Singular and combined effect of postharvest treatments on viability and reproductive ability of phyllosticta citricarpa infections.

Schreuder, Wouter (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

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.

AFRIKAANSE OPSOMMING: Sitrus is wêreldwyd een van die belangrikste vrugtegewasse en word tans geproduseer in meer as 100 lande. Suid-Afrika is een van die grootste uitvoerders van vars sitrus, met ongeveer 40% van die produksie uitgevoer na Europese markte. Pakhuise is onderworpe aan streng programme om uitvoergehalte te beheer deur die handhawing van hoë vrugkwaliteit en voorkoming van na-oes vrugbederf. Sitrus swartvlek (SSV) (veroorsaak deur Phyllosticta citricarpa (McAlpine) van der Aa) is hoofsaaklik 'n kosmetiese siekte wat die uitvoerkwaliteit van vrugte verswak. Alhoewel dit nie na-oes bederf veroorsaak nie, word dit egter in sommige lande beskou as ‘n kwarantyn organisme. Ten spyte van wetenskaplike bewyse tot die teendeel, word handelsbeperkinge teen SSV vrugte steeds opgelê, byvoorbeeld die nul toleransie van SSV letsels op vrugte uitgevoer na die Europese Unie. Produksie areas waar SSV voorkom volg streng vooroes programme om te verseker dat vrugte uit sulke boorde aan hierdie nul toleransie vereistes voldoen. Die grootste gevaar rondom swartvlek is die teenwoordigheid van latente, asimptomatiese infeksies op geoesde en verpakte vrugte Sulke infeksies word soms eers uitgedruk lank na na-oes behandeling, verkoeling en verskeping van die vrugte. Vorige studies het aangedui dat na-oes behandelings simptoomuitdrukking vertraag, met gevolglike swartvlekbeheer deur die vermindering van letseluitdrukking en die lewensvatbaarheid van piknidiospore. Die doel van hierdie studie is om die meer onlangse pakhuis protokolle en swamdoders, asook alternatiewe swamdoders, te evalueer vir hul uitwerking teen latente swartvlek infeksies en hul effek op die reproduktiewe potensiaal van letsels. Vrugte met swartvlekletsels, asook asimptomatiese vrugte met latente infeksies, was onderwerp aan standaard pakhuissanitasie, swamdoderbehandelinge en koelkameropberging (alleenstaande en gekombineer). Daarna was dit geïnkubeer onder omstandighede wat simptoomuitdrukking bevoordeel. Die volledige pakhuisbehandeling, tesame met ‘n kouestoor tydperk, het aansienlike beheer van latente infeksies gegee. Oor die algemeen was die voortplantingsvermoë van letsels baie swak, met minder as 2.1% van alle letsels op beide Valencia en Eureka suurlemoene wat uiteindelik piknidia ontwikkel het. Drie alternatiewe alleenstaande behandelings het potensiaal getoon vir die beheer van latente infeksies: FLU, kaliumsorbaat and Propirly 270 EC (PPZ + PYR). FLU en Propirly 270 EC het matige tot aansienlike beheer oor latente infeksies op beide Valencia lemoene en Eureka suurlemoene uitgeoefen. Kaliumsorbaat het matige beheer oor latente swartvlek infeksies op beide Valencia lemoene en Eureka suurlemoen uitgeoefen. Die kombinasie van epidemiologiese vereistes vir piknidiospoor vrystelling, tesame met die resultate van hierdie proewe, dui daarop dat ge-oesde vrugte is nie 'n noemenswaardige epidemiologiese weg bied vir die verspreiding van swartvlek nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101054
This item appears in the following collections: