Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by browse.metadata.advisor "Erasmus, A."

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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.
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    Optimisation of postharvest drench application of fungicides on citrus fruit
    (Stellenbosch : Stellenbosch University, 2016-03) Christie, Charmaine; Erasmus, A.; Fourie, P. H.; Lennox, C. L.; Stellenbosch University. Faculty of Agrisciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: South Africa is the 2nd largest exporter of fresh citrus, after Spain, worldwide. Delays to the packline, i.e. degreening, can result in substantial postharvest decay such as green mould caused by Penicillium digitatum (PD). Pre-packline aqueous fungicide drench application is an important tool to minimize postharvest losses before degreening, which provides a favourable environment for infection. Sour rot, caused by Geotrichum citri-aurantii (GC), becomes an infection risk after rainfall and the availability of effective fungicides against this pathogen is limited. Thiabendazole (TBZ), pyrimethanil (PYR), guazatine (GZT) and 2,4-dichlorophenoxyacetic acid (2,4-D) are applied during drenching in South Africa for the control of postharvest diseases on citrus, although this application has not yet been standardized and guazatine use is restricted to certain export markets; GZT is the only fungicide in the drench mixture that is effective against sour rot. Therefore the aim of this study was to improve our understanding of drench application in terms of the influence of infection age, fruit orientation (pole), treatment exposure time and the addition of adjuvants and sanitisers on disease control. Lemon, Satsuma mandarin and navel orange fruit were drenched with TBZ and PYR (1000 μg.mL-1 each) at different exposure times (14 s, 28 s and 56 s) and inoculated with PD 0, 6, 12, 18, 24, 30, 42, 48 and 54 h before (curatively) and 24 h after (protectively) treatment. Sporulation inhibition and residue loading were evaluated. Lemon and Satsuma mandarin fruit were exposed to a lower drench volume compared to navel orange fruit (26.5 and 64.3 L.min⁻¹, respectively). Batch differences played a significant role in green mould control with lemon and Satsuma mandarin fruit requiring treatment by 33.1 to 44.5 h and 23.8 to 32.1 h infection age, respectively, to gain 90% control. Exposure time only became significant with ≥ 30 h old infections on navel orange fruit at the higher drench volume used, with control declining more rapidly for fruit drenched at shorter exposure times. Control on navel orange fruit differed as much as 30.2% between exposure times with 54 h old infections and > 90% control was achieved by drenching fruit before 27 h. Protective control was generally effective (> 90%). These results support the proposition to drench all citrus types ≤ 24 h in order to reduce the risk for green mould decay development as sporulation inhibition was poor (< 50%) and fruit batches differed as much as 8 to 12 h in infection age for similar control levels. Valencia orange fruit were drenched with TBZ, PYR and 2,4-D (1000, 1000 and 250 μg.mL-1, respectively; calyx-end facing upward, sideways and downwards) at 41.0 L.min⁻¹ for 18 s with different adjuvant concentrations (0.0, 0.025, 0.05, 0.1 and 0.2 μl.mL⁻¹). Almost no differences were evident between concentrations, other than a negative effect on residue loading, deposition quantity and green mould control at the highest adjuvant concentration tested. Fruit orientation was however significant, with fruit facing calyx-end upward resulting in higher residue levels, curative green mould control, deposition quantity and quality compared to the stylar-end. Since sour rot inoculum levels can accumulate in the drench solution with dirt from fruit during drenching, Chlorine (Cl; 80 μg.mL-1) and hydrogen peroxide/peracetic acid (HPPA; 0.6%) efficacy was compared for the control of GC spores (CFU.mL-1) in solution without reducing fungicide persistence and efficacy. Wounded navel orange fruit were drenched with TBZ, PYR, GZT and 2,4-D (1000, 1000, 500 and 250 μg.mL-1, respectively) during commercial packhouse trials with Cl or HPPA (80 μg.mL-1 and 0.6%, respectively) used as shock treatments at each bin stack (two bins) containing bin no. 1, 50, 100 and 150. Fungicide persistence and green mould infection (environmental inoculum) was similar regardless of whether sanitisers were present or not. Green mould infection increased by bin 150 (4.6 – 5.4% difference). Different sanitiser concentrations (0, 20, 40, 60 and 80 μg.mL-1 Cl or 0.00, 0.01, 0.10, 0.30 and 0.60% HPPA) were combined with a mixture of TBZ, PYR and 2,4-D (1000, 1000 and 250 μg.mL-1, respectively) and