Browsing by Author "Christie, Charmaine"

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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.