Optimisation of postharvest drench application of fungicides on citrus fruit

Christie, Charmaine (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

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.

AFRIKAANSE OPSOMMING: Suid-Afrika is wêreldwyd die tweede grootste uitvoerder van vars sitrus vrugte. Vertragings vanaf oes na die paklyn, vir onder andere ontgroening, kan lei tot aansienlike na-oes verliese, veral weens groenskimmel wat deur Penicillium digitatum (PD) veroorsaak word. ‘n Voor-paklyn stortstelsel is 'n belangrike instrument om na-oes verliese te beperk voor ontgroening, wat 'n gunstige omgewing vir infeksie ontwikkeling bied. Suurvrot, wat veroorsaak word deur Geotrichum citri-aurantii (GC), raak ‘n probleem in tye van hoë reënval en die beskikbaarheid van doeltreffende swamdoders teen hierdie patogeen is beperk. Thiabendazole (TBZ), pyrimethanil (PYR), guazatine (GZT) en 2,4-dichlorofenoksie-asynsuur (2,4-D) word aangewend in die voor-paklyn stortstelsels in Suid-Afrika vir die beheer van na-oes siektes op sitrus. Hierdie proses is nog nie gestandariseer nie en GZT gebruik is tot sekere uitvoer markte beperk; GZT is die enigste swamdoder wat effektief in die stortstelselmengsel teen suurvrot is. Die doel van hierdie studie was om die begrip van stortaanwending te verbeter in terme van die invloed van infeksie ouderdom, vrugoriëntasie, blootstellingstyd aan behandeling en die toevoeging van benatters en ontsmettingsmiddels om siektebeheer te verbeter. Suurlemoen, Satsuma manderyn en navel lemoen vrugte is gestort met TBZ en PYR (1000 μg.mL-1 elk) met verskillende blootstellingstye (14 s, 28 s en 56 s) en geïnokuleer met groenskimmel 0, 6, 12, 30, 42, 48 en 54 h voor (kuratief) en 24 uur na (beskermend) behandeling. Spoorvormingsinhibisie en residu-lading is ook geëvalueer. Suurlemoen en Satsuma manderyn vrugte is aan 'n laer stortingsvolume in vergelyking met navel lemoen vrugte blootgestel (26.5 en 64.3 L.min⁻¹, onderskeidelik). Vruglotverskille het ‘n beduidende rol in groenskimmel beheer met suurlemoen en Satsuma mandaryn vrugte gespeel, en behandelings van onderskeidelik 33.1 – 44.5 en 23.8 – 32.1 h oue infeksies was nodig om 90% beheer te kry, afhangende van die vruglot. Blootstellingstyd het eers beduidend geraak met ≥ 30 h ou infeksies in navel lemoen vrugte, met die hoër stortvolumes, met beheer wat vinniger afneem vir vrugte gestort met korter blootstellingstyd. Beheervlakke het verskil van 30,2% op 54 h oue infeksies en > 90% beheer is behaal op vrugte wat binne 27 h na infeksie behandel is. Beskermende beheer was oor die algemeen effektief (> 90%). Hierdie resultate ondersteun die aanbeveling om alle sitrus tipes ≤ 24 h na-oes te stort om so die risiko vir groenskimmel ontwikkeling te beperk. Spoorvorminginhibisie was in die algemeen swak (<50%). Om die effek van ‘n benatter te bepaal, is Valencia lemoen vrugte met TBZ, PYR en 2,4-D (1000, 1000 en 250 μg.mL-1, onderskeidelik) teen 41.0 L.min⁻¹ vir 18 s met verskillende benatter konsentrasies (0.0, 0.025, 0.05, 0.1 en 0.2 μl.mL⁻¹) gestort. Geen verskille is ondervind behalwe 'n negatiewe uitwerking op residu-lading, neerslag hoeveelheid en groenskimmel beheer teen die hoogste getoetste benatter