Doctoral Degrees (Plant Pathology)

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Browsing Doctoral Degrees (Plant Pathology) by Author "Baloyi, Mahlatse Annabella"

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    Inoculum ecology of petri disease fungi in grapevines of South Africa
    (Stellenbosch : Stellenbosch University, 2016-12) Baloyi, Mahlatse Annabella; Halleen, Francois; Mostert, Lizel; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Petri disease is among the important grapevine trunk diseases affecting lifespan and productivity of young vines worldwide. Infection result in poor vine stand in newly established vineyards and a general vine decline. Pathogens causing this disease are known invaders of susceptible pruning wounds. The knowledge of when aerial spore inoculum of these pathogens are released in vineyards has not been reported in South Africa, and this result in growers pruning without the knowledge of whether that would coincide with periods of high aerial spore concentration. This study aimed at investigating when aerial spores of Petri disease pathogens are released, and to determine their source of inoculum. Knowledge regarding spore release in South African vineyards was determined for two seasons in 2012 and 2013. Spore traps were affixed to arms of infected vines in six vineyards and two rootstock mother blocks. Results showed the occurrence of Petri disease pathogens throughout the year and Phaeomoniella chlamydospora and Pm. minimum were trapped in all vineyards. A total of 14 Phaeoacremonium species were identified from the different blocks. Spore release was shown to coincide with pruning and suckering activities, however, there was no positive correlation between rainfall and spore release events. The occurrence of Petri disease pathogens fruiting bodies was determined by surveying six vineyards and two rootstock mother blocks between 2012 and 2014. Dead wood from diseased vines were collected for microscopic examination. Phaeomoniella chlamydospora pycnidia were found in all vineyards and rootstock mother blocks surveyed. Perithecia of Pm. minimum were only found in vineyards of Stellenbosch P2 and B3, Rawsonville and a rootstock mother block in Slanghoek. Additionally, mating studies with isolates of Pm. australiense and Pm. scolyti were conducted in vitro. After seven and eleven months fertile perithecia of Pm. australiense and Pm. scolyti were observed, respectively. Crosses of both species corresponded to a heterothallic mating system. This study gives the first report of the occurrence of pycnidia of Pa. chlamydospora and perithecia of Pm. minimum in South African vineyards and rootstock mother blocks and also the first description of sexual morphs of Pm. australiense and Pm. scolyti. The pathogenic status of 10 Phaeoacremonium species found in South African vineyards was studied. Fresh pruning wounds of a nine-year-old Cabernet Sauvignon vineyard were inoculated with 104 conidia/ml of each fungus per wound and assessed after 18 months. All inoculated isolates successfully colonized pruning wounds causing lesions significantly different from the negative control and were re-isolated at varying percentages ranging from 28.57% to 85.71%. The study confirmed the capability of all tested Phaeoacremonium species to infect grapevine pruning wounds and cause lesions. The genetic diversity and mode of reproduction were assessed using microsatellite markers and also by determining the mating type distribution of aerial trapped spores of Pm. minimum. In total 320 Pm. minimum isolates were assessed with Mat1-2 specific-primers. Both mating types of Pm. minimum were found in all eight vineyards. An equal distribution of MAT1-1 and MAT1-2 were found in six of the vineyards, but not in the Paarl A and Wellington populations. Primers for dinucleotide microsatellite loci were designed and 15 microsatellite loci were identified to be polymorphic and could thus be used to assess the genetic diversity of the Pm. minimum isolates. A total of 134 multilocus genotypes (MLGs) were observed of which 115 were observed once and 19 genotypes were observed either two or more times. The presence of the same MLG in a vineyard at different collection times, supports the presence of asexual reproduction, and the widespread distribution of MLGs is most probably due to infected nursery planting material. The total gene diversity (H) was high with a mean of 0.58 across all populations. Analysis of molecular variance indicated that 94% of the genetic variation was distributed within populations and only 6% between populations. High and significant population differentiation values were only obtained when Paarl Z was compared to Stellenbosch P2. This study confirms the importance of infected planting material that can distribute similar MLGs over long distances. Therefore, the management of Petri disease needs to focus on ensuring clean mother vines and nursery plants.