Further optimisation of in-line aqueous application of imazalil to control citrus green mould caused by Penicillium digitatum

Savage, Catherine (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

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.

AFRIKAANSE OPSOMMING: Suid Afrika het ‘n suksesvolle sitrusbedryf. Penicillium digitatum, die swampatogeen wat groenskimmel veroorsaak, is ‘n bedreiging vir vars sitrusvrugte. Imazalil (IMZ) is die belangrikste swamdoder in die bekamping van hierdie patogeen. Die pH en temperatuur van die oplossing, asook blootstellingstyd van die vrugte aan die oplossing is belangrik wanneer die sulfaat vorm van IMZ gebruik word. Navorsing het ons kennis van IMZ verbreed, maar verdere ondersoek van toepaslike veranderlikes is nodig, asook ‘n alternatiewe aanwendingsmetode. Die beheer van groenskimmelinfeksies en sporulasie deur IMZ na aanwending met ‘n verhitte vloedtoediener is ondersoek. Verskillende oplossingsveranderlikes het ingesluit pH (3; 4; 5; 6), temperatuur (45; 55; 65°C) en konsentrasie (250 of 500 μg.mL-1), na ‘n blootstellingstyd van 8 s. Residue het toegeneem met toenemende pH, temperatuur en konsentrasie. Die meeste residuwaardes was tussen 0.4 en 3.0 μg.g-1. Behandelings by pH 6 het hoër residue by 55 en 65°C gelaai, met die maksimum residulimiet (MRL) van 5 μg.g-1 omtrent deurgaans oorskry. Residulading deur die vloedtoediener was genoegsaam en het goeie genesende, sowel as beskermende beskerming verleen. Sporulasie inhibisie van groenskimmel was ook gekoppel aan residulading, met volledige inhibisie teen hoër residuladings. Die vloedtoediener gee effektiewe toediening van IMZ. Die swamdoderbad is die mees algemene IMZ toedieningsmetode in Suid Afrika. Die vermoë van IMZ om groenskimmel te beheer in ‘n koue bad teen 10°C is ondersoek en vergelyk met baddens teen omgewingstemperatuur en 35°C. Die oplossingstemperatuur het geen noemenswaardige effek gehad op die vermoë van IMZ om 24 uur-oue groenskimmel infeksies te beheer nie, met alle temperature wat tot meer as 80% beheer gelei het. Sporulasie inhibisie en residulading het toegeneem met toenemende pH en temperatuur van die oplossing, asook blootstellingstyd. Sporulasie inhibisie in pH 3 baddens was < 50%, ongeag die temperatuur, met volledig inhibisie behaal teen 35°C en pH 6, alhoewel die IMZ MRL oorskry is teen langer blootstellingstye (> 45 s). Die in vitro oorlewing van Rhizopus stolonifer is bestudeer teen verskeie watertemperature (10°C to 65°C) en vir blootstellingstye van 1 of 60 minute, asook nà ‘n pasteurisasie stap. Tussenbehandelings het die byvoeging van òf IMZ òf groenskimmelspore ingesluit, met IMZ wat oënskynlik ‘n noemenswaardige effek het op spooroorlewing gehad het. Dit het nie gegeld vir oplossings onder 35°C nie, maar temperature van 45, 55 en 65°C met veral 60 min blootstelling het ‘n noemenswaardige verlaging in Rhizopus spooroorlewing tot gevolg gehad het. Volledige uitwissing van Rhizopus is behaal met 65°C en die pasteurisasie stap. Ten einde swamkontaminasie in die swamdoderbad te beheer, behoort pakhuise IMZ in verhitte oplossings (circa 45°C) aan te wend en/of moet hulle oornag pasteuriseer. Imazalil residuvlakke op sitrus kan verhoog word met verhoging van die oplossing se pH, temperatuur, konsentrasie of verlenging van blootstellingstyd. Die meeste behandelings gee uitstekende infeksiebeheer en slegs ‘n lae residuwaarde is voldoende om ‘n groenskimmelinfeksie te genees of voorkom. Sporulasie inhibisie, daarteenoor, is nòù gekoppel aan die residuvlakke. Beide die verhitte vloedtoediener en die swamdoderbad het goeie groenskimmelbeheer gegee. Die opbou van kontaminante in die oplossing kan uitgewis word by hoë temperature.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100931
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