Thesis (M. A. Agriculture (Plant Protection)) -- University of Limpopo, 2019 / Potato (Solanum tuberosum L.) is highly susceptible to root-knot (Meloidogyne species) nematodes, with no known nematode resistant genotypes. In Limpopo Province, two cucurbitacin-containing phytonematicides had been researched and developed. The active ingredients of the cucurbitacin-containing phytonematicides are cucurbitacins, which are allelochemicals that could induce phytotoxicity on crops being protected against nematode damage. The objectives of this study were to determine: (1) mean concentration stimulation point (MCSP) of Nemarioc-AL phytonematicide on potato cultivar ꞌMondial G3ꞌ for managing M. javanica and (2) application interval of Nemarioc AL phytonematicide on potato cultivar ꞌMondial G3ꞌ. Sprouted tubers were planted in 10 cm deep/pot with each pot filled with steam-pasteurised soil and Hygromix at 3:1 (v/v) ratio in the field under microplot conditions. After 100% emergence (2 weeks), each plant was inoculated with 5 000 M. javanica eggs and second-stage juveniles (J2). Seven treatments, namely, 0, 2, 4, 8, 16, 32 and 64% Nemarioc-AL phytonematicide were arranged in a randomised complete block design, with 11 replications. In Objective 2, four treatments, namely, 1, 2, 3 and 4 weeks were arranged in randomised complete block design, with 15 replications. Plant variables and nutrient elements were subjected to the Curve-fitting Allelochemical Response Data (CARD) model to generate biological indices used to compute MCSP using the relation MCSP = Dm + Rh/2 and the overall sensitivity value (∑k). The MCSP for plant variables and nutrient elements, were empirically derived as 4.31% and 1.33%, with the ∑k of 18 and 4 units, respectively. Nematode variables and increasing concentrations of Nemarioc-AL phytonematicide exhibited negative quadratic relations where eggs, J2 in soil and roots and total population (Pf) were optimised at xv 14.43, 28.23, 23.30 and 13.55%. To conduct Objective 2 which is application interval, empirically derived MCSP value of 4.31% from Objective 1 was used. Application interval was optimised using the concept of 1, 2, 3, and 4 weeks in weeks-per-month-of-30-days. The application interval of 4.31% was established at 2.43 weeks which translated to 18 days [(2.43 weeks/4 weeks) × 30 days]. All nematode variables in Objective 2 were not significantly different at all intervals. In, conclusion Nemarioc-AL phytonematicide can be used at 4.31% concentration to control nematodes population densities without being phytotoxic to crops at 18 days application interval. / National Research Foundation (NRF) , Agricultural Research Council (ARC) and the Flemish Interuniversity Council of Belgium

In this study, surveys were conducted in pea and potato fields in North Dakota and Central Minnesota to investigate the incidence and abundance of plant-parasitic nematodes in these fields. Moreover, the effect of the pin nematode, Paratylenchus nanus, on plant growth and yield of six field pea cultivars was determined under greenhouse conditions. Similarly, the influence of lesion nematode, Pratylenchus penetrans, and wilt fungi, Fusarium oxysporum alone and together on growth and yield of potato cultivar ‘Red Norland’, was evaluated in microplots under field conditions. The results indicate Paratylenchus spp. and Pratylenchus spp. are the most frequent nematodes, respectively, in pea and potato fields. Pin nematodes reproduced on field pea cultivars and caused up to 37% reduction in plant height and 40% reduction in yield. Additionally, both P. penetrans and F. oxysporum alone, and together had significant negative effect on growth and yield of potato.