GC spores (≈ 3.175 × 104 spores.mL-1) for 1, 3 and 60 min exposure during in vitro trials. Fungicide concentration was generally not influenced by sanitisers although sanitisers, however, did not persist after 60 min in solution exposed to fungicides. Only HPPA could completely reduce sour rot inoculum (0.0 CFU.mL-1) after 1 – 3 min as Cl was not as effective at the high pH levels (> 10) of the solution. During in vivo trials, green mould inoculated (24 h before treatment) and wounded fruit were drenched with TBZ, PYR and 2,4-D (1000, 1000 and 250 μg.mL-1, respectively) and GC spores (similar to in vitro trials) containing either 80 μg.mL-1 Cl or 0.3% HPPA with the addition of 0, 500 or 1000 μg.mL-1 kaolin, used to simulate dust accumulation during drenching. Sanitiser addition mostly did not affect solution concentration and green mould control, although HPPA treatments improved sour rot control on Valencia and Nadorcott mandarin fruit and resulted in improved green mould control on Nadorcott mandarin fruit; the lower level of kaolin (500 μg.mL-1) tested in this study improved green mould and sour rot control in some cases. Timeous drench application (≤ 24 h) provides effective green mould control whereas exposure time and adjuvant concentration requires further investigation in order to improve fungicide retention and distribution throughout highly congested fruit bins. Since drench pH is not regulated, HPPA was superior to Cl at high pH levels (> 10) for reducing sour rot infection and inoculum levels in solution, although further research is required to determine shock treatment intervals (within 60 min) required and potential side effects.
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    Singular and combined effect of postharvest treatments on viability and reproductive ability of phyllosticta citricarpa infections.
    (Stellenbosch : Stellenbosch University, 2017-03) Schreuder, Wouter; Fourie, P. H.; Erasmus, A.; Du Plooy, W.; Lennox, Cheryl L.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Citrus is one of the most important fruit crops globally and is currently being produced in over 100 countries. South Africa is one of the biggest shipping exporters of fresh citrus, with approximately 40% exported to European markets. Packhouses have rigorous export quality control programmes to maintain quality and prevent postharvest decay during the shipping period. Citrus black spot (CBS) (caused by Phyllosticta citricarpa (McAlpine) van der Aa) is mostly a cosmetic disease that reduces the aesthetic quality of fruit and does not cause postharvest decay. However, P. citricarpa is regarded as a quarantine organism in certain countries, and despite scientific evidence to the contrary, trade restrictions are imposed, such as the zero tolerance for CBS lesions on fruit exported to European Union. Whilst fruit may be exported from areas where CBS occurs, very strict preharvest control programmes must be followed to ensure fruit production in orchards meet the zero tolerance requirements. The biggest danger surrounding CBS is the presence of latent, asymptomatic infections in harvested and packed fruit, which can sometimes manifest on the fruit long after packhouse treatment, cold storage and shipping. Previous studies have indicated that postharvest treatments delay symptom expression and control CBS by reducing lesion and pycnidiospore viability. The objective of this study was to evaluate the effect of more recent protocols and fungicides used in packhouses, as well as alternative fungicides, against latent CBS infections, including the reproductive potential of the lesions. Fruit with CBS lesions, as well as asymptomatic fruit with latent infections, were subjected to standard packhouse sanitation, fungicide treatment and cold storage (singularly and combined), and incubated at conditions that enable expression of latent infections. The full packhouse treatment along with storage period gave significantly control of latent infections. The over all reproductive ability of lesions were very low, with less than 2.1% of all lesion that formed on both Valencia’s and Eureka lemons developing pycnidia. Three alternative single treatments showed potential to control latent infections: FLU, potassium sorbate and Propirly 270 EC (PPZ + PYR). Treatment with (respectively) FLU and Propirly 270 EC resulted in moderate to significant control of latent infections on both Valencia oranges and Eureka lemons. Potassium sorbate moderately controlled latent CBS infections in both Valencia oranges and Eureka lemon trials. The combined epidemiological requirements for pycnidiospore release along with results from trials conducted in the current study indicate that harvested fruit is not an epidemiologically significant pathway for the spread of CBS.