Avovantage – On-orchard fruit rot mitigation

Completed, Fruit quality, Fruit rots

Summary

The outcomes of this project aimed to reduce the percentage of unsound fruit as well as further develop best practice guidelines based on practical, sustainable and effective disease management strategies for the management of fruit rots on orchard. The work was being led by Plant & Food Research and ran from 2019 to 2022.

Some key findings have been made which growers can implement to improve fruit quality, however further work is required validating and extending practices.

Fruit rots caused by fungi are a major cause of quality issues in the export market for New Zealand avocados. Rots caused by at least five fungal species have become a significant concern.

Detailed information from 10 tagged trees on each of the 17 Bay of Plenty ‘Hass’ avocado orchards has been collected over the last three years, along with orchard management practices and library tray fruit assessments to measure rot incidence. Results support current best practice and identify some important new avenues to follow up:

• Key findings

  • There is a high degree of variability in postharvest rot incidence between individual trees within blocks over time. For the orchard manager this means it’s not about rot-challenged blocks – it’s all about rot-challenged trees within blocks. For example, from up to 170 trees across all participant blocks, 31 trees were classed as consistently rot-challenged for three consecutive years in a row. In contrast, there were 27 trees that were consistently classed as rot-managed, with 5 percent or less postharvest rots for three consecutive years in a row.

• • A significant positive link was observed between canopy density and postharvest rots. The greater the canopy density the higher the risk of postharvest rots. A visual tool has been developed for growers to assess canopy density and a threshold score of 60 or less was recommended to rot-challenged participants to reduce fruit rots. The full guide is available from the NZ Avocado industry website by searching for ‘visual canopy density handout’.

• • A significant negative link was observed between the number of copper-based sprays applied per season and postharvest rots. Blocks with less postharvest rot issues applied more copper-based sprays than rot-challenged blocks. Blocks that applied six or more copper-based sprays had fewer postharvest rot issues. The spray diary analysis also highlighted what can happen if best practice was not followed. One rot-managed block substantially decreased its copper use over the three-years of this project, from eight applications (2019, Year 1) to just one (2022, Year 3). As a consequence, percent of unsound fruit from the 10 trees in this orchard block increased from 5% (Year 1) to 43% (Year 3). The correlation between copper use and unsound fruit isn’t perfect so other management practices are important to compliment the use of copper sprays.

• • Potential new information on the timing of copper-based sprays. Analyses of spray diaries showed that copper-based spray applications largely coincided with pesticide applications made by the participating orchard managers rather than specifically targeting postharvest rot control. One pattern that emerged from this analysis was that in contrast with rot-challenged blocks, the rot-managed blocks generally applied copper-based sprays in December (after flowering), February, March, April, May or July, August and September (pre-harvest).  The timing of copper based fungicide application is likely relevant to fungal rot control and these windows of application provide a starting point for consideration.

• • A new field trial was established by the Avovantage project to field test selected commercially available registered biofungicides for postharvest rot reduction. The goal of this part of the project was to identify a product/s that could replace copper-based sprays in the near future. Fruit from this trial will be harvested late 2022 and assessed for rots with results reported shortly after.

• • One key finding from the biofungicide trial was that the dominant fungal pathogens present on flowers belonged to the Colletotrichum acutatum complex of species which is responsible for the majority of postharvest fruit rots. This indicates that fungal infection may occur very early in fruit development. Future studies are needed to confirm a positive link between flowering infections and postharvest rots.

• • Identification of a link between pathogen inoculum in the tree and risk of postharvest rots. In one season it was identified that rot-challenged trees had more mummified fruit and old floral tissues (necrotic inflorescences) left in the tree compared with rot-managed blocks. This was not consistent for all seasons but it is known that dead necrotic tissue harbours fungal rot species highlighting the need for it to be managed through regular pruning. 
• • In the final year of the project the Avovantage and soil characterisation projects were combined to investigate links between fruit macro and micro nutrients and unsound fruit. There has been only one season of data but the research team identified a significant positive link between fruit skin nitrogen content and postharvest rots. The fruit nutrition and postharvest rot findings from the Avovantage sites needs to be repeated to have confidence that this relationship is consistent between growing seasons.