Thesis (MScAgric)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The potato can be considered as one of the most important food crops in many African countries. The potential of this crop is reflected in the large increase in area of production where Africa showed the highest rate of growth within the developing world over the past twenty years. The multiplication rate of potatoes is very low compared to other crops. Therefore it is essential to investigate methods of increasing the number of minitubers produced from disease free in-vitro plantlets. There is a number of potato propagation procedures that are currently being used worldwide to multiply seed potatoes. As aeroponics is still a relatively new technique that has not been researched extensively for the production of minitubers, a study relating to the production practises including the nutritional requirements of potato minitubers produced in a aeroponic system were undertaken. Potato plantlets, cv BP1, were grown aeroponically at two different densities (20 and 30 plants/m2) and four harvest intervals (7, 10, 14 and 18 days). The interaction between harvesting intervals and plant densities did not influence plant growth, minituber quality or yield. Best results were realised when harvesting every 7 days with a higher total tuber number over the growing period. Harvest interval also influenced the phosphorus and copper concentration in minitubers. To study the effect of Calcium (Ca) application rate, potato plantlets of cultivars Up-to-date, Mnandi, Buffelspoort and BP1 were grown at four different Ca levels (8.40, 6.75, 5.10 and 3.45 meq/L). The interaction between Ca application levels and cultivars significantly influenced the percentage stolon branching. BP1 had more stolons at the lowest Ca application level and Buffelspoort had more stolons at the full Ca application levels. However, low Ca treatments produced the highest yield. The minituber number and weight harvested were three times more for Mnandi. An aeroponic study on the irrigation frequency (20, 30, 40 and 50 minutes interval) was conducted on four potato cultivars (Up-to-date, Mnandi, Buffelsoort and BP1). Significant differences were noted in the interaction between irrigation frequencies and cultivars for the percentage tuberised plants and stolon and tuber dry mass. When irrigated every 40 minutes, 48% of the Buffelspoort plants produced tubers. Plant height was also significantly affected by the interaction between irrigation frequencies and potato cultivars, with Mnandi producing taller plants when irrigated every 30 minutes. Total tuber number and tuber fresh and dry weight was higher at the irrigation frequency of 20 minutes. The interaction between irrigation frequencies and cultivars on the response to macro and trace elements was not significant for sodium and iron but was for phosphorus, potassium, calcium, zinc and aluminium. A field study was conducted in a greenhouse where potato seed of BP1 obtained from the first trial were graded into different sizes (Small: >20, medium: 20–40 and large: > 40 mm of diameter) and stored at 3 different temperatures (3, 16 and 25oC) for 2 supplementary months before being planted. Sprouting capacity was mostly influenced by temperature regardless of other factors applied to potato seed minitubers such as harvest intervals and sizes. The higher storage temperature of 25oC resulted in tubers with a higher number of sprouts, longer sprouts and with a higher sprouting capacity. / AFRIKAANSE OPSOMMING: Aartappels is een van die belangrikste voedselgewasse in baie lande in Afrika. Die potensiaal van die gewas word gereflekteer in die groot toename in produksie areas, met Afrika wat die vinnigste van al die ontwikkelende lande gegroei het die laaste 20. In vergelyking met ander gewasse is die tempo van planvermeerdering by aartappels baie stadig. Dit is dus essensieel om metodes te ondersoek wat sal help om die aantal miniknolle wat per in vitro plantjie verkry kan word te verhoog. Daar is heelparty plant vermeerderings prosedures wat tans wêreldwyd gebruik word om saad aartappels te vermeerder. Aangesien aeroponika nog steeds ʼn relatiewe nuwe tegniek is wat nog nie ekstensief ondersoek is vir die verbouing van miniknolle nie, is ‘n studie geloods om te kyk na die produksie praktyke, wat insluit die voedingsbehoeftes van aartappel miniknolle in ʼn aeroponika sisteem. Aartappel plantjies, kultivar, BP1, is aeroponies verbou by twee plant digthede (20 en 30 plante/m2) en vier oesintervalle (7, 10, 14 en 18 dae). Die interaksie tussen oesintervalle en plantdigtheid het geen effek gehad op plant groei, miniknol kwaliteit of opbrengs nie. Die beste resultate is verkry waar die knolle elke 7 dae geoes is met ‘n hoër totale aantal knolle oor die groeiseisoen. Die oesinterval het ook ‘n effek gehad op die fosfaat en koper konsentrasie van die miniknolle. Om die effek van die Kalsium (Ca) toedieningspeil te ondersoek is aartappel plantjies; kultivars Up-to-date, Mnandi, Buffelspoort en BP1 gekweek by vier verskillende Ca peile (8.40, 6.75, 5.10 en 3.45 meq/L). Die interaksie tussen Ca toedienings peile en kultivars het ‘n beduidende effek gehad op die persentasie stolon vertakking. BP1 het meer stolons gehad by die laagste Ca toedieningspeil en Buffelspoort het meer stolons gehad by die volle Ca toedieningspeil. Die hoogste opbrengste is egter waargeneem by die laagste Ca toedieningspeil. Die aantal miniknolle en oes massa was drie keer meer vir Mnandi. ‘n Aeroponiese studie op die besproeiingsfrekwensie (20, 30, 40 en 50 minuut intervalle) is gedoen met vier aartappel kultivars (Up-to-date, Mnandi, Buffelsoort en BP1). Beduidende verskille is opgemerk in die interaksie tussen besproeiings frekwensie en kultivars vir die persentasie plante met knolle en stolon en knol droë massa. Met besproeiings elke 40 minute het 48% van die Buffelspoort plante knolle produseer. Plant hoogte is ook beduidend beïnvloed deur die interaksie tussen besproeiingsfrekwensie en aartappel kultivar met Mnandi plante wat hoër was wanneer dit elke 30 minute besproei is. Die totale aantal knolle en knol vars- en droë massa was hoër wanneer daar elke 20 minute besproei is. Die interaksie tussen besproeiings frekwensie en kultivars op die makro- en mikro element inhoud van die knolle was nie beduidend vir natrium en yster nie, maar wel vir fosfaat, kalium, kalsium, sink en aluminium. ‘n Potproef is gedoen in ‘n kweekhuis waar aartappel saad van BP1 verkry vanaf die eerste proef nadat knolle verdeel is in verskillende grootte klasse (klein: < 20mm, medium: 20-40mm en groot: >40mm) en gestoor is by drie verskillende temperature (3, 16 en 25oC) vir 2 addisionele maande voor plant. Spruit ontwikkelings kapasiteit was meestal beïnvloed deur temperatuur ten spyte van ander behandelings soos oes intervalle en knol grootte. Die hoër bergings temperatuur 25oC het aanleiding gegee tot knolle met ‘n hoër aantal spruite, langer spruite en ‘n hoër spruit ontwikkelings kapasiteit.

