No description available.

No description available.

No description available.

No description available.

No description available.

Thesis (MScAgric (Plant Pathology))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Maize is the most important cereal crop produced in southern Africa. Maize producers, processors, and consumers in the region, however, are affected by Fusarium ear rot, a disease caused primarily by the fungal pathogen Fusarium verticillioides that reduces grain quality and potentially contaminates the grain with mycotoxins (fumonisin). Due to the threat of fumonisin to human and animal health, and the economic losses associated with reductions in grain quality, strategies aimed at the prevention of Fusarium ear rot and fumonisin contamination are required. These preventative strategies should be focused on protecting the crop prior to harvest, as damage is known to occur in the field before storage. Chapter 1 provides the reader with a broad overview of maize production in southern Africa, the disease Fusarium ear rot caused by F. verticillioides, and the contamination of grain with fumonisins. Potential disease management practices are summarised, and the role of host resistance and its underlying mechanisms emphasised. Finally, the use of plant breeding and resistance elicitors as methods to enhance host resistance in maize towards Fusarium ear rot and fumonisin contamination are discussed in detail. The planting of maize genotypes with enhanced host resistance potentially offers the most efficient method to reduce Fusarium ear rot and mycotoxin contamination. If plant breeding is to be used to enhance resistance, sources of genetic resistance are required. These sources would ideally be in the form of locally adapted maize genotypes, such as inbred lines. In Chapter 2, maize inbred lines used in local breeding programmes, which are adapted to the production conditions in southern Africa, were evaluated as potential sources of resistance to Fusarium ear rot and fumonisin contamination. If inbred lines with good genetic resistance were to be identified they could be used by breeding programmes to develop commercial maize cultivars with resistance to Fusarium ear rot and fumonisin. Activation of resistance responses in normally susceptible maize genotypes using resistance elicitors could provide a novel management strategy for Fusarium ear rot control, as no commercial cultivars with complete resistance to this disease have been identified in southern Africa. Elicitors have previously been found to induce resistance to plant pathogens, mostly in dicotyledonous crops, but the ability of a range of elicitors to reduce Fusarium ear rot and fumonisin contamination in maize has not been investigated. In Chapter 3, a variety of chemical elicitors that induced resistance in other plant-pathogen systems were selected based on the different defence pathways that they stimulate, and evaluated in field and greenhouse trials. Three commercial maize hybrids were included in the trial, conducted at two different field sites, and the elicitors were tested for their ability to reduce Fusarium ear rot and fumonisin contamination of grain, as well as for their effect on yield. These elicitors could be applied in the field as part of an integrated disease management programme, are environmentally friendly, and would be affordable to commercial producers that produce the majority of maize in South Africa.

Aflatoxin contamination of crops is frequent in warm regions across the globe, including large areas in sub-Saharan Africa. Crop contamination with these dangerous toxins transcends health, food security, and trade sectors. It cuts across the value chain, affecting farmers, traders, markets, and finally consumers. Diverse fungi within Aspergillus section Flavi contaminate crops with aflatoxins. Within these Aspergillus communities, several genotypes are not capable of producing aflatoxins (atoxigenic). Carefully selected atoxigenic genotypes in biological control (biocontrol) formulations efficiently reduce aflatoxin contamination of crops when applied prior to flowering in the field. This safe and environmentally friendly, effective technology was pioneered in the US, where well over a million acres of susceptible crops are treated annually. The technology has been improved for use in sub-Saharan Africa, where efforts are under way to develop biocontrol products, under the trade name Aflasafe, for 11 African nations. The number of participating nations is expected to increase. In parallel, state of the art technology has been developed for large-scale inexpensive manufacture of Aflasafe products under the conditions present in many African nations. Results to date indicate that all Aflasafe products, registered and under experimental use, reduce aflatoxin concentrations in treated crops by > 80% in comparison to untreated crops in both field and storage conditions. Benefits of aflatoxin biocontrol technologies are discussed along with potential challenges, including climate change, likely to be faced during the scaling-up of Aflasafe products. Lastly, we respond to several apprehensions expressed in the literature about the use of atoxigenic genotypes in biocontrol formulations. These responses relate to the following apprehensions: sorghum as carrier, distribution costs, aflatoxin-conscious markets, efficacy during drought, post-harvest benefits, risk of allergies and/or aspergillosis, influence of Aflasafe on other mycotoxins and on soil microenvironment, dynamics of Aspergillus genotypes, and recombination between atoxigenic and toxigenic genotypes in natural conditions.

