Pre and post-harvest monitoring of Alternaria spp. in orchards with historically high incidence of dry core rot

Since no method to quantify the presence of Alternaria spp. within the seed cavity exist, the aim of this study was to develop and test a method to detect and quantify the accumulation of Alternaria spp on blossoms and in the seed cavity of susceptible Top Red apples. The developed method was subsequently used to firstly determine whether blossom distance from the orchard floor has an influence in the inoculum levels of Alternaria spp., and secondly to monitor the accumulation of Alternaria spp. fungal species in the seed cavity of susceptible cultivars during production and storage.

We quantified Alternaria spp. and specifically Alternaria alternata on blossoms located at different distances from the orchard floor and in the extracted seed cavity of fruit at different maturities or stored for different periods in industry simulated cold storage conditions. The fungal inoculum was quantified based on the amount of DNA present using quantitative real- time PCR.

We could accurately quantify the Alternaria spp. inoculum levels on blossoms and in the seed cavity. There is an inoculum gradient from the orchard floor to the top of the tree with higher inoculum present in blossoms closest to the floor, indicating the importance of the orchard floor as inoculum source. There was also a gradual build-up of Alternaria spp. in the seed cavity up to two months in storage, after which there was a gradual decline. The decline could potentially be attributed to the methodology used will be adapted in future analysis. In all samples analysed there were no significant difference observed between total Alternaria spp present and Alternaria alternata, indicating that the predominance of Alternaria alternata.

The aim of the project was to develop methods to detect and quantify the most prominent Alternaria spp. on blossoms and in the seed cavity of Top Red apples. In this regard the project was successful and could accurately quantify the Alternaria spp. inoculum levels on blossoms and in the seed cavity. Furthermore, we could show that there is an inoculum gradient from the orchard floor to the top of the tree with higher inoculum present in blossoms closest to the floor. There was also a gradual build-up of inoculum within the seed cavity which was highest two months into storage, after which it declined. The decrease in inoculum after two months can be attributed to the experimental layout inadvertently out-selecting fruit with higher inoculum levels and the method will be adapted for future studies. The project has greatly advanced our ability to detect and quantify Alternaria spp. within Top Red production, providing tools to greatly reduce the economic impact of DCR.