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Short and long term solutions for postharvest decay of apple by gray mold

Book - Dissertation

Apple (Malus x domestica) is a seasonal fruit that can be harvested in autumn. To provide consumers with a year round supply of high quality apples the fruit is stored with controlled atmosphere at low temperature. During storage fungal-caused decay may occur, resulting in losses. These losses are currently controlled through chemical pesticides, but increasing public concerns about safety for people and environment are putting this practice under pressure. Therefore, we wanted to investigate hot water dipping of apple fruit as a viable alternative. This work aims at filling in knowledge gaps and bridging the gap between practical and fundamental knowledge on the subject. To this end, we first conducted an exploratory study on which postharvest pathogens prevail on apple and if pathogen incidence could be linked to fruit characteristics in chapter 2. Results showed that Penicillium spp. had the highest incidence followed by Fusarium, Botrytis, Neonectria and Monilinia. Fruit mass significantly correlated with rot incidence, most likely due to a relation between fruit mass and pre/postharvest factors such as fruit ripening stage. In chapter 3 and 4 we investigated how to best carry out artificial inoculation studies by testing whether the used disinfection method and the position used for inoculation on the fruit has an effect, respectively. Two prevalent disinfection methods, i.e., ethanol and sodium hypochlorite, were compared to no disinfection and results demonstrate that both treatments significantly affected the fruit surface and physiology. Comparing inoculation positions on the fruit led to the conclusion that susceptibility is linked to cell density, likely due to a link with biosynthetic capacity per tissue volume. In chapter 5 we then applied this knowledge to perform an RNAseq time course investigation of apple inoculated with B. cinerea. Inoculated fruit displayed a clear activation of the main hormonal pathways jasmonate, ethylene and salicylate, and had a strong induction of secondary metabolic pathways for the biosynthesis of phenylpropanoids, terpenoids and alkaloids. Furthermore, timely activation of terpenoid biosynthesis seemed to be linked to reduced susceptibility. In chapter 6 we then applied this knowledge to investigate the effect of a hot water dip (HWD) on fruit defense and see if key defense responses exhibit a dose-response relationship with duration of the treatment to allow optimization of this parameter. Unfortunately, our results indicate that, although HWD has a direct effect on the pathogen and provide protection in this way, it had no or even an adverse effect on fruit defense in the case of `Jonagored' apples. Key defense genes such as phenylalanine ammonia lyase and squalene monooxygenase did show a dose-response relationship with duration of the treatment where longer durations led to a higher reduction in expression.
Publication year:2020
Accessibility:Closed