Aflatoxins are potent carcinogens produced by the fungus Aspergillus flavus Link:Fr and are a significant preharvest problem in maize production in Texas, the southern US, and subtropical climates. Several sources of maize germplasm are available which reduce preharvest aflatoxin accumulation, but many of these sources lack agronomic performance for direct use as a parent in commercial hybrids. Tropical germplasm is a source of both resistance to aflatoxin accumulation resistance and agronomic performance traits. The goal of this study was to investigate germplasm for traits to reduce preharvest aflatoxin accumulation. The specific objectives of this research were: 1) to validate QTL estimates previously identified in lines per se and estimate new QTL associated with reduced aflatoxin accumulations and agronomic traits; 2) to evaluate agronomic characteristics of selections from a RIL population in testcrosses at multiple locations across Texas; and (3) to release agronomically desirable germplasm sources with reduced risk to preharvest aflatoxin accumulation. A total of 96 QTLs were detected across fourteen measured traits using an RIL population of 130 individuals in testcross hybrids evaluated in five environments. Three QTL detected in per se analyses were also detected in hybrid testcrosses. Previously unreported QTL were detected on chromosomes 3, 4, 8 and 9. Within each of the two years, neither subset of the RIL testcross hybrids produced grain yields equal to commercial hybrid checks in these trials, but one testcross in 2008 produced grain yield within 10 percent of commercial check hybrids and in 2009, five RIL testcrosses produced grain yield within 17 percent of the commercial check hybrids. Although RIL testcrosses did not yield more than the commercial checks, they will be a source of germplasm for reduced aflatoxin. Improved sources of maize germplasm lines Tx736, Tx739, and Tx740 have been selected for adaptation to southern US and Texas growing environments with traits that reduce aflatoxin accumulation. Each of the lines in testcross accumulated significantly fewer aflatoxins than commercial hybrids in the trial.

Market information services established in 1999 were aimed at the promotion of market efficiency through provision of information across the nation. While the responsible bodies have improved the knowledge of prices, information exchange and flow, as a result of competition between markets, is not known and questions of market effectiveness still stand. This study examines market efficiency based upon response to price signals across Ugandan markets. We focus on information exchange for maize and beans among 16 key markets. We study weekly price data from the first week of 2000 to the last week of 2003 from each of the sixteen markets. Each commodity is studied separately using Vector Autoregessions (VARs) and Directed Acyclic Graphs (DAGs). The two techniques are widely used to show market risk and causal relations in time series data. While results are presented individually for each commodity, the markets are comparable. In determining market efficiency, we test for stationarity of the data, explore the magnitude of forecast error decompositions over time across markets, and observe the patterns of communication based on DAGs. We find that markets are more efficient in exchanging information on maize than beans. Communication of data is mostly between markets in eastern, western, and central parts of Uganda. Overall, markets are very slow in reacting to information in the short run.Information from the Mbale and Iganga markets, which are located in areas of high production, is very valuable in the maize trade. However, of the two markets, it is data from the Mbale market, located near the border with Kenya, which is of paramount importance. Specifically, price is discovered in Mbale in the maize trade. Our results also show the Gulu market, which is situated in an insecure zone, to be very responsive to price signals over the long run. In the case of beans, it is the price signals from Tororo and Jinja that cause more disruption in most of the markets. Price is discovered in these two markets. A majority of the markets is more affected by data from Jinja than Tororo. This segmentation in market price discovery suggests an existing market failure. Arua and Gulu are found to be the least responding markets in regards to price signals for beans. We do not find information from the Kampala market to be important in either the maize or beans trade.

Understanding water stress responses in maize and the genetic factors controlling these traits has direct application to breeding and genetic study. To understand water stress responses in more detail, this thesis characterizes a novel B73 ethyl methanesulfonate induced wilting maize mutant. This mutant is shown to express a wilting phenotype under both field and greenhouse conditions, exhibiting leaf roll, stunted stem growth and reduced fertility. Germination is unaffected in mutant seeds and no abnormal seedling phenotypes are observed. Onset of the phenotype is between V3 and V6 growth stage, and corresponds with a disruption of stem elongation. To characterize the genetic elements underlying the wilting phenotype, F2 and F3 mapping populations were phenotyped and genotyped. The mutant phenotype was found at variable frequencies between populations; ¼ or less of all mapping population members were observed as wilting, implicating a single genetic factor controlling the mutant phenotype. Background effects of the non-mutant parent and residual EMS mutations are hypothesized to be responsible for the unexpected frequency of wilting observed. Single nucleotide polymorphism and microsatellite markers localized the wilted mutant allele to a 7.4-Megabase region on the long arm of chromosome 7. The phenotype results and rough mapping data provide a base for further study and cloning of the mutant allele.