In recent years two new diseases, brown spot and black pit, have been observed on potatoes in South Africa. Brown spot symptoms appear on the foliage as small brown lesions, whereas black pit symptoms appear on the tubers as small dark sunken lesions. In this study the causal organism of brown spot and black pit of potatoes in South Africa was determined. During initial isolation, one fungus and two bacteria were isolated, which were included in the trial. Only the treatments where the fungal isolate was used in inoculation resulted in the development of brown spot lesions. The causal organism of brown spot and black pit were identified as Alternaria alternata which is consistent with other research. To better understand the spread of Alternaria alternata between the plants and/or tubers a pot trial was conducted. It was observed that when planting an inoculated seed tuber brown spot may develop on foliage. But the daughter tubers harvested from plants infected with A. alternata will not necessarily develop black pit. Daughter tubers are most likely infected by A. alternata during harvesting and black pit lesions develop in high humidity in storage. Cultivar resistance is one of the most important measures in controlling plant diseases. Cultivar susceptibility of thirteen South African potato cultivars (Avalanche, Buffelspoort, BP1, Fabula, Fianna, Frodo, Hertha, Labadia, Lanorma, Mondial, Pentland Dell, Up-To-Date and Van Der Plank) was evaluated. Pot trials showed that all the evaluated cultivars are susceptible to infection by Alternaria alternata. Various crops (tomatoes, cabbage, mustard, wheat, oats, tobacco and maize) were assessed to determine the host range of Alternaria alternata (potato pathotype) in rotation crops in South Africa potato growing regions. Of the crops evaluated, the pathogen was able to infect only tomato crops. Only wheat, maize and oats can safely be used in the crop rotation in South Africa, as various potato pathogens attack cabbage, mustard and tobacco plants. This study will lead to a better understanding of brown spot and black pit diseases of potatoes in South Africa and globally. The study emphasise the need for further research which will help to reduce brown spot and black pit diseases of potato. / Dissertation (MSc Agric)--University of Pretoria, 2013. / gm2014 / Microbiology and Plant Pathology / unrestricted

Rhizoctonia solani inoculum can be present either as soil- or tuber-borne sclerotia or hyphae. Although both inoculum sources play a role in disease development, it is not clear which of the two is more important. Successive cultivation of potato crops increases R. solani soil inoculum load resulting in an escalation in disease incidence and severity. The use of tolerant cultivars, however, can effectively reduce inoculum levels thereby decreasing disease intensity. Four pot trials were conducted; the objective of the first two pot trials was to determine the effect of tuber and soil-borne inoculum and stolon inoculations on disease development in sandy and clay loam soils. The second two pot trials were aimed at determining susceptibility levels of five cultivars. Two field trials were planted over two growing seasons in the same soils, using three inoculum levels. Results from the pot trials showed that tubers harvested from inoculated sandy soils developed significantly more disease than those harvested from clay loam soils. Of the three inoculum sources, stolon inoculation and seed-borne inoculum resulted in significantly more disease on progeny tubers than those from R. solani spiked soils. Although none of the cultivars proved to be tolerant to R. solani, BP1 was less susceptible to R. solani at temperatures between 21-26oC. More severe disease symptoms were observed under cooler temperatures on all cultivars. Results from the field trial showed the cultivation of potatoes in the same soil over two growing seasons resulted in an increase in diseased (black scurf) tubers. Furthermore, black scurf was most severe on tubers from soils infested with the highest concentration of inoculum. There were significant disease severity differences, with initial soil inoculum levels being directly proportional to final disease severity. Future studies in South Africa should focus on investigating the genetic composition of various cultivars; the effect of soil type and pH on the pathogenicity of R. solani and the use of molecular diagnostic tools to detect and quantify R. solani in soils. / Dissertation (MSc)--University of Pretoria, 2012. / Microbiology and Plant Pathology / unrestricted

Potato (Solanum tuberosum) is regarded as one of the single most important vegetable crops in South Africa, with an average annual production of 2 million metric tons. The potato industry contributes to an average of $ 0.37b worth of potatoes annually. Over the years, potato production in South Africa has been affected by, among other factors, diseases and plant parasitic nematodes particularly root-knot nematodes (Meloidogyne spp.). In infected potato fields, root-knot nematodes cause great damage to the crop leading to substantial losses in yield and compromised produce quality. The direct and indirect damage caused by Meloidogyne species results in revenue loss due to a high number of table and processing potatoes rejected in markets both locally and internationally. The presence of resistance breaking Meloidogyne populations, the withdrawal of methyl bromide and lack of commercially grown resistant cultivars suggests that growers are likely to experience more losses in the future. Furthermore, distribution of seed tubers harbouring root-knot nematodes, which may also be asymptomatic, inadvertently facilitates transmission of these parasites to new areas thus perpetuating the problem. Therefore, for the potato industry to adequately address the threat of root-knot nematodes, accurate identification and quantification of root knot nematode levels in the field as well as in seed tubers is of importance. Currently most methods of identifying Meloidogyne species largely rely on the use of morphological traits. However, it can be a challenge to accurately differentiate between closely related species using morphology and other classical methods. To resolve this, recent trends globally have focused on the development of DNA-based diagnostics to rapidly and accurately identify different Meloidogyne species. This study therefore sought to; (a) develop a PCR-based diagnostic tool for accurate detection and identification of various Meloidogyne species parasitizing potatoes in South Africa; (b) use this tool to map their distribution and; (c)develop real-time PCR (qPCR) techniques for accurate quantification and characterization of tropical Meloidogyne species from infected potato tubers. In this study, of the 78 composite potato tuber samples collected from various potato growing regions across seven provinces, 24% were found infected with M. javanica, 23% with M. incognita, 17% with M. arenaria, 14% with M. enterolobii, 3% M. chitwoodi, 1% M. hapla and 1% as M. artiellia. The identity of the remaining 17% could not be established. The three tropical species; M. javanica, M. incognita and M. arenaria were identified as the dominant species, occurring almost in every region sampled. Meloidogyne hapla and M. enterolobii occurred in Mpumalanga and KwaZulu–Natal respectively while M. chitwoodi was isolated from two growers located within the Free State. In the study the use of HRMC and real-time PCR was also developed for identification and quantification of tropical Meloidogyne species infesting potato tubers. Using these two techniques, we were able to show that Meloidogyne arenaria populations produced specific melting peaks (79.3183± 0.0295°C, P < 0.05) thus distinguishing themselves from M. incognita (79.5025± 0.0224°C, P < 0.05) and M. javanica (79.96 ± 0.0459°C, P < 0.05). Real-time PCR was also able to detect 1.53/100th of a nematode using second stage juveniles. / Dissertation (MSc)--University of Pretoria, 2012. / Microbiology and Plant Pathology / unrestricted

