Browsing by Author "Koopman, Trevor Ashley"

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    Molecular characterisation and pathogenicity of the South African venturia inaequalis (apple scab) population
    (Stellenbosch : Stellenbosch University, 2016-12) Koopman, Trevor Ashley; Lennox, Cheryl L.; Tobutt, K. R.; Bester, Cecilia; Meitz-Hopkins, Julia C.; Stellenbosch University. Faculty of AgriSciences. Department of Plant Pathology.
    ENGLISH ABSTRACT: Venturia inaequalis, the causal agent of apple scab, is one of the most important fungal diseases on apple, causing major economic crop losses in almost all the apple growing regions world-wide. The disease is mainly controlled by fungicides and requires between 6 and 15 applications, depending on the weather conditions, to effectively control the disease in South Africa. In this study, V. inaequalis samples were collected over two seasons from symptomatic leaves and fruit from the four principal apple growing regions of South Africa namely the Koue Bokkeveld, Elgin and the Upper and Lower Langkloof areas. This was to assess the genetic diversity, pathogenicity, virulence and determination of races occurring in these four South African V. inaequalis populations. A subset (129 and 139 isolates) of different apple scab isolates from the four apple growing regions were sequenced for specific gene areas of V. inaequalis including polymorphic virulence factors. Nucleotide variation was found within the ITS (four haplotypes) and ABC2-NB2 (six haplotypes) gene regions for the South African population. Four non-synonymous polymorphisms were detected in the ABC-NB2 gene region. Another subset of the ABC-NB2 gene region samples (20) was used to sequence two candidate effector genes (THN- and Vice16 genes) and polymorphism of between two to five nucleotide changes were found. The population structure of V. inaequalis from four South African apple growing regions was determined for the first time. Two sub-populations were found, with the Elgin population different from the others, when V. inaequalis isolates were genotyped with seven SSR- markers. Due to sexual outcrossing most of the genetic differences in the V. inaequalis population were found within the populations. High gene flow was observed due to the many migration events that had occurred between the regions. On the other hand, identical genotypes (clones) were found from two seasons in two regions, indicating overwintering asexual inoculum (conidia). In this study the V. inaequalis races occurring in South Africa were determined for the first time. The accessions ‘Royal Gala’ (0), ‘Golden Delicious’ (Rvi1), A723 (Rvi10) and ‘Hansen’s Baccata#2’ (Rvi12) were found to be susceptible with abundant sporulation on the leaves 14 days after inoculation. Single spore V. inaequalis isolates were used to inoculate different apple cultivars without dominant resistance genes. The single spore isolates differed with respect to their ability to infect and sporulate on these different cultivars. A qPCR method was also used to determine the infection level of the single spore isolates, and the results of the visual scoring and qPCR were comparable. The qPCR method seems to be reliable in detecting fungal DNA in asymptomatic host tissue and is useful for testing virulence between apple scab isolates.