The variation for embryo production in anther culture of Solanum phureja was examined as a function of maximum greenhouse temperature prior to bud harvest and innate responsiveness among anthers within a bud. S. phureja clones PP5, AD2-4, A3P2-6 and AD3-4 were grown in a greenhouse under a 16 h photoperiod. The temperature was monitored continuously using a thermograph. Buds were collected from PPS and AD2-4 and the anthers were cultured in two groups of five flasks. In the first group, each flask contained the 30 anthers from 6 buds; the second group, each flask contained 1 anther from each of 30 buds -- a total of 30 anthers per flask. Significantly smaller coefficients of variation were observed for the second group, suggesting that the variation for embryogenic capacity among buds was greater than that among anthers within a bud. Variation in embryo yield as a function of greenhouse temperature for clones A3P2-6 and AD3-4 was examined by stepwise regression analysis. Embryogenic capacity of clone A3P2-6 was adversely affected by high temperatures (31-37°C) that occurred 2 and 7 days before bud harvest. However similarly high temperatures appeared to enhance the androgenic response of clone AD3-4. Regeneration rate of anther-derived embryos over three subcultures to fresh regeneration medium was examined as a function of anther donor or clone, cold pretreatment of embryos, and morphological classification of embryos. Only clonal origin significantly affected regeneration. Regeneration rate declined on each serial subculture. The frequency of regenerable embryos varied from 12.5% for clone BARD 1-3 to 46.0% for clone A3P2-6. Flow cytometric analyses were performed on several anther-derived monoploids of S. phureja to examine the frequency of nuclei at the 1x, 2x, and 4x levels within and among clones. Significant variation was found among duplicate cultures of the individual clones, but this variation was small enough to allow the detection of significant differences among the clones. Monoploid cell frequency ranged from 22.3% to 35.7%. Diploid cell frequency ranged from 48.6% to 59.9%. Tetraploid cell frequency ranged from 11.9% to 25.3%. Several families of anther-derived monoploid clones of S. phureja were analyzed for differences among clones within a family and among families. Significant differences were found in both categories. Finally, unstained protoplasts of monoploid S. phureja clone AM3 were sorted based on forward angle light scatter (FALS) and autofluorescence. Fractions selected for low FALS and weak autofluorescence appeared to be selectively enriched for monoploid protoplasts. / Master of Science

Thesis (M. A. Agriculture (Plant Protection)) -- University of Limpopo, 2019 / Available potato (Solanum tuberosum L.) cultivars do not have any genotype that is resistant to the root-knot (Meloidogyne species) nematodes. Due to the susceptibility of potato cultivars to Meloidogyne species, alternative management strategies had to be researched and developed after the withdrawal of methyl bromide from the agro-chemical markets, amongst which were the cucurbitacin-containing phytonematicides. However, of the available application methods of phytonematicides, the ground leaching technology (GLT) and botinemagation technology were not suitable for use in most high-rainfall potato-producing regions, where production is under rain-fed conditions. The objective of the study, therefore, was to determine whether pre- and post-emergent application of Nemafric-BG phytonematicide would have effects on growth of potato and suppression of M. javanica population densities. Parallel pot trials of pre- and post-emergent application of Nemafric-BL phytonematicide were conducted under greenhouse conditions in autumn (February-April: Experiment 1) 2017 and validated (Experiment 2) in 2018. Each plant was inoculated with 3000 M. javanica eggs and second-stage juveniles (J2). Five treatments, namely, 0, 2, 4, 8 and 16 g concentration of Nemafric-BG phytonematicide, arranged in randomised complete block design, were either applied mixed with seed tubers for pre-emergent or spread on the soil surface after emergence for post-emergent trials. In all cases, plant growth variables were assessed using the Curve-fitting Allelochemical Response Data (CARD) model, whereas nutrient elements (Fe, K, Na and Zn) and nematode variables were assessed using analysis of variance, with data subjected to lines of the best fit. In pre-emergent application trial, plant height (R2 = 0.98) and fresh root mass (R2 = 0.99) exhibited quadratic relations, characterised by density dependent growth patterns with increasing concentrations of Nemafric-BG xv phytonematicide in Experiment 1, similar trends were also observed on plant height (R2 = 0.99) and root mass (R2 = 0.99) in Experiment 2. In contrast, in post-emergent application trial, plant height (R2 = 0.97), fresh root mass (R2 = 0.99) and dry shoot (R2 = 0.98) exhibited quadratic relations in Experiment 1, which ascribed to DDG patterns, similar trends were also observed in Experiment 2 on plant height (R2 = 0.99), fresh root mass (R2 = 0.96) and dry shoot (R2 = 0.99) of potato cv. ꞌMondial G3ꞌ. In pre-emergent application trials, Mean Concentration Stimulation Point (MCSP) = 24.18 and 7.82 g, respectively, in Experiment 1 and Experiment 2, with ∑k being equivalent to 20 and 6 units for potato to the product, respectively, in Experiment 1 and Experiment 2. In contrast, post-emergent application trials, MCSP = 9.87 and 12.10 g, respectively, in Experiment 1 and Experiment 2, whereas the ∑k value for potato to the product was 11 and 6 units, respectively in Experiment 1 and Experiment 2. Increasing concentrations of the phytonematicide significantly (P ≤ 0.05) affected the selected nutrient elements. In pre emergent application trials, K (R2  =  0.96) Na (R2  =  0.90) and Zn (R2  =  0.83) each with increasing Nemafric-BG phytonematicide concentrations exhibited positive quadratic fashion, while Fe (R2  =  0.87) exhibited negative quadratic relations in Experiment 1. In Experiment 2, K (R2  = 0.99), Na (R2  = 0.90) and Zn (R2 =  0.97) contents each in leaf tissues against the increasing concentrations of the phytonematicide exhibited negative quadratic relations, while Fe (R2  = 0.88) exhibited positive quadratic relations. In post emergent trials, Fe (R2 = 0.91, Na (R2 = 0.90) and Zn (R2 = 0.99) contents in leaf tissues against increasing Nemafric-BG phytonematicide concentration exhibited negative quadratic relations, whereas K (R2 = 0.86) exhibited positive quadratic relation in Experiment 1. In Experiment 2, Fe (R2 = 0.93), K (R2 = 0.92), Na ( R2  = 0.79) and Zn (R2 xvi = 0.89) contents in leaf tissues of potato exhibited positive quadratic, respectively. In pre emergent trial for Experiment 1, eggs in roots (R2 = 0.78), J2 in roots (R2 = 0.85), J2 in soil (R2 = 0.97) and Pf (R2 = 0.78) of M. javanica against increasing pre-emergent application concentrations of Nemafric-BG phytonematicide exhibited negative quadratic relations, characterised by DDG patterns. Similar trends were observed on eggs in roots (R2 = 0.82), J2 in roots (R2 = 0.99), J2 in soil (R2 = 0.84) and Pf (R2 = 0.85) in Experiment 2. In contrast, in post-emergent application trial, eggs in roots (R2 = 0.87), J2 in roots (R2 = 0.99), J2 in soil (R2 = 0.91) and Pf (R2 = 0.99) of M. javanica against increasing post emergent application concentrations of Nemafric-BG phytonematicide also exhibited negative quadratic relations in Experiment 1, which ascribed to DDG patterns. Similar trends were also observed on eggs in roots (R2 = 0.72), J2 in roots (R2 = 0.68), J2 in soil (R2 = 0.85) and Pf (R2 = 0.83) in Experiment 2. Results from the study demonstrated that Nemafric-BG phytonematicide stimulated plant growth at lower concentration and the product does not have any detrimental effects in accumulation of nutrient elements in leaf tissues. Therefore, it is concluded, that the product could be applied at the recommended rates of 7.82 and 9.87 g/plant in pre and post-emergent application, respectively, for the management of root-knot nematodes, provided the active ingredient does not accumulate in potato tubers or have any detrimental effects in accumulation of nutrient elements in tubers and temper with nutritional value of potatoes.

