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Plant Pathology

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Browsing Plant Pathology by Author "Bara, Gracian Takudzwa."

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    Scirtothrips aurantii faure (thysanoptera: thripidae) population dynamics, biological control, and the characterisation of bracharoa mixta (snellen) (lepidoptera: erebidae) and wind in scarring avocado, persea americana miller (lauraceae)
    (2020) Bara, Gracian Takudzwa.; Laing, Mark Delmege.
    Avocado (Persea americana miller) production is a growing, multibillion rand, export oriented industry in South Africa employing tens of thousands of people and contributing significantly to the local GDP. To remain competitive in the global market, the South African avocado industry needs to consistently produce high quality fruit. However, poor quality fruit remains one of the biggest challenges to export. The aim of this study was to identify, describe and quantify the role played by biotic and abiotic factors in scarring avocado fruit, as well as to propose mitigatory approaches. The lack of current, updated and scientifically evidenced documentation is a shortcoming addressed in this thesis with a goal to enable policy makers, growers and academics to minimize scarring losses. Using a modified beat cup method, the avocado thrips spectrum during flowering and early fruiting was found to include Frankliniella occidentalis (Pergande), Scirtothrips aurantii Faure, Thrips gowdeyi (Bagnall), Thrips pusillus Bagnall, Thrips tenellus Trybom, Haplothrips gowdeyi (Franklin), Haplothrips bedfordi Jacot-Guillarmod and Megalurothrips sjostedti (Trybom). Tea filter paper (74 μm pore size) and coffee filter paper (53 μm pore size) were determined to be effective thrips screen material for use in thrips exclusion trials. Fruit sampling using naturally infested fruitlets confirmed that avocado is a natural host to S. aurantii (the South African citrus thrips). This is the first study to demonstrate that avocado is a natural host to this pest. The confirmation of S. aurantii, an established and well-studied pest of citrus, in South African avocado fruit, forms the basis for tailor-made IPM programs in avocado fruit and allows for parallels and comparisons to be drawn on its biology, ecology and management from the citrus industry. To monitor the presence and abundance of S. aurantii in avocado orchards during early fruiting, several different coloured sticky traps and placements (border vs interior) were trialled. Yellow sticky traps were found to be effective, with the incidence and distribution of the pest tending to be random, with numbers high during the spring flush, declining thereafter and picking up in summer. This means that when monitoring for S. aurantii, yellow sticky traps can be set-up randomly in an avocado orchard during early fruiting. The spring flush and early fruiting period present a small window of opportunity when control measures can be implemented (if necessary), to prevent economic scarring damage on the young, developing fruit. Several pre-harvest diseases are known to inflict serious economic damage to avocado fruit, among them avocado scab (Sphaceloma perseae Jenkins). Having been confirmed in South Africa previously, S. perseae was initially suspected as the cause of a scarring injury called “wind damage” by growers. However, morphological and DNA fingerprinting did not confirm its presence. Cladosporium spp. were repeatedly isolated from typical scars on avocado fruit but they were neither directly infectious, nor infectious in wounds created by mechanical abrasion (to simulate thrips and wind damage), and pathogenicity was not demonstrated. Thrips and wind abrasion were identified as major quality constraints accounting for 30 % scarring damage, a loss factor of 13.72 % and a combined revenue loss of 5.57 %. Revenue losses in the order of 1.49 % are incurred annually due to S. aurantii downgrading (3.86 % loss factor). Cultivar differences were observed, with ‘Pinkerton’ and ‘Carmen®-Hass’ being the most susceptible cultivars. Proximity to macadamia trees was also found to increase incidence of S. aurantii scarring damage. Avocado growers are therefore advised to take steps to minimise scarring damage by siting avocado orchards away from prevailing and dominant winds, macadamia trees, as well as putting in place suitable windbreaks. Towards the end of the 2018/2019 avocado season, unidentified Lepidoptera larvae were observed scarring avocado fruits and defoliating leaves in Wartburg, KwaZulu-Natal, South Africa. Using morphological and molecular techniques, Bracharoa mixta Snell. (Lepidoptera: Erebidae) (tussock moth) was identified, DNA barcoded (GenBank MN527963) and voucher specimens (voucher I.D AcP 9636) were deposited for future referencing. This is the first report of tussock moths scarring avocado fruit. Potential revenue losses of up to ZAR 1352.90/t (2.26 % revenue loss) through downgrading are possible because this insect is capable of causing economic loss, in sporadic, isolated epizootics. The implications of these findings are that the insect has been identified, characterized and its biology studied, and this will aid in rapid detection and implementation of IPM should there be a further outbreak. Dispersal/Emergence (D/E) traps and fruit infestation indices were used to monitor S. aurantii populations during the critical early fruiting period and soil insecticide applications were applied to control below-ground life stages. Use of fruit infestation indices is recommended ahead of D/E traps for monitoring. Control using soil drenches of biologicals and synthetic insecticides was not demonstrated, probably because only a small percentage of the thrips pupated in the ground (18.40 % under laboratory conditions). The low percentage of S. aurantii pupating on the ground means that soil application of non-systemic insecticides and biocontrol agents is not a viable control option.