Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by browse.metadata.advisor "Du Plooy, G. W."

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    Further optimisation of in-line aqueous application of imazalil to control citrus green mould caused by Penicillium digitatum
    (Stellenbosch : Stellenbosch University, 2017-03) Savage, Catherine; Du Plooy, G. W.; Erasmus, A.; Lennox, C. L.; Fourie, P. W.; Stellenbosch University. Faculty of AgriScience. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: South Africa has a successful citrus export industry. A threat to fresh citrus fruit is the fungal pathogen Penicillium digitatum causing green mould. Imazalil (IMZ) is the most important fungicide to combat green mould. Solution pH and temperature, and exposure time of the fruit to the solution, are important when using the sulphate form of IMZ. Research has increased our understanding of IMZ use, but further variables need to be investigated, along with an alternative application method. The control of green mould infection and sporulation by IMZ were tested using a heated flooder. Solution variables included the effects of pH (3; 4; 5; 6), temperature (45; 55; 65°C), and concentration (250 or 500 μg.mL-1) in a time of 8 s. Residues increased with increasing pH, temperature and concentration. The majority average residues loaded were between 0.4 and 3.0 μg.g-1. Treatments at pH 6 loaded higher residues at 55 and 65°C, where the maximum residue limit (MRL) of 5 μg.g-1 was almost always exceeded. The flooder loaded adequate residues, offering good curative and protective control. Sporulation inhibition of green mould was also linked to residues, and complete inhibition was achieved at the higher residue levels. The flooder was an effective applicator of IMZ. The fungicide bath is the most common IMZ application method in South Africa. The ability of IMZ to control green mould was investigated in a cold bath of 10°C and compared to ambient temperature and 35°C baths. Solution temperature had no significant effect on IMZ’s ability to cure 24 hr old green mould infections with all temperatures providing control above 80%. Sporulation inhibition and residue loading increased as solution pH, temperature, and exposure time increased. Sporulation inhibition was < 50% in pH 3 baths, irrespective of temperature, complete inhibition was obtained at 35°C and pH 6, but the IMZ MRL was exceeded at longer exposure times (> 45 s). The survival of Rhizopus stolonifer was studied in vitro at various water temperatures (10°C to 65°C) for exposure times of 1 or 60 min, and after a pasteurisation step. Sub-treatments included the addition of IMZ fungicide or green mould spores, with IMZ seemingly having a significant effect on Rhizopus spore survival. The same was not true for solutions at temperatures below 35°C, however, temperatures of 45, 55 and 65°C, particularly after a 60 min exposure, caused a significant reduction in Rhizopus spore viability. Complete Rhizopus eradication was achieved with 65°C and the pasteurisation step. In order to control fungal contaminants in the fungicide bath, packhouses need to apply IMZ in heated solutions (circa 45°C) and/or pasteurize fungicide baths overnight. Imazalil residue levels on citrus can be increased by increasing solution pH, temperature, concentration or exposure time. Most treatments gave excellent infection control and only a low residue is necessary to cure or prevent a green mould infection. Residue levels were closely linked to the level of sporulation inhibition achieved. Both the flooder and dip tank offered good green mould control. Contaminants that build up in solution can be eradicated at high temperatures.