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A series of studies are presented on the genetic evaluation of cultivated potato (Solanum tuberosum L.) to improve the accuracy and efficiency of selection at various stages of a breeding programme. The central theme was the use of correlated data, such as relationship information and spatial and across-trial correlations, within a linear mixed modelling framework to enhance the evaluation of candidate genotypes and to improve the genetic response to selection. Analyses focused on several social and economically-important traits for the enhancement of the nutritional value, disease resistance and yield of potato tubers. At the formative stages of a breeding scheme, devising a breeding strategy requires an improved understanding of the genetic control of target traits for selection. To guide a strategy that aims to enhance the micronutrient content of potato tubers (biofortification), univariate and multivariate Bayesian models were developed to estimate genetic parameters for micronutrient tuber content from a breeding population generated from crosses between Andean landrace cultivars. The importance of the additive genetic components and extent of the narrow-sense heritability estimates indicated that genotypic 'individual' recurrent selection based on empirical breeding values rather than family-based selection is likely to be the most effective strategy in this breeding population. The magnitude of genetic correlations also indicated that simultaneous increases in important tuber minerals, iron and zinc, could be achieved. Optimising selection efficiency is an important ambition of plant breeding programmes. Reducing the level of candidate replication in field trials may, under certain circumstances, contribute to this aim. Empirical field data and computer simulations inferred that improved rates of genetic gain with p-rep (partially replicated) testing could be obtained compared with testing in fully replicated trials at the early selection stages, particularly when testing over two locations. P-rep testing was able to increase the intensity of selection and the distribution of candidate entries across locations to account for G×E effects was possible at an earlier stage than is currently practised. On the basis of these results, it was recommended that the full replication of trials (at the first opportunity, when enough planting material is available) at a single location in the early stages of selection should be replaced with the partial replication of selection candidates that are distributed over two locations. Genetic evaluation aims to identify genotypes with high empirical breeding values (EBVs) for selection as parents. Using mixed models, spatial parameters to target greater control of localised field heterogeneity were estimated and variance models to account for across-trial genetic heterogeneity were tested for the evaluation of soil-borne powdery scab disease and tuber yield traits at the early stages of a selection programme. When spatial effects improved model fit, spatial correlations for rows and columns were mostly small for powdery scab, and often small and negative for marketable and total tuber yield suggesting the presence of interplot competition in some years for tuber yield traits. For the evaluation of powdery scab, genetic variance structures were tested using data from 12 years of long-term potato breeding METs (multi-environment trials). A simple homogeneous correlation model for the genetic effects was preferred over a more complex factor analytic (FA) model. Similarly, for the MET evaluation of tuber yield at the early stages, there was little benefit in using more complex FA models, with simple correlation structures generally the most favourable models fitted. The use of less complex models will be more straightforward for routine implementation of potato genetic evaluations in breeding programmes. Evaluations for (marketable) tuber yield were extended to multi-location MET data to characterise both genotypes and environments, allowing a re-evaluation of New Zealand MET selection strategies aimed at broad adaptation. Using a factor analytic mixed model, results indicated that the programme’s two main trial locations in the North and the South Islands optimised differentiation between genotypes in terms of G×E effects. There was reasonable performance stability of genotypes across test locations and evidence was presented for some, but limited, genetic progress of cultivars and advanced clonal selections for tuber marketable yield in New Zealand over recent years. The models and selection strategies investigated and developed in this thesis will allow an improved and more systematic application of genetic evaluations in potato selection schemes. This will provide the basis for well informed decisions to be made on selection candidates for the genetic improvement of potato in breeding programmes.