La papa es el tercer cultivo más importante del mundo, después del arroz y el trigo. El Perú es considerado como el principal productor de papas en Latinoamérica. A pesar de ello, en los últimos años se ha registrado un incremento en la demanda de importaciones de papas prefritas congeladas, motivo por el cual la presente investigación tiene como principal objetivo determinar los factores que influyen en las importaciones de papas prefritas congeladas, subpartida nacional 2004.10.00.00, desde Estados Unidos en el período 2015 - 2018, dentro de los cuales se determinan como los principales: costo de importación, requisitos no arancelarios, tipo de cambio y producto bruto interno. El estudio tuvo un enfoque mixto, el cual se basó en la técnica de entrevistas semiestructuradas realizadas a actores pertenecientes al sector público, sector privado, gremios y organismos relacionados. Mientras que, para el análisis cuantitativo, se utilizó el modelo de regresión lineal. Finalmente, el resultado obtenido en el estudio determina que el costo de importación y los requisitos no arancelarios son los factores que influyen significativamente en las importaciones de papas prefritas congeladas desde Estados Unidos. / Potatoes are the third most important crop in the world, after rice and wheat. Peru is considered the main producer of potatoes in Latin America. Despite this, in recent years there has been an increase in the demand for imports of frozen pre-fried potatoes. In that sense, the main objective of this research is to determine the main factors influencing imports of frozen pre-fried potatoes, national subheading 2004.10.00.00, from the USA from 2015-2018. For this, different factors have been identified in the chosen theoretical framework which the following are listed: import costs, non-tariff treatment, exchange rate and GDP (Gross Domestic Product). The study has a mixed approach, which was based on the technique of semi-structured interviews, carried out with actors from the public sector, from the private sector, from business associations and related organizations. While, for the quantitative analysis, the linear regression model was conducted. Finally, the results obtained in the following study determines that import costs, and non-tariff treatment are the main factors that influence on imports of frozen pre-fried potatoes from the United States of America from 2015-2018. / Tesis

This research project was undertaken to determine if an evaporation pan would accurately predict evapotranspiration when placed in an actual irrigated field situation. Two potato fields in Southern Idaho with different micro-climates and soil types were used in this study. The in-field evaporation pan method was compared with the gravimetric method and the Jensen-Raise and modified-Penman climatic methods. Yield and quality responses were evaluated by varying the amount of sprinkler-applied water so that three distinct moisture regimes were evaluated. It was also necessary to evaluate recent crop coefficient (Kc) curves on potatoes to see if the Kc values predicted evapotranspiration (Et) accurately when related to the evaporation pan or climatic methods. The results showed that the in-field evaporation pan method predicted Et as well as or better than the climatic methods. During July and August, the evaporation pan reading times a Kc of 0.95 predicted Et extremely well when compared with measured actual Et. Crop coefficient curves were developed for both fields by dividing measured actual Et by the evaporative pan reading. The two Kc curves were very similar for the entire growing season. Established Kccurves did not predict actual Et with accuracy or consistency. Yield and quality was definitely correlated with the amount of applied water. The dry moisture regime for both fields received 37 percent less water than the normal plots (which were watered to gravimetric and evaporation pan depletion levels) and resulted in a yield reduction of 34 percent. The quality (percent number one potatoes) was decreased by about 50 percent with reduced water.

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