Building resilience to drupelet disorder on rubus (RB14003)
What was it all about?
This project investigated the causes, mechanisms and potential management strategies for red drupelet reversion in blackberries. Red drupelet reversion is a disorder of blackberries where drupelets that are black revert to red, usually after the fruit has been harvested and placed into cool storage. This reduces both the visual and physical quality of the fruit.
The project ran from 2015 to 2019 and involved laboratory trials that investigated the underlying physical and chemical changes associated with the disorder, and then field trials that assessed the effects of nitrogen application rate, harvest technique, environmental conditions at harvest and postharvest storage conditions on the incidence and severity of the reversion.
From the laboratory trials, the project found that colour change is caused by a decrease in anthocyanin concentration (the compound that pigments food with red/purple/blue colouring), reduced cellular integrity, reduced drupelet firmness and lower pH levels. The researchers found that susceptibility to the reversion disorder is influenced by genetics, with evidence that cultivar firmness, cell wall formation and susceptibility to post-harvest weight loss can influence incidence and severity of reversion.
The field trials then confirmed that stresses, such as warm temperatures during harvest, are linked to high rates of red drupelet reversion, being linked to an increase in the mechanical injuries incurred during harvest and handling. The project team also found that an increase in drupelet reversion can also be caused by excessively high nitrogen application rates and rapid temperature changes in postharvest storage.
The project made the following recommendations for growers:
- Reduce double and rough handling of fruit whilst harvesting. If practical, fruit should be harvested directly into punnets and care should be taken during transport to minimise damage from vibrations.
- Limit handling of blackberries at higher temperatures, as temperatures exceeding 23°C during handling and transport will significantly increase the incidence and severity of red drupelet disorder. If practical, fruit should be harvested during the early morning or evening, and harvest avoided completely on extremely warm days if possible.
- Reducing mechanical injury to fruit through punnet design and postharvest technologies should be a priority. Unnecessary fruit-on-fruit contact could be reduced by using punnets that only contain one layer or fruit.
- Once cooled, fruit should remain cool to reduce susceptibility.
For the industry as a whole, the researchers suggested that development of varieties with low susceptibility to red drupelet disorder should be pursued as skin firmness, texture and water loss are correlated with susceptibility to the disorder.
This project was a strategic levy investment in the Hort Innovation Raspberry and Blackberry Fund
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