Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by Subject "Antimicrobial polymers"

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    Understanding citrus replant disease in South Africa with the aim of developing a Methyl Bromide free management strategy
    (Stellenbosch : Stellenbosch University, 2018-03) Swart, Laurika; Van Niekerk, Jan; McLeod, Adele; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Citrus fruits are one of the most important fruit crops after deciduous fruit and vegetables cultivated in South Africa. The Citrus industry in South Africa, is seen as the third largest horticultural industry and is considered one of the most important horticultural crops due to its economic export value and local consumption. South Africa is currently the eleventh largest citrus producer in the world, and second largest in the Southern hemisphere. The growth of citrus production and the development of new cultivars in South Africa has become a priority that growers establish orchards on sites where citrus has been cultivated for many years. With the establishment of a new orchard symptoms associated with replant disease have been observed on these newly planted citrus trees like in many other parts of the world. The casual agents associated with citrus replant disease in South Africa have been regarded as the citrus nematode, Tylenchulus semipenetrans and the soilborne pathogens, Phytophthora nicotianae and Phytophthora citrophthora. Symptoms can be characterised by the appearance of small leaves, the formation of gummosis and trees showing low vigour with short internodes. Previous studies conducted in South Africa on citrus replant disease did not focus on the characterisation of fungal organisms present in replant sites that may play a role in replant disease. Therefore, the aim of this study was determining whether the basis of citrus replant disease is biotic or abiotic. This study also aimed to identify the exact pathogens involved and investigating the molecular characterisation of oomycetes. Phylogenetic analyses of Fusarium spp. associated with replant soils was conducted to determine the diversity of Fusaria associated with citrus in South Africa. Soil samples was collected from four orchards, aged between 37 and 47 in two major citrus producing areas. Representative soil samples from each orchard was used for nematode extraction and revealed that both juvenile and female citrus nematodes (T. semipenetrans) was present in the soil. The soil was subjected to six different treatments. The treatments included steam sterilisation, a 20% soil dilution, a mefenoxam, difenoconazole and cadusafos drench treatment and an untreated control. After treating the orchard soil two Carrizo citrange seedlings was planted per pot and left to grow for seven months in a glasshouse. Prior to planting and at trial evaluation the seedling length and weight was determined to compare the growth of the seedlings. Statistical analyses based on the growth response of citrus, revealed that all the treatments to some extent led to an increase in seedling growth. The steam sterilisation treatment showed to have the best effect on seedling growth eliminating pathogens in the soil, followed by the untreated control, mefenoxam, cadusafos, difenoconazole and soil dilution treatment. None of the biocide treatments indicated the involvement of specific biological agents targeted by each biocide. Isolations from the roots of the control bioassay seedlings in all orchards showed that isolates belonging to the ‘Fusarium solani’ species complex (FSSC) dominated in all orchards. Fusarium oxysporum species complex (FOSC) isolates and oomycetes (Phytophthora nicotianae, Phytophthora citrophthora and Pythium irregulare) were also associated with the citrus roots at more or less similar frequencies within each orchard. Identification of Fusarium spp. was based on the phylogenetic analyses of the translation elongation factor 1-alpha (TEF) and RNA polymerase II second largest subunit RPB2 gene region. Phylogeny of the FSSC isolates showed that the citrus isolates grouped into four clades including a Neocosmospora solani clade (25 isolates), Neocosmospora croci clade (one isolate), an unnamed Fusarium spp. clade (13 Isolates) with F. falciformis as the most related known Fusarium spp., and another clade (one isolate) containing an unnamed Fusarium species. The citrus FOSC isolates grouped within the F. oxysporum phylogenetic species II. The FOSC citrus isolates were furthermore polyphyletic and distributed among two subclades, previously designated as Clade 3 (11 isolates) and Clade 4 (two Isolates). This study showed that the growth response (weight and length increases) of the seedlings in the bioassay are biological in nature. Phytophthora nicotianae, Phytophthora citrophthora, Pythium irregulare, Fusarium spp. within the FSSC and FOSC as well as the citrus nematode Tylenchulus semipenetrans are all shown to be associated with citrus tree roots and replant soil. The management strategies indicate that the applications could be possible substitutes for methyl bromide soil fumigation in orchards earmarked for replant. But ultimately the effective prevention of citrus replant disease using non-methyl bromide fumigation is dependent on knowing what is present in the soil and making the correct decisions based on this knowledge.