Thesis. (M. A. Agriculture (Agricultural Economics)) -- University of Limpopo, 2022 / Food security, poverty and hunger issues, as well as methods of addressing remain a concern for many South Africans. Smallholder farmers' agricultural production is seen as the key to simultaneously alleviating poverty and ensuring food security, especially in rural areas. The sweet potato crop is commonly produced by smallholder farmers in rural areas as a staple in many South African households with the potential to reduce hunger and poverty. Nevertheless, just like other crops, the sweet potato is impaired by external factors such as extreme weather conditions, insects, pests and diseases, thus threatening food security. The most destructive pest to sweet potatoes acknowledged in the literature is the sweet potato weevil (SPW), which can cause between 5-100% in areas where it is not controlled. While there are many SPW control measures Entomopathogenic Nematodes (EPNs) are emerging as one of the Integrated Pest Management (IPM) bio-control techniques that have shown promise in controlling SPW infestations in South Africa and globally. This study conducts a socio-economic analysis of smallholder sweet potato production and analyses the acceptability of EPNs as bio-control measures against the SPW in the Gauteng, Limpopo and North West Provinces of South Africa. This was done through an assessment of farmers‘ knowledge, attitudes, perception and practices (KAPP analysis), exploration of the acceptability of EPNs by farmers, determination of and factors influencing profitability and technical efficiency. Primary data was collected from 119 respondents who were selected through non-probability sampling techniques; purposive, census, and snowball. The analytical tools used to analyse the data were descriptive statistics, Gross Margin Analysis, Multiple linear regression model, Data Envelopment Analysis (DEA) and the Tobit regression model. From the results, an average knowledge score of 2.30 based on a 3–point Likert scale revealed that sweet potato farmers are knowledgeable of the SPW, the impacts and the control measures. Despite this level of knowledge, the farmers were impartial about the attitudes and perceptions regarding the SPW and the control measures. This was based on the findings of a 5-point Likert scale, which yielded average scores of 2.53 and 2.74, respectively. The study also revealed that the majority of the farmers prefer the use of indigenous and physical practices to control SPW. With regards to acceptance of the EPNs bio-control innovation towards control of the SPW, a mean Composite Index of Acceptancy (CIA) of 0.77 revealed the willingness of farmers to accept the EPNs as a bio-control measure. A Gross margin of R9 552.37 indicates that sweet potato farming is generally profitable, and this is influenced by socio-economic factors such as marital status, employment status, sweet potato output per cycle and access to machinery. On the other hand, while sweet potato farming was found to be profitable, the DEA score of 0.09 reveals that these farmers are technically inefficient. Their technical inefficiency is influenced by sweet potato output per cycle, gross margins, farm size, and access to credit, employment status, and chemical use. Based on these findings, the study recommends farmers‘ support through capacity development initiatives for the sweet potato farmers with regards to general economics of sweet potato production and marketing to maximise and sustain their revenue generation, as well as their general efficiency. In addition, increased training and awareness of the EPNs and their benefits as bio-control measures towards SPW infestation will work towards changing farmers‘ mindset with regard to SPW control measures. / Department of Social Innovation (DSI) and United States Agency for International Development (USAID)

Thesis (M.Sc. (Horticulture)) --University of Limpopo, 2022 / Worldwide, water scarcity, land degradation and unavailability of agricultural lands have posed serious threats on food security and income generation. Hunger, micronutrient deficiencies and starvation are likely to go up in Africa due to increasing urbanisation and rise in global food prices. As a result, it is important to identify innovative vegetable growing-based strategies, which can increase access to food by utilising the limited space available in rural and urban areas. Therefore, sack gardening technology could be a solution to both the lack of arable land and water scarcity in urban and rural areas. The objective of the study was to determine whether axillary bud number will have an effect on agronomic parameters and yield of sweet potato cv. 'Blesbok' produced using the sack gardening technology. Treatments, namely, 1, 2, 3, 4, and 5 number of axillary buds per stem cutting, were arranged in randomised complete block design, with 10 replicates. One bud (1) was used as a control. Different stem cuttings measuring from 10 to 30 cm long with approximately one (1) to six (6) axillary buds were collected from mature sweet potato plantation at a certified nursery from cv. 