konsentrasie. Vrug oriëntasie het egter ‘n beduidende rol gespeel, met hoër residu-vlakke, kuratiewe groenskimmel beheer, neerslag hoeveelheid en kwaliteit op vrugte wat kelk-end opwaarts gewys het, in vergelyking met die teenoorgestelde end van dieselfde vrug. Siende dat suurvrot inokulumvlakke in die stortstelsel oplossing saam met stof van vrugte tydens stortaanwending kan opbou, is chloor (Cl; 80 μg.mL-1) en waterstofperoksied / asynsuur (HPPA; 0,6%) se doeltreffendheid vir beheer van GC spore (CFU.mL-1) in oplossing vergelyk, sowel as om te toets dat die swamdoderkonsentrasie en doeltreffendheid daarvan nie verminder word nie. Gewonde navel lemoen vrugte is met TBZ, PYR, GZT en 2,4-D (1000, 1000, 500 en 250 μg.mL-1, onderskeidelik) gedurende kommersiële pakhuisproewe gestort, met Cl of HPPA (80 μg.mL-1 en 0.6%, onderskeidelik) wat toegedien is as skokbehandelings in die oplossing by elke vrugkratstapel (twee kratte) wat kratnommers 1, 50, 100 en 150 ingesluit het. Swamdoderbehoud en groenskimmel infeksie (vanweë omgewingsinokulum) was soortgelyk ongeag die eenwoordigheid van ontsmettingsmiddel. Groenskimmel infeksie het verhoog by krat 150 (4.6 – 5.4% verskil). Tydens in vitro proewe is verskillende ontsmettingsmiddel konsentrasies (0, 20, 40, 60 en 80 μg.mL-1 Cl of 0.00, 0.01, 0.10, 0.30 en 0.60% HPPA) met 'n mengsel van TBZ, PYR en 2,4-D (1000, 1000 en 250 μg.mL-1, onderskeidelik) en GC spore (≈ 3,175 × 104 spore.mL-1) gekombineer vir 1, 3 en 60 min blootstellingtyd. Swamdoderkonsentrasies is oor die algemeen nie beïnvloed deur ontsmettingsmiddels nie, maar ontsmettingsmiddels het egter nie in oplossing bly voortbestaan na 60 min blootstelling nie. HPPA kon suurvrot inokulum heeltemal uitwis (0,0 CFU.mL-1) na 1 – 3 min en Cl was nie so effektief in die hoë pH vlak (> 10) van die oplossing nie. Tydens in vivo proewe is groenskimmel geïnokuleerde (24 h voor behandeling) en gewonde vrugte gestort met ‘n mengsel van TBZ, PYR en 2,4-D (1000, 1000 en 250 μg.mL-1, onderskeidelik) en GC spore (soortgelyk aan in vitro proewe) wat 80 μg.mL-1 Cl of 0,3% HPPA bevat het, asook 0, 500 of 1000 μg.mL-1 kaolin. Die ontsmettingsmiddel het meestal geen negatiewe invloed op swamdoder konsentrasie en groenskimmel beheer gehad nie, alhoewel HPPA behandelings suurvrotbeheer op Valencia en Nadorcott manderyn vrugte verbeter het, asook verbeterde groenskimmelbeheer op Nadorcott manderyn vrugte. In sommige gevalle het die laer vlak van kaolin (500 μg.mL-1) gelei tot verbeterde groenskimmel en suurvrot beheer. Tydige stortbehandeling (≤ 24 h) lewer doeltreffende groenskimmel beheer, terwyl blootstellingstyd en benatter konsentrasie verder ondersoek moet word om swamdoderwerking en verspreiding deur dig-verpakte vrugkratte te verbeter. Met die wete dat die pH vlakke van stortstelseloplossings nie gereguleer word nie, is HPPA ‘n beter ontsmettingsopsie teen die hoë pH-vlakke (> 10) in stortoplossings. Verdere navorsing is nodig om skokbehandelingsintervalle en moontlike newe-effekte van ontsmettingsmiddels te bepaal. Behoorlike vermenging van oplossings is ook noodsaaklik vir verbeterde swamdodereenvormigheid in oplossing en die daaropvolgende residu-lading.

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