Thesis (M. Agricultural Management (Animal Production)) -- University of Limpopo, 2016 / Cultivated potato (Solanum tuberosum L.) cultigens do not have resistant genotypes to root-knot (Meloidogyne species) nematodes. Currently, efforts are underway to introgress nematode resistance in potato breeding programmes, whereas other environment-friendly nematode management strategies are being assessed in various cultigens. Nemafric-BL and Nemarioc-AL phytonematicides have being researched and developed for managing the root-knot nematode whereas Biocult Mycorrhizae are intended to enhance crop productivity through improved absorption of P, which is inherently low in most South African soils. The objectives of the study, therefore, were: (1) to determine the interactive effects of Nemacur (N), Biocult Mycorrhizae (B) and Nemarioc-AL or Nemafric-BL phytonematicide (P) on population densities of M. javanica and growth of potato plants, (2) to investigate the effects of Nemacur (N), Velum (V), Biocult Mycorhizae (B) and Nemarioc-AL or Nemafric-BL phytonematicide (P) on population densities of M. javanica and growth of potato plants. For the microplot experiment, potato cv. ‘Mondial G3’ seeds were sown in 25 cm-diameter plastic pots with 5 000 ml steam-pasteurised river sand and Hygromix-T at 3:1 (v/v) growing mixture in autumn (March-May) 2015. Pots were buried 80% deep into the soil in with 0.5 m inter-row and 0.5 m intra-row spacing. Potato cv. ‘Mondial G3’ seeds were dipped in a mixture of Mancozeb with a wettener for disease management prior to sowing. Appropriate treatments were applied soon after emergence of leaves. Each plant was inoculated by dispensing a mixture of 5 000 eggs and M. javanica J2. Eight treatments, control (N0B0P0), Nemacur (N1B0P0), Biocult (N0B1P0), phytonematicide (N0B0P1), Nemacur × Biocult (N1B1P0), Nemacur × phytonematicide (N1B0P1), Biocult × phytonematicide (N0B1P1) and Nemacur × Biocult × phytonematicide (N1B1P1), were arranged in a randomised complete block xxvi design (RCBD) with 8 replications (n= 64). Under field conditions the study was conducted in summer (October 2015 - January 2016), with 30-cm furrows dug and potato seeds placed in the soil with 30 cm inter-row and 40 cm intra-row spacing. The four treatments, namely, (1) untreated control, (2) Nemacur or Velum (3) Biocult Mycorrhizae and (4) Nemarioc-AL or Nemafric-BL phytonematicide, were arranged in RCBD, replicated three times for the Velum experiment and five times for the Nemacur experiment. At 56 days after inoculation, the second order interaction (N1B1P1) was highly significant (P ≤ 0.01) for eggs in root and total nematodes, contributing 13 and 12% to total treatment variation (TTV) of the two variables, respectively, in the Nemarioc-AL phytonematicide study. Relative to untreated control, the second order interaction (N1B1P1) reduced eggs in root and total nematodes by 42 and 36%, respectively. In both Nemarioc-AL and Nemafric-BL phytonematicide experiments, the combination of phytonematicide and Biocult Mycorrhizae reduced gall rating. Nemacur, Biocult and Nemarioc-AL phytonematicide, the treatment effects were highly significant on eggs, J2 in root and total nematodes, contributing 53, 68 and 57% to TTV of the three variables, respectively. Nemacur, Biocult and Nemafric-BL phytonematicide treatments each was not significant (P ≤ 0.05) for nematodes variables. Both treatments for Nemacur, Biocult and Nemarioc-AL or Nemafric-BL phytonematicides were significant for gall rating, contributing 92 and 70% to TTV of the variable, respectively. In Nemarioc-AL phytonematicide, relative to the untreated control, gall rating was reduced by 48 to 56%, whereas in Nemafric-BL phytonematicide the variable was reduced by 33 to 56%. In the Velum study, Biocult and Nemarioc-AL or Nemafric-BL phytonematicide, the treatment effects in both experiments were highly significant (P ≤ 0.01) on eggs in root, contributing 88% to TTV of the variable. Both treatments from Nemarioc-AL xxvii and Nemafric-BL phytonematicides had no significant effects on all plant variables measured. In microplot, the second order interaction (Nemacur × Biocult × Nemarioc-AL phytonematicide) was highly significant for nematode eggs in root and total nematode. In a three-way matrix, the N1B1P1 interaction had the highest effects on eggs, followed by Biocult alone, then Nemacur alone and then the phytonematicide. The same trend was observed in the three-way matrix for total nematodes. However, in two-way matrix for eggs, Biocult