'Blesbok'. A total of eight (8) stem cuttings were planted in sacks consisting of growing media mixture of steam pasteurised loam soil and compost (3:1 v/v), 4 on top side of the sack and the remaining four on the side-holes of the sack. Insect pests were scouted and monitored weekly, while diseases were managed using spraying programmes as in commercial sweet potato production systems. Two weeks after transplanting, each plant was fertilised with 5 g nitrogen (N), phosphorus (P), and potassium (K) 2:3:2 (26) + 0.5% Zinc (Zn) + 5% sulphur (S) and 5% calcium (Ca). Number of axillary buds had significant effects on vine length, fresh shoot mass, number of shoots and dry shoot mass contributing 59%, 58%, 45% and 58% in total treatment variation (TTV), respectively, whereas axillary bud numbers had no significant effects on fresh tuberous root mass, chlorophyll content, vine stem diameter, tuberous root diameter, tuberous root length, dry tuberous root mass and number of tuberous roots. Relative to control (one bud), two (2), three (3), four (4), five (5) number of axillary buds decreased vine length, fresh shoot mass and dry shoot mass by 8-37, 30-55 and 26-55%, respectively. However, relative to control (one bud), number of axillary buds increased number of shoots by 8-17%. The quadratic relationship models were explained by 95, 92, 96 and 98% of vine length (VL), number of shoots (NS), fresh shoot mass (FSM) and dry shoot mass (DSM), respectively. In conclusion, findings of the study demonstrated that a single (1) or two (2) axillary bud(s) on stem cuttings of sweet potato CV. „Blesbok‟ showed better results for both yield and growth parameters as compared to three (3), four (4) and five (5) buds. / AgriSETA

Thesis (MBA)--Stellenbosch University, 2001. / ENGLISH ABSTRACT: This study evaluates the use of information technology as a method to reduce production risk for irrigated potatoes. Risk in agriculture is discussed under climatic, market or price and production related risk. Production related risk covers the disciplines of planting, irrigation, fertilisation, disease and pest control as well as harvesting. It is in the above mentioned disciplines that information technology could possible be applied to reduce production risk in irrigated potatoes. The nature of this study can therefor be described as the investigation of the economic justification of information technology as a method to reduce production risk in irrigated potatoes. A brief introduction to put potato production in world and South African context into perspective, is provided. In South African context the importance and position of potato production relative to other crops is discussed. To better understand the economic conditions, under which potatoes are produced in South Africa, a financial breakdown of production cost for irrigated potatoes over regions is given. It is important to understand the economics of potato production before a proper assessment of thepossible benefits of information technology can be made. Various principles and instruments involved with information technology, as it applies to potato production under irrigation are also discussed. Irrigation scheduling as a discipline in which information technology can be applied, is also discussed. It is important to understand that irrigation scheduling can be based on two underlying principles, i.e. atmospheric modelling and soil moisture measurement. Various examples of measurement instruments are discussed. Climatic based disease modelling and petiole sampling as a method to determine nutritional status in the potato plant, was also discussed. A brief discussion of computerised agriculture management software was included to conclude the discussion on the principles and instruments available for information technology in irrigated potato production. The use of fertiliser scheduling, irrigation scheduling and climatic modelling to reduce fungal diseases in potatoes, as information technology methods, are explained in greater detail. It is shown that recommended levels for various nutrients do exist and that petiole analysis as a method to determine these levels at various growth stages, can be applied successfully. It is further argued that with various methods of irrigation scheduling, proper decision making about the amount and timing of irrigation needed, can be possible. The Plant-Plus system as a method to better control Late Blight on potatoes, is discussed. The results obtained through a commercial trial proves the validity of climatic modelling as a method of information technology to reduce production risk on potatoes, specifically the risk associated with Late Blight. The existence of and results obtained through the equipment and methods previuosly mentioned, raises the question of the economic viability of information technology at farm level. To investigate the perception of growers with regard to the applicability of information technology to reduce production risk and their readiness to adopt such . -- technology, a questionnaire was sent to 40 commercial potato growers throughout South Africa. Growers were tested on their perception with regard to the use of information technology on aspects such as financial management, irrigation scheduling, fertilisation, climatic measurement for fungal modelling as well as a profile analysis of the grower. The results of the survey are tabulated and analysed. Growers were on average positive about the value of information technology and