Doctoral Degrees (Plant Pathology)

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Browsing Doctoral Degrees (Plant Pathology) by Author "Halleen, Francois"

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    Characterisation of Cylindrocarpon spp. associated with black foot disease of grapevine
    (Stellenbosch : Stellenbosch University, 2005-12) Halleen, Francois; Crous, Pedro W.; Fourie, Paul H.; Stellenbosch University. Faculty of Agrisciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: During the past few years a drastic reduction has been noted in the survival rate of grafted grapevines in nurseries, as well as in young vineyards in the Western Cape Province of South Africa. Circumstantial evidence suggested that Cylindrocarpon spp., which cause black foot disease of grapevine, were associated with this decline. Black foot disease of grapevine is a relatively new, and as yet poorly known disease affecting vines in various countries where grapevines are cultivated. Primary aims of this research have been (1) to conduct nursery surveys in order to determine which fungi are involved in the decline phenomenon, with special reference to the involvement of Cylindrocarpon spp., (2) to identify and characterise the organisms believed to be the causal organisms of black foot disease, and (3) the development of control and/or management strategies to prevent or eradicate Cylindrocarpon infections. Nursery grapevines were sampled at different stages from three commercial nurseries in the Wellington area of the Western Cape Province and were investigated during the 19992000 season by means of destructive sampling. The first samples were taken in September from callused cuttings prior to planting in nurseries. After planting, asymptomatic rooted cuttings were selected from nurseries after 3, 6 and 9 months. Isolation studies clearly demonstrated that different “Cylindrocarpon spp.” infected cuttings from nursery soils. These species rarely occurred in rootstock propagation material prior to planting. At the time of planting, the susceptible basal ends (especially the pith area) of most of the nursery cuttings are partly or even fully exposed. Callus roots also break during the planting process, resulting in small wounds susceptible to infection by soilborne pathogens. The isolation studies revealed that the first infections occurred in the roots, followed by infections of the rootstocks. These infections increased progressively during the course of the growing season. Substantial variation in cultural and morphological characters was observed among the Cylindrocarpon isolates obtained from the nursery survey, as well as from isolations that were made from diseased grapevines. Morphological and phylogenetic studies were conducted to identify these “Cylindrocarpon spp.” and to establish their association with black foot disease. Sequences of the partial nuclear large subunit ribosomal DNA (LSU rDNA), internal transcribed spacers 1 and 2 of the rDNA including the 5.8S rDNA gene (ITS), and partial β-tubulin gene introns and exons were used for phylogenetic inference. Phylogenetic analyses confirmed the diversity observed among the isolates and four Cylindrocarpon-like species were identified. One of these species was initially identified as Cylindrocarpon destructans. However, further research revealed C. destructans to represent a species complex. Grapevine isolates of “C. destructans” proved to be identical to the ex-type strain of Cylindrocarpon liriodendri, which also produced a teleomorph, Neonectria liriodendri in culture. A second species was newly described in this study as Cylindrocarpon macrodidymum (Neonectria macrodidyma). The two remaining Cylindrocarpon-like species were placed in a new genus, Campylocarpon. The two species were named Campylocarpon fasciculare and Campylocarpon pseudofasciculare. Pathogenicity studies confirmed that all four species were able to reduce root and shoot mass significantly. Knowledge obtained pertaining to the disease cycle of black foot disease suggest that suitable management strategies should focus on prevention of primary infection in nurseries. However, at present, no fungicides are registered for control of this disease in South African vineyards or nurseries. Thirteen fungicides were screened in vitro for mycelial inhibition of these pathogens. Prochloraz manganese chloride, benomyl, flusilazole and imazalil were the most effective fungicides tested, and were subsequently included in semi-commercial field trials. Basal ends of grafted cuttings were dipped (1 min) in various chemical and biological treatments prior to planting in open-rooted nurseries. Black foot pathogens were not isolated from grafted cuttings prior to planting in nurseries. Additional treatments involved soil amendments with Trichoderma formulations and hot water treatment (50°C for 30 min) of dormant nursery grapevines. Field trials were evaluated after a growing season of eight months. The incidence of black foot pathogens was not significantly and/or consistently reduced by the majority of chemical or biological treatments. However, these pathogens were not isolated from uprooted plants that were subjected to hot water treatment. It is therefore recommended that hot water treatment of dormant nursery plants be included in an integrated strategy for the proactive management of black foot disease in grapevine nurseries.