Herbivory was the most prevalent form of disturbance. Levels of disturbance were higher in the dry season, and percentage bare area estimated was above 50%, compared to the wet season. Thirty-six (36)IAPs were recorded in the park, mostly near water bodies and in low densities. To determine vulnerability of the vegetation communities to invasion, Spearman's correlation coefficient r ,(rho) was calculated for the number and coverlabundance of I N S and diversity and coverlabundance of native species, disturbance level, bare area and altitude. Although favorable growing conditions seemed to play an important role, each community responded differently in both seasons. For MNP, an invasion is not imminent given the limited diversity and density of IAPs, unfavorable growing conditions most of the year and selection through herbivory Still, it is recommended to study the vegetation of Meru National park, effects of herbivores and outline the management of IAPs in the park's management plan.

Ascertaing metabolic pathway sites in Orobanche aegyptiaca that can be inhibited using anti-metabolites in vitro

Author: Mgutu, Allan Jalemba

Awarding University: Kenyatta University, Kenya

Level : MSc

Year: 2004

Holding Libraries: Kenyatta University Moi Library ;

Subject Terms: Weeds ; Weed control ; Striga ; Orobanche aegyptiaca ; Biochemistry ;

Abstract:

Some parasitic angiosperms cause considerable damage to food and fodder plants resulting in very low yield or none at all. The pernicious root parasites Striga and Orobanche are among the most devastating weeds and are not effectively controlled due to various reasons. The principal interest of this study was the biochemical understanding of the growth and development processes of the weeds. In this study, biochemical pathways of Orobanche aegyptiaca that can be inhibited with anti-metabolites were investigated using in vitro experimental systems. In vitro cultures of Orobanche aegyptiaca were established both on solid and liquid media with potential use in bioassays and biochemical studies. A modified MS medium composition was adopted as most suitable and consisted of a 10-fold reduction in ammonium nitrate. The medium was further supplemented with 2 mg/L 2,4-dichlorophenoxy acetic acid and 0.1 mg/L kinetin as plant growth regulators and standard MS vitamins. The effect of anti metabolites inhibiting amino acid, fatty acid, carotenoid, cellulose and folate biosyntheses, photosynthesis and tubulin polymerization were determined by measuring the growth of the in vitro tissues and elongating radicles. Branched chain amino acid biosynthesis was the most sensitive pathway to inhibition among all the investigated biosynthesis pathways. Glutamine biosynthesis and/or ammonium assimilation was equally sensitive to inhibition in the in vitro tissues. The significance of amino acid biosyntheses in Orobanche aegyptiaca was shown when the activities of the specific enzymes were also found to be quite sensitive to inhibition by the same anti-metabolites. Acetolactate synthase in branched chain amino acid biosynthesis was effectively inhibited by all the ALS targeting anti-metabolites used i.e. chlorsulfuron (IC5o 1.0 nM), imazapyr (IC5o 0.88?M) and imazaquin (IC5o 1.97 ?M). Despite the common inhibitory activity, imazaquin was completely ineffective on the radicles. Glutamine synthetase was sensitive to glufosinate when partially purified (IC5o 9.1 ?M). Cellulose biosynthesis was sensitive to inhibition by dichlobenil in callus tissues (IC5o0.1 nM) and elongating radicles (IC5o 10 nM) only. Cell suspensions seemed to have developed the ability to grow without real cell walls. Inhibitors of microtubulin polymerization were only effective in inhibiting growth of callus tissues. Folate biosynthesis was mildly sensitive to inhibition by sulfadiazine and asulam possibly due to a competent folate recycling system and/or sufficient stored reserves. Inhibition of fatty acid, carotenoid and plastoquinone/tocopherol biosyntheses, and photosynthesis, had little effect on growth of the tissues.