Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by Subject "Apple orchards -- South Africa"

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    Optimizing pesticide spraying in high density apple orchards in South Africa, using Venturia inaequalis as model pathogen.
    (Stellenbosch : Stellenbosch University, 2019-04) Rebel, Philip; McLeod, Adele; Van Zyl, Gideon J.; Wessels, Bekker J. P.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Calculating pesticide dose rates, and the use of dosing models, are important for ensuring that pesticide spray applications yield effective disease control in tree crops. A dosing model that has been used for many years is South African apple orchards, is the tree row volume (TRV) pesticide dosing model. In recent years, the change to high density apple orchards has brought into question the efficacy of the model. A dosing system that is known to be suitable for use in high density apple orchards, is the Marktgemeinschaft Bodenseeobst (MABO) dosing model. Photo-macrophotography combined with image analyses and the use of a yellow fluorescent pigment is known to be accurate for assessing spray deposition parameters (quantity, uniformity and quality). The fluorescent particle coverage (FPC%) of the pigment is an effective tracer of mancozeb, the main fungicide used for controlling apple scab in South Africa. It is currently unknown what spray deposition quantities are required for controlling the destructive apple scab pathogen Venturia inaequalis. Benchmark models can be used to determine spray deposition quantities required for disease control. The current study showed that the MABO model resulted in spray deposition parameters (quantity, uniformity and quality) that were comparable to those of the TRV model in high density apple orchards (4m and 3.5m-row-width) in South Africa, by using image analyses and a fluorescent pigment. In the 4m-row-width orchards the MABO model did not differ significantly from the TRV model. The use of two different volumetric airflow rates (VAR, 28000 m³/h [low] and 36000 m³/h [high]) in the orchards (4m- and 3.5m-row width) furthermore yielded similar deposition parameters for both models. In the 3.5 m-row-width orchards, the MABO model yielded a significantly higher deposition quantity than the TRV model; either in canopy tops or for the whole canopy. No clear trends were seen in model performance with regards to deposition uniformity and quality. The MABO model on average resulted in a 40 to 28.5% spray cost saving relative to TRV, depending on the spray volume used. The deposition quantities achieved with the MABO and TRV models in apple orchards were above (0.70- 4.7 FPC%) those required for controlling apple scab with mancozeb, based on a benchmark laboratory model developed in the current study. The benchmark model, developed using thermal infrared imaging (TIRI) disease quantification, showed that 0.40 %, 0.79 % and 1.35 FPC% corresponded to 50, 75 and 90% control respectively. However, when the model was developed using quantitative real-time PCR (qPCR) for disease quantification, lower benchmarks were obtained (0.10, 0.20 and 0.34 FPC% respectively). Both benchmark models showed that mancozeb yielded high levels of disease control at very low concentrations; the FPC% values of the FPC90 (90% control) corresponded to 0.3 to 0.45 times that of the registered mancozeb concentration in South Africa. To conclude, the MABO model can be used as a cost effective and grower friendly dosing model in high density apple orchards in South Africa. The model uses a constant water delivery rate and pesticide concentration to treat multiple orchards differing in size by only adjusting forward speed. The more conservative TIRI benchmark model is recommended for evaluating mancozeb depositions, rather than the qPCR benchmark model, since apple scab is such a high risk and destructive disease. The mancozeb deposition benchmark values established in this study will be valuable for assessing the efficacy of spray applications made in future research trials and grower applications. Due to the low benchmark values identified for mancozeb, future studies should also investigate benchmark values for other contact fungicides as well as for mesosystemic and systemic fungicides used for controlling apple scab world-wide.