Post-harvest rind pitting studies on ‘Valencia’ orange

Thesis (MSc)--Stellenbosch University, 2016.

ENGLISH ABSTRACT: Post-harvest rind pitting is a non-chilling related physiological rind disorder that affects various citrus cultivars and reduces fruit value. This disorder is characterised by the collapse of the flavedo sub-epidermal cells, however the main cause of this disorder is unknown, but it is aggravated by changes in relative humidity (RH) and rind water status. Studies were conducted on ‘Turkey’ and more susceptible ‘Benny’ valencia oranges in Limpopo and Mpumalanga South Africa. The effect of fruit position, maturity and size on fruit susceptibility to this disorder was investigated and it was found that fruit from the outside of the canopy are more susceptible to this disorder probably due to greater exposure to variation in environmental conditions than fruit from the inside of the canopy. More mature fruit were also found to be slightly more susceptible, however size did not influence incidence of this disorder. Various plant growth regulators were also evaluated to prevent pitting. The application of the synthetic auxins 2,4-dichlorophenoxy acetic acid (2,4-D) and 3,5,6 trichloro-2-pyridiloxyacetic acid (3,5,6 TPA) at 50 % petal drop (2,4-D) or after physiological fruit drop (2,4-D or 3,5,6-TPA) reduced the incidence of post-harvest pitting. Application of s-abscisic acid 1 week before harvest was also found to reduce incidence of this disorder, however gibberellic acid applied in January did not reduce the incidence of post-harvest rind pitting. A systemic fungicide thiabendazole (TBZ) which reduces the incidence of chilling injury also reduced post-harvest pitting incidence if applied before fruit were subjected to stress inducing environmental conditions. Pre-harvest foliar application of TBZ 1 week before harvest and post-harvest dip treatments directly after harvest reduced post-harvest weight loss and incidence of this disorder. A citrus industry survey was conducted to estimate the financial impact of this disorder at foreign and local markets on producers. Markets generating higher prices had a lower tolerance for incidence of post-harvest rind pitting than lower priced markets and are therefore seen as high-risk. Due to this large reduction in market price for fruit with the disorder, treatments found during this study might be cost effective.

AFRIKAANSE OPSOMMING: Gepokteskil is 'n na-oes fisiologiese skildefek wat nie met koueopberging by lae temperature gedurende geassosieer word nie en kan verskeie sitruskultivars affekteer en die waarde van vrugte verminder. Die defek word gekenmerk deur die ineenstorting van die sub-epidermale flavedo selle en alhoewel die hoof oorsaak van die defek nie bekend is nie word dit vererger deur variasie in relatiewe humiditeit (RH) en skil water status in die na-oes omgewing. Hierdie studie was op 'Turkey’ en die meer vatbaar ‘Bennie' valencia lemoene in Limpopo en Mpumalanga Suid-Afrika gedoen. Die effek van voor-oes faktore soos vrug posisie, -volwassenheid en -grootte op die vatbaarheid van vrugte vir hierdie defek is ondersoek. Dit is bevind dat die vrugte aan die buitekant van die blaardak meer vatbaar is vir hierdie defek, waarskynlik as gevolg van groter blootstelling aan variasie in omgewings toestande as vrugte vanaf die binnekant van die blaardak. Daar is ook gevind dat meer volwasse vrugte ʼn hoër vatbaarheid vir die defek het, maar dat vruggrootte dit nie beïnvloed nie. Die effektiwiteit van verskeie plant groei reguleerders om die voorkoms van gepokteskil te verminder is geëvalueer. Die toediening van sintetiese ouksiene 2,4-dichlorofenoksie asynsuur (2,4-D) en 3,5,6 trichloro-2-piridiloksi asynsuur (3,5,6 TPA) by 50 % blomblaarval (2,4-D) of na fisiologiese vrug val (2,4-D en 3,5,6-TPA) verminder die voorkoms van na-oes gepokteskil. So ook het die toediening van s-absisiensuur een week voor-oes die voorkoms van hierdie defek verlaag, maar daarteenoor het gibberelliensuur in Januarie geen effek op die voorkoms van na-oes gepokteskil gehad nie. Dit is al voorheen bewys dat 'n sistemiese swamdoder thiabendasool (TBZ) koueskade kan verminder, en TBZ het ook die voorkoms van na-oes gepokteskil verminder mits dit voor stres-induserende omgewings toestande aangewend word. TBZ toediening een week voor-oes as ʼn blaar bespuiting of as ʼn doop behandeling direk na oes verminder gewig verlies en die voorkoms van die afwyking. ʼn Sitrusbedryf opname was gedoen om die geskatte finansiële impak van gepokteskil in buitelandse en plaaslike markte op produsente te bepaal. Markte wat hoër pryse aanbied het ʼn laer toleransie vir die voorkoms van na-oes gepokteskil as markte wat laer pryse aanbied en word gesien as hoë risiko. As gevolg van ʼn drastiese afname in markprys vir vrugte met gepokteskil kan van die behandelings in die studie moontlik koste effektief wees.