indicated that under certain conditions, they were prepared to implement such technology. Margins in commercial potato production are under severe pressure and are in fact shrinking. Case studies presented in this investigation and results from the pilot survey indicate that the use of information technology is not only important for the reduction of risk in potato production but also imperative for sustainable and profitable potato production. It can therefore be concluded that, based on the results obtained in the pilot questionnaire, a statistical valid sample would probably support the findings in the pilot study. The pilot study indicated that reliable, affordable and practical information technology, as it has been presented in this investigation, does have a place in modern day irrigated potato production. / AFRIKAANSE OPSOMMING: Hierdie studie evalueer die gebruik van informasie tegnologie as 'n metode om produksie risiko in die verbouing van aartappels onder besproeiing, te verminder. Risiko in landbou word onder klimatologiese, mark of prys en produksie verwante risiko bespreek. Produksie verwante risiko kan weer onderverdeel word in risiko wat verband hou met plant of vestiging, besproeiing, bemesting, plaag en pes beheer asook oes. Dit is dan in die bogenoemde dissiplines waar informasie tegnologie moontlik aangewend kan word om produksie verwante risiko te verminder. Die kern van hierdie ondersoek kan gevolglik saamgevat word as 'n ondersoek na die ekonomiese regverdigbaarheid van informasie tegnologie as 'n metode om produksierisiko in die verbouing van aartappels onder besproeiing, te verminder. Daar word kortliks verwys na aartappel verbouing in wêreld en Suid Afrikaanse konteks. In Suid Afrikaanse konteks is die relatiewe belangrikheid van aartappelverbouing in vergelyking met ander gewasse uitgelig. Ten einde 'n beter begrip vir die ekonomiese omstandighede waaronder aartappels in Suid Afrika verbou word, word 'n afbraak van produksiekoste oor streke verskaf Dit is belangrik om die ekonomie van aartappelproduksie te verstaan voordat 'n deeglike evaluering van die moontlike voordele wat informasie tegnologie kan inhou, onderneem kan word. Verskeie beginsels en instrumente in die aanwending van informasie tegnologie, soos van toepassing in die verbouing van aartappels onder besproeiing, word ook toegelig. Hieronder word besproeiingskedulering as 'n metode bespreek. Dit is verder belangrik om te besef dat besproeiingskedulering op hoofsaaklik twee beginsels berus naamlik atmosferiese modellering en fisiese grondvog meting. Verskeie voorbeelde van meetinstrumente word bespreek. Klimatologiese siektemodellering en petioolontledings as 'n metode om die voedingstatus van aartappelplante te bepaal, word ook bespreek. Die gebruik van bemestingskedulering, besproeiingskedulering en klimatologiese modellering vir die beheer van laatroes op aartappels, as metodes van informasie tegnologie, word in groter besonderhede toegelig. Daar is aangetoon dat aanbevole peile vir verskeie nutriënte bestaan. Deur die neem van gereelde petiool ontledings kan hierdie peile suksesvol gedurende die opeenvolgende fenologiese stadia van die aartappelplant, gehandhaaf word. Daar is verder aangevoer dat deur die gebruik van verskeie metodes van besproeiingskedulering, deeglike besluitneming rakende die hoeveelheid en tydsberekening van 'n besproeiing, moontlik is. Die Plant-Plus sisteem as 'n metode om laatroes op aartappels te beheer, is toegelig. Resultate behaal in 'n kommersiële aanplanting het die geldigheid van klimatologiese modellering as 'n metode om produksierisiko - veral die risiko geassosieer met laatroes - te verminder, onderskryf. Die bestaan van resultate en instrumente soos voorgehou in Hoofstukke 3 en 4, onderskryf of bevestig die vermoede dat informasie tegnologie wel aangewend kan word om produksierisiko in aartappelverbouing te verminder. Die vraag is egter of dit ekonomies op plaasvlak aangewend kan word. Ten einde die persepsie met betrekking tot - die toepaslikheid en gereedheid van produsente in dié verband te evalueer, is 'n loodsvraelys na 40 respondente, wat die kommersiële aartappelprodusente regdeur Suid Afrika verteenwoordig, gestuur. Respondente is ge-evalueer met betrekking tot hul persepsie rakende die gebruik van informasie tegnologie in die dissiplines van finansiële bestuur, besproeiingskedulering, nutriëntskedulering en klimatologiese modellering vir die beheer van laatroes. Respondente was oor die algemeen positief met betrekking tot die nut van informasie tegnologie en dat onder seker voorwaardes, hulle dit sal gebruik. Marges in kommersiële aartappelverbouing is onder ernstige druk en is inderdaad besig om te krimp. Gevallestudies wat in hierdie ondersoek voorgehou is, asook resultate verkry uit die vraelys, dui aan dat informasie tegnologie nie alleen belangrik is vir die vermindering van risiko in aartappelverbouing me, maar ook noodsaaklik IS vir volhoubare en winsgewende aartappelproduksie. Dit kan gevolglik gestel word dat resultate verkry uit die loodsvraelys, daarop mag dui dat 'n statisties verifieerbare steekproef moontlik kan uitwys dat betroubare, bekostigbare en praktiese informasie tegnologie, soos voorgehou in hierdie ondersoek, wel 'n bestaansreg in die moderne verbouing van aartappels onder besproeiing het.