outperformed Nemacur, as was the phytonematicide on J2. In another microplot study, the second order interaction (Nemacur × Biocult × Nemafric-BL phytonematicide) was significant for J2 in soil and roots, with the three-way matrix showing, that Biocult alone had higher effects than the N1B1P1 interaction on J2 in root. A three-way matrix also showed that Nemacur was outperformed by the phytonematicide alone, Biocult alone and the interactions on J2 in soil. In conclusion, Nemarioc-AL and Nemafric-BL phytonematicides could each be used with Biocult Mycorrhizae in the management of population densities of M. javanica in potato production since the impact from Nemacur which is a synthetic nematicide does not have that much difference from that of phytonematicides interacted with Biocult Mycorrhizae. / Agricultural Research Council

Thesis (M.Sc. (Soil Science)) -- University of Limpopo, 2019 / Phosphorus (P) deficiency has been reported in 30- 40% of global arable land, which poses a huge threat in potato production because of its critical role in the early vegetative development and tuber formation. The use of low cost ground phosphate rock (GPR) as an alternative P fertilizer source has gained recognition. Although GPR contains high P percentage, its direct application is less beneficial immediately due to its low reactivity which makes P unavailable for plant uptake. In this experiment, GPR was co-composted with cattle and poultry manure in order to enhance P acquisition by the potato crop. The aim of this study was to evaluate the potential of phosphocompost application as a cheaper alternative P-source for potato production. The experiment was conducted on Mondial and Valor… potato cultivars at the University of Limpopo Syferkuil Experimental farm in 2015 and 2016. Poultry (PM) and cattle manure (CM)-based phospho-compost mix ratios of 8:2 and 7:3 were applied at 0, 20, 40, 80 and 120 kg P/ha. The trial was laid out in a split plot arrangement fitted into a randomised complete block design with treatments replicated three times. Results indicated statistically significant effects of phospho-compost types on soil pH and available P content at both flowering and harvesting growth stages in 2015 and 2016 with the higher available P content found in the PM-based phospho-composts. In both seasons, highly significant differences in fresh and dry leaf samples among phospho-compost types were obtained. Highly significant season x compost type interaction effects were also recorded on leaf biomass as well as the 2015 tuber weight, with highest tuber weight obtained in plots that received PM7:3-based phospho-compost at 80 t/ha rate. Notwithstanding the non-significant effect of compost type on tuber yield in 2016, higher yield was obtained from PM8:2. Although the grading of tubers showed no significant response to phospho-compost application; the difference between small and medium tubers obtained from 2016 trial was significantly affected by phosphocompost application rates. The CM8:2 mix ratio gave the highest baby tubers (16.87%) while PM7:3 mix ratio gave the highest (36.32%) medium tubers. The grading of the potato tubers revealed a mostly class 1 dominated by baby, small and medium size tubers in the 2015 harvest while the 2016 harvest was also mostly class 1 but dominated by small, medium and large-small size tubers. Tuber size and class were vi most favored by the PM-based phospho-compost applications in both planting seasons. None of phospho-compost types and application rates had significant effect on the measured nutrient concentrations of both plant parts. However, the differences in nutrient concentrations across seasons and plant parts were significant except for Ca. The measure tissue P concentration from the 2016 trial was within the required range suggesting that phospho-compost utilization, particularly the poultry manure-based, in potato production can be beneficial in addressing P deficiency. The PM8:2 mixed ratio resulted in increased soil available P content, potato tuber yield in 2016 and the P concentration across the two plant parts evaluated. The concentration of soil available P and tissue P showed increases with higher application rates albeit non-significance. Future research on the optimum application rate is suggested on a wide range of soils for the various phospho-compost types. / Potato SA and the National Research Foundation (NRF)

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)

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)