Investigation of environmentally friendly postharvest treatments to extend the storage life of soft citrus cv. Clementine

Abstract

The aim of this thesis was to develop an effective integrated disease management (IDM) program to reduce Penicillium digitatum (Pers.: Fr.) Sacc. and P. italicum Wehmerdecay in Clementine mandarins, while still maintaining overall fruit quality. Three alternative treatments were investigated for inclusion into the IDM programme: hot water dips (HWD), biocontrol and modified atmosphere packaging (MAP). In Chapter 3, the effect of hot water dips on host defence reactions was investigated in order to improve host resistance to green- (P. digitatum) and blue mould (P. italicum) in fruit. The 70 kDa heat shock protein (HSP) was used as target molecule to detect increased defence reactions in fruit subjected to various HWD regimes. The effect of these heat treatments on total phenolic content (TPC) and phenylalanine ammonia-lyase (PAL) activity was also investigated. Maximum 70kDa HSP production occurred in fruit subjected to 52°C for 5 min. four days after exposure to heat treatment. Treatments exceeding 55°C resulted in peel damage. Results further indicated that the enzyme PAL was also involved in the heat shock response and that increased HSP production was assiociated with reduced PAL activity. Although increased HSP production was induced by some treatment combinations in the IDM trial, it could not be correlated with protection against decay in this study. The efficacy and mode of action of the bacterial biocontrol agent, Bacillus subtilis (Ehrenberg) Cohen strain PPCB002 (PPCB002), against green- and blue mould was investigated in Chapter 4. Although in vitro studies indicated that PPCB002 was effective against Penicillium pathogens through a variety of mechanisms, similar efficacy could not be achieved during in vivo experiments with artificially inoculated ‘Nules’ Clementine. Therefore, a commercially formulated product, Innovacure (Candida saitoana Nakasa and Suzuki), was included in further investigations. The Innovacure formulation, as well as its combination with PPCB002 could successfully reduce disease incidence and severity of both pathogens. The combination of the two biocontrol formulations was selected for incorporation into the IDM trial, since the two products or organisms have different modes of action. Finally, a suitable MAP type was selected in Chapter 5 and combined with the previously selected hot water dip (HWD) and biocontrol treatments. A bioriented polypropylene (BOPP) packaging type with 0.00565% perforated material was selected among three different packaging materials as the most suitable for Clementine. Although desiccation could effectively be reduced by this packaging type, it was associated with high levels of decay. Under fluctuating conditions, extreme O2 and CO2 levels also developed within the packaging which resulted in peel damage on some fruit. This MAP type is therefore not recommended for Clementine. A HWD at 52°C for 5 min. combined with a mixture of PPCB002- and Innovacureformulations, was effective against green and blue mould under lower-disease pressure conditions. However, rind breakdown was detected on some of the fruit subjected to this treatment combination. In addition, the biocontrol treatment showed variability in performance and lack of curative action, especially at ambient temperature (Chapter 3). It is therefore recommended that future studies continue with either the HWD treatment or the Innovacure formulation, in combination with other, more effective biocontrol formulations and/or other disease control strategies.