Sweetpotato (Ipomoea batatas (L.) Lam.) is an important food crop in East Africa including Kenya. The crop incurs high yield losses in production due to biotic (insect pests and diseases) and abiotic (drought, and heat) constraints. Among abiotic constraints, drought is the most important. Prolonged periods of drought in arid and semi-arid areas of Kenya have led to reduced quantity and quality of sweetpotato storage roots and in severe cases caused total crop failure. The objectives of this study were to: 1) determine sweetpotato production system constraints and farmers’ coping strategies; 2) evaluate sweetpotato clones for yield performance and drought tolerance; 3) analyse genotype x environment interaction and stability for storage root yield of selected clones, 4) determine mechanisms of drought tolerance in sweetpotato, and 5) determine combining ability and heterosis for yield and drought tolerance traits under managed drought stress conditions. To determine the production constraints and farmers’ coping strategies, a survey was conducted in central, eastern and western Kenya. Out of 345 farmers interviewed, 60% were women, and 40% men. Farm sizes ranged from 0.4-0.8 ha, with 90% of sweetpotato cultivated on 0.2 ha or less. The main sweetpotato varieties were Vitaa, Kabonde and Bungoma and the majority of farmer’s used their own conserved planting material which was conserved by leaving them in the field after harvest. About 35% of the farmers identified weevils as the major pest, and sweetpotato virus disease (SPVD) as the major disease, while 28% of the farmers identified drought as a major constraint. The farmers used clean seed, high yielding varieties, high planting density, and manure application as the main strategies to cope with sweetpotato production constraints. Eighty four sweetpotato clones were evaluated under managed drought stress environments at KARI-Kiboko and KARI-Thika. Drought reduced the fresh weight of storage roots (FSR) (72.5%), fresh biomass weight (FB) (74.0%), marketable fresh storage root (MFSR) (80.7%), number of storage roots (NSR) (24.5%), days to permanent wilting point (DPWP) (0.3%), but seemed to increase percent root dry matter (% RDM) (-4.7%), harvest index (HI) (-2.6%), and chlorophyll content (CC) (-2.7%). Across the environments, genotypes 194555.7 (1.06), 421066 (1.05), Chingovu (0.94), 420014 (0.91), Excel (0.9), 199062.1 (0.87) and Unawazambane06-01 (0.81) gave higher FSR yields (kg plant-1) than the local checks. Genotypes Nyarmalo and Polista were among the lowest yielding in the irrigated and non-irrigated conditions. Clones W119, 441725, and Xiadla-xa-kau were the highest yielding under the drought stress conditions. The performance of 24 improved clones was evaluated in replicated trials at KARI-Thika and KARI-Kiboko using 24 sweetpotato clones grown under managed drought stress conditions for two seasons. AMMI, GGE biplots and regression analyses were conducted to determine stability of the clones. Mean FSR was significantly different (P < 0.001) in the two research sites with the environment contributing to 92.7% of the total variation, genotype 1.8%, and interactions 0.4%. AMMI and GGE biplots, and regression indicated the most stable clones to be 441725, Unawazambane06-01 and 189150.1, while Xiadla-xa-kau was the least stable. To gather more information on drought tolerance mechanisms expressed in the sweetpotato genotypes, an experiment was conducted in the greenhouse using clones expressing different levels of drought tolerance in the field. The results showed more under-developed roots (pencil roots) in the drought stressed regimes than in the irrigated regimes and in the drought susceptible genotypes. Drought tolerant genotypes produced more FSR and NSR. The number of vine branches (NVB), vine tip pubescence (VTP) and mature leaf pubescence (MLP) increased with reduction of drought stress however, drought tolerant clones had reduced NVB. Drought stressed clones had shorter basal vine length (BVL), reduced CC, and reduced leaf growth (LG). Drought stress reduced growth of vines in terms of internodes length, internodes diameter, vine length, petioles length, and leaf CC. Overall, for the first time, the study demonstrated that drought stress in the first three months after planting, leads to the proliferation of non-edible pencil roots, which do not become edible storage roots even when drought stress is removed. Therefore, water is critical during this period for improved sweetpotato storage root yield, probably as a drought tolerant mechanism. Combining ability for yield and drought tolerance of 15 F1 sweetpotato families generated through a half diallel mating of six parents was evaluated at KARI-Kiboko in 2012. Significant (P≤0.05) general combining ability (GCA) and specific combining ability (SCA) effects were recorded for root yield in both drought stress and no stress conditions, indicating that both additive and dominance gene effects were important in the inheritance of resistance to drought stress. Progenies from families G2, G5, G7, G8, G10, G12 and G15 had good SCA for fresh storage root yield, total biomass, number of days to permanent wilting point (DPWP), harvest index, and drought stress index (DSI). Progeny 8 from family G4, 5 and 8 from G15, had the highest mid and best parent heterotic effect (117-270%) for fresh storage root yield in both drought stress and no stress conditions. Progeny 