Susceptibility of apple fruits (Malus x domestica Borkh.) to the postharvest pathogen Colletotrichum fioriniae: cultivar differences and correlation with fruit ripening characteristics

Colletotrichum spp. infections are an increasing threat to the fruit production industry worldwide, as a postharvest disease. Gaining knowledge on the susceptibility of apple fruit cultivars to Colletotrichum species is essential for specific breeding programs aiming to decrease the disease susceptibility of commercial apple cultivars. We investigated fruit susceptibility of nine apple cultivars at harvest and after storage, and studied the relationship of the susceptibility with fruit ripening characteristics. An accurate artificial inoculation technique, by wounding and injecting the spore suspension, was used to consistently follow-up the relative susceptibility symptoms of fruits. The nine apple cultivars studied, cvs. Pinova, Nicoter, Nicogreen, Golden Delicious, Gala, Elstar, Jonathan, Idared and Topaz, showed clear differences in susceptibility to C. fioriniae, main species causing problems in Europe, based on lesion expansion growth rate data. Cultivar Nicoter and Nicogreen were evaluated as less susceptible cultivars, while cv. Golden Delicious and cv. Pinova were classified as high susceptible cultivars. After three months of storage all fruit cultivars showed a higher susceptibility than at harvest. We further explored the link between fruit ripening characteristics and postharvest pathogen developments. Results showed significant higher Brix, ethylene production and respiration and significantly lower firmness and titratable acids after storage, with the extent of these changes being cultivar dependent. The ripening parameters respiration and Brix had a positive and the firmness a negative correlation with the lesion expansion growth rate of C. fioriniae after artificial wound inoculations. These experiments help to give a better insight in the susceptibility of different cultivars. Moreover, our data support the hypothesis that the extent of the necrotrophic fungal development depends on the fruit ripening stage.