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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Effect of soil factors on parasitic nematodes of sugarcane in KwaZulu- Natal, South Africa.

Dana, Pelisa. January 2004 (has links)
Nematicides are not only expensive and unaffordable to small-scale farmers but are also harmful to the environment as they kill both the target organisms and non-target micro and macro-organisms, thereby destabilising the ecosystem. Most developed countries have or are in the process of banning use of chemicals for pest management, implying that agricultural products from developing countries using pesticides will not be marketed in the developed countries. In former studies, it was shown that plant parasitic nematodes posed serious problems in sugarcane fields as their attacks on sett roots during germination period decreased sett root weights, delayed bud germination or led to fewer buds germinating as most buds abort. Those that germinate later are then faced with competition for food, space and light from the "older" shoots and often die. The attacks on shoot roots may lead to inefficient uptake of water and nutrients by the plant thereby leading to stunted plants. Fewer and shorter sugarcane stalks due to nematode attacks result in poor yield. In this study, it has been shown that a large number of endoparasites would be needed to reduce sett root weights. Agricultural systems based on monoculture are rarely successful in the long term and because sugarcane fields have been monocultured for a very long time, they are losing their productive capacity and this is termed "Yield Decline". Instead of using nematicides, alternative methods can be used for the management of nematode communities. Research has shown on other plants that nematode communities dominated by Helicotylenchus dihystera are less pathogenic to the plants than other ectoparasitic nematodes, e.g., Xiphinema elongatum and Paratrichodorus spp. A study conducted as a pot experiment showed that sugarcane grown in soil with high H. dihystera grew taller and produced greater root and aerial biomass than one grown in X elongatum infested soil. To induce a nematode community dominated by H. dihystera in the field, two strategies were followed: (i ) abiotic factors that influence the nematode's environment were identified. Certain elements found in soil and sugarcane leaves were found to be correlated to certain species, e.g., H. dihystera was negatively correlated to soil sulphur, medium and coarse sand while X elongatum was positively correlated to these soil types and soil elements. Sugarcane leaves with high levels of Ca, Zn, Cu and Fe were found in areas with high percentages of H. dihystera while the reverse was true for X elongatum. (ii) organic amendments were used to improve the sugarcane growth, modify the environment and decrease competition among species within a community. Application of organic matter to the soil improves soil properties such as water infiltration, water holding capacity, erodibility and nutrient cycling, increases suppressiveness of soils to plant parasitic nematodes and stimulates other anti-nematode micro-organisms, e.g., nematode-trapping fungi. Organic amendments were therefore used in this study not only as screens to protect sugarcane roots from nematode attacks but also to manipulate nematode communities for the less pathogenic species, H. dihystera. In a field study where organic amendments were used, plots treated with filter cake, thume + filter cake, trash + filter cake, filter cake + furfural and Temik (aldicarb) had high percentages of H. dihystera while control plots had high percentages of X elongatum. However, the change in relative proportion of H. dihystera by certain treatments was not followed by an average increase in yield, probability because of the overall variability. The yield results, however, showed that for all treatments, including control, the highest yields corresponded to plots with higher H. dihystera proportions, conflicting the initial hypothesis. As a result, if an organic amendment that can substantially increase the relative proportions of H. dihystera can be found, a substantial increase in yield can be expected. Although the organic amendments did not successfully manipulate the nematode communities for the less pathogenic species, H. dihystera, plots with higher yield were those that had high H. dihystera percentages in their nematode communities. / Thesis (Ph.D.)-University of Natal, Durban, 2004.
2

Evaluation of sugarcane varieties for resistance to ratoon stunting disease.

McFarlane, Sharon Anne. January 2003 (has links)
Ratoon stunting disease (RSD), caused by the bacterium Leifsonia xyli subsp xyli, is well established in most sugarcane growing regions of the world and is considered to cause more yield losses worldwide than any other sugarcane disease (Hughes, 1974). In South Africa, field trials have demonstrated that yield reductions under rainfed conditions can exceed 40% in highly susceptible varieties (Bailey and Bechet, 1986). When cane is grown under irrigation, yield losses are less noticeable but still significant in many varieties (Bailey and Bechet, 1995). It is estimated that RSD currently results in a one percent reduction in industrial production in South Africa and between 10 and 20% in other African countries where South African varieties are grown (Bailey and McFarlane, 1999; Rutherford et al., 2003). For many years, the reaction of different sugarcane varieties to RSD has been based on large, replicated yield loss trials grown over a number of years under rainfed and irrigated conditions. Although these trials provide valuable information, they are time-consuming and require large areas of uniform land. They are therefore not suitable for incorporation into a routine disease screening programme in which large numbers of genotypes are assessed for their reactions to the important diseases occurring in the industry. As a result, the susceptibility of new commercial varieties to RSD is only known several years after release to the growers. The main objective of this study was to establish a suitable method to reliably evaluate sugarcane genotypes for RSD resistance as part of the plant breeding and selection programme. Emphasis was placed on the use of the tissue blot immunoassay (TBlA) developed by Harrison and Davis (1988) and modified by Davis et al (1994), in relation to the more traditional methods of variety assessment, such as the rate of spread of RSD in the field at harvest and yield loss trials. Although the immunoassay protocol was not altered, slight modifications to the blotting procedure resulted in clearer blots that were easier to interpret. Internode position and the age of the cane were shown to have a marked effect on the extent of colonisation and ultimately the RSD resistance rating. A trial investigating the effect of the extent of colonisation on the rate of spread of RSD at harvest was conducted and showed that the relationship between spread and colonisation was highly significant. This indicated that RSD spread more rapidly through varieties such as N14 and N22 that supported high populations of L. xyli subsp xyli. The control plots in the same trial provided useful information on the extent of colonisation in the twelve varieties planted. In another trial, the effect of RSD on the yield components of six commercially grown varieties was investigated and TBIA was also conducted to compare the two methods of variety assessment. The relationship between yield loss and the extent of colonisation was significant in both the plant and first ratoon crops. TBIA produced consistent results and the ranking of the six varieties was virtually identical, despite the different growing conditions during the two crop cycles. In an attempt to screen large numbers of genotypes under controlled glasshouse conditions, .TBIA was also tested on RSD-infected sugarcane transplants (seedlings). The results of this trial were variable and could not be reliably used as a screening tool. Based on the findings of this study, TBIA has now been adopted as a quicker and cheaper alternative to immunofluorescence microscopy for diagnosing RSD in sugarcane transplants. More importantly, TBIA has been accepted as a method of screening genotypes routinely for resistance to RSD and the first screening trial was planted in November 2002. It will now be possible to inform sugarcane growers of the RSD status of the new varieties as they are released, enabling them to make more informed decisions on how to manage each variety. This information will also be valuable when selecting parents in the crossing programme, with a long term view of improving the general resistance of commercially grown varieties to RSD. This should ultimately result in a substantial reduction in RSD levels in the industry. / Thesis (M.Sc.)-University of Natal, Durban, 2003.

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