6, and 7 from family G10, had the highest mid and best parent heterotic effect (165-234%) for fresh total biomass yield under no drought stress conditions. In conclusion, the progenies from families G2, G5, G7, G8, G10, G12 and G15 that had high yield and biomass specific combining ability under drought and no drought stress indicated that they could be having drought tolerant genes, and therefore could be incorporated into advanced drought screening trials with the aim of releasing the best performing drought tolerant varieties. Secondly, the findings in this study lay a foundation for sweetpotato breeding programmes on drought tolerance. Thirdly, for the first time, this study uniquely combines yield performance, combining ability estimates, days to permanent wilting point and heterosis under contrasting moisture regimes to unmask the gene action of drought tolerance in sweetpotato, a milestone in science. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.

Alternaria leaf petiole and stem blight is an important disease of sweetpotato (Ipomoea batatas (L.) Lam.) causing yield losses in both landraces and improved cultivars. The most important species causing economic yield loss in Uganda are Alternaria bataticola and A. alternate with A. bataticola the most aggressive and widely distributed. The study was conducted to: i) establish farmer-preferred sweet potato attributes, production constraints and Alternaria leaf petiole and stem blight awareness; ii) evaluate Ugandan sweet potato germplasm for Alternaria leaf petiole and stem blight resistance; iii) determine the mode of inheritance of resistance to Alternaria leaf petiole and stem blight and storage root yield components of sweet potato through estimation of the general combining ability (GCA) of the parents and the specific combining ability (SCA) of the parents for each cross; and iv) determine the adaptability and farmer acceptability of selected F1 genotypes across environments. The participatory rural appraisal was conducted to establish farmer preferences and production constraints revealed that farmer preferred sweet-potato traits were high yield, sweetness (taste), early maturity, high dry mass, resistance to pests and diseases, and in-field root storability after maturity. A majority of the farmers considered Alternaria leaf petiole and stem blight a serious production constraint causing yield loss of over 50%. The main control measures against the disease were roguing of infected plants, spraying with fungicides, use of healthy planting materials and planting resistant genotypes. Thirty sweet potato land races and improved cultivars were evaluated for Alternaria blight severity; yield, dry mass, harvest index, sweetpotato weevil (Cylas spp.) damage and sweetpotato virus disease at two sites (Namulonge and Kachwekano) over three seasons (2010B, 2011A, 2011B) under Alternaria inoculum and fungicide spray treatments. Landrace Shock was more resistant to Alternaria blight than Tanzania, the resistant check. Genotypes NASPOT 1, NASPOT 7, New Kawogo and Dimbuka were the most susceptible. Thirty two F1 families were generated from 16 parents in two sets in a North Carolina II mating scheme. The families were evaluated at two sites using a 5 x 7 row-column design with two replications. There were significant (P<0.05) differences among the families in Alternaria blight severity. Both GCA and SCA mean squares (MS) for Alternaria blight were highly significant (P<0.001) but the predominance of GCA sum of squares (SS) for Alternaria blight at 67.4% of the treatment SS versus 32.6% for SCA SS indicated that additive effects were more important than the non-additive effects in controlling this trait. For the yield components, the GCA MS were significant (P<0.05) and accounted for more than 60% of the treatment SS except for percentage dry mass composition where SCA SS accounted for 53.0% of the treatment SS implying that non-additive genetic effects were slightly more important than additive for this trait. Some parents that had desirable high, negative GCA effects for Alternaria blight produced families with undesirable positive SCA effects and the reverse was also true. This implied that the best parents should not be chosen based on GCA effects alone but also on SCA effects of their best crosses. The promising F1 genotypes selected from previously evaluated crosses together with one Alternaria blight resistant check (Tanzania) and one susceptible check (NASPOT 1) were evaluated at three sites (Namulonge, Kachwekano and Serere) using a randomised complete block design with three replications. Scientists and farmers evaluated the agronomic performance and also quality traits of the genotypes before and at harvest. Genotypes G14, G16, G24, G29, G49, G59 and G69 were the most stable across the sites for low Alternaria blight severity and can, therefore, be recommended for further evaluation under both low and high disease pressure areas. Genotypes G67, G13, G14, G24, G29 and G53 were the most high yielding and stable across the sites and were therefore the most widely adapted. In the participatory selection, before harvest and at harvest, Spearman’s rank correlation of the scientists and farmers’ mean ranking of the genotypes at each site was positive and significant. This indicated that the scientists in the study were capable of selecting for farmer preferred traits. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.

Text in English / Potato is an important source of energy to most micronutrient malnourished affected population in South Africa. Improvements through bio-fortification can therefore enhance access to essential micronutrients. The study was aimed at determining the level of variability of iron and zinc concentration among 20 potato genotypes as a preliminary step for future breeding program. The materials were evaluated using inductively coupled plasma optical emission spectrometry. Statistical analysis indicated significant (P<0.001) variation of Fe and Zn among the genotypes. The average concentration ranges from 34.67 to 76.67 mg kg-1 and 12.88 to 66.1 mg kg-1 for iron and zinc respectively. The best performing genotypes were cultivar Mnandi, Hertha, Buffelspoort and breeding lines-N105-1, 00-S100-33 and 03-627-50. Iron concentration was positively correlated with Zinc concentration. The study showed that enough variability of Fe and Zn concentration exist among the evaluated genotypes, which can be exploited for use in potato bio-fortification breeding programme. / Agriculture, Animal Health and Human Ecology / M.Sc. (Agriculture)

Sweetpotato (Ipomoea batatas, L.) is one of the important sources of carbohydrates and economic income in Mozambique. As with most of the food crops in Mozambique, it is usually produced by small-scale farmers under dryland conditions. Despite the importance of the crop, the storage root yields are still low and it is difficult to keep planting material (vines) for the next planting season. One of the major challenges to production is drought stress. Drought stress affects sweetpotato by retarding aboveground growth, reducing total root yield, percentage of dry mass, and reducing the quality of the roots as a result of the increase in damage caused by the sweetpotato weevil (Cylas formicarius). The objective of this study was to identify sweetpotato genotypes tolerant to drought particularly amongst the orange fleshed types which can be used in breeding programmes to improve the drought tolerance of genotypes grown in Mozambique. To this end, 48 genotypes were evaluated in both field and greenhouse studies conducted at Umbeluzi Research Station (26º 03’ S, 32ºC 23’ E; 12 masl) located about 30 km from Maputo city. The field trial was a three replicate, α-design with split-plots. Genotypes were the wholeplot treatment factor and irrigation levels were the sub-plot treatment factor. The three irrigation levels imposed were: nonstressed plants irrigated from planting to 120 DAP; moderately stressed, plants irrigated until 60 DAP; and severely stressed, plants irrigated until 30 DAP. In the greenhouse trial the 48 genotypes were grown in wooden boxes arranged in a two replicate, randomized complete block design. The plants were exposed to water stress from 10 DAP to the end of experiment at 60 DAP. Genotypes were significantly different for all traits, namely: survival %, vine vigour, aboveground biomass, total and commercial root yield, total fresh biomass, harvest index, β-carotene content, % dry mass, dry mass yield, incidence of sweetpotato virus disease, and incidence of weevil damage. Irrigation levels were significant for the traits: survival %, vine vigour, aboveground biomass, total and commercial root yield, total fresh biomass, harvest index, β-carotene content, % dry mass, and dry mass yield. Irrigation levels were not significant for incidence of sweetpotato virus disease and incidence of weevil damage. The genotypes x irrigation levels interaction was significant for: total and commercial root yield, and incidence of weevil damage; and not significant for: survival %, vine vigour, aboveground biomass, total fresh biomass, harvest index, β-carotene content, % dry mass composition, dry mass yield and incidence of sweetpotato virus disease. The mean dry mass yields across irrigation levels of the national breeding lines and introduced genotypes were higher than the landrace genotypes. Most of the national breeding lines had higher β-carotene content than the introduced and landrace genotypes. The landrace genotypes had relatively higher % dry mass composition compared to the national breeding lines. The stress tolerance index (STI) separated the 48 sweetpotato genotypes evaluated in the field trial into three groups: drought tolerant (high STI); moderate drought tolerant (intermediate STI); and drought sensitive (low STI). Under moderate stress, yield potential (Yp) and yield in a stress environment (Ys) were highly significant, positively correlated with Mean productivity (MP), Geometric mean productivity (GMP), Stress tolerance index (STI) and Tolerance index (TOL). Under severe stress the same correlations were reported. Under moderate and severe stress, the correlation between stress tolerance index (STI) and Stress susceptibility index (SSI) was significant and negative. In the greenhouse trial, differences between genotypes in vine length increment, vine diameter increment, leaf width increment and number of nodes vine-1 were significant (P<0.05). Vine length, vine diameter, leaf width and length increments either increased or were reduced due to water stress. Less than 10% increment in vine length (between 25 and 50 DAP) was recorded in MGCl01, Atacama, Cordner, Beauregard, and CN1448-49. Higher than 40% vine length increment was recorded in Jonathan and UNK-Malawi, Naspot, MUSG0614-24, Resisto, K566632, Tainung64, Ejumula and MUSG0623-09. Vine diameter decreased in Manhissane and MUSG0616-18. No change in leaf length in Tacna and Jonathan and in leaf width in Xihetamakote and Resisto-Nairobi was recorded. The longest petiole length at 30 DAP was recorded by Tacna and the shortest by Nhacutse4. The longest internode length was recorded in 199062.1. Similar to petiole length, Nhacuste4 reported the shortest internode length. The highest number of primary vines was recorded by MUSG0608-61 and lowest by Beauregard. Of the 48 genotypes exposed to water stress, 18 survived until the end of the greenhouse experiment at 60 DAP and were therefore considered to be drought tolerant. / Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, 2011.