What causes red drupes?

Red drupe in Prime-Ark 45. Likely due to excessive rain, These drupes never turned black.

Usually all the drupelets on a ripe blackberry fruit are uniformly black. Sometimes the fruit can have drupes that are white, tan or red. White and tan drupelets have been discussed before (https://teamrubus.blogspot.com/search?q=white+drupelet). 

The focus of this post is to discuss the occurrence of red drupes in blackberry fruit. This reddening can occur after and before fruit is harvested.  There are several known causes of red drupes and some conditions that are speculation. No matter what the cause, excessive amounts of red drupes can result in rejection of fruit in wholesale markets.

After harvest

Reversion. This is the most common cause of red drupes.

Symptoms. Drupes are black at harvest and turn red after harvest.

What is known. Research conducted by Max Edgley from the University of Tasmania looked at several factors including nitrogen rates, physical damage during harvest and transportation to the cooler, air temperatures during harvest, slow or fast cooling of fruit after harvest.  

Key Findings from the University of Tasmania study:

• Physical damage (bruising, impact and fruit compression) during harvest and shipping is the main cause of red drupelet disorder
• High nitrogen fertigation during harvest can significantly increase the amount of fruit with red drupelet disorder post-harvest
• Fruit core temperatures exceeding 23C (73F) at harvest significantly increase the amount of red drupelet post-harvest
• Harvest times, techniques, and shipping conditions can be manipulated to reduce incidence of red drupelet disorder
• A step-cooling process reducing the rate of cooling post-harvest has been effective in reducing incidence of the disorder
• They also found that early season fruit is more prone to this disorder. 

https://www.utas.edu.au/tia/research/research-projects/projects/red-drupelets-on-blackberries2

https://www.utas.edu.au/__data/assets/pdf_file/0015/1110435/Blackberry-red-drupelet-fact-sheet-2017.pdf 

Addedum from Alejandra Salgado (PhD U Ark):  a weak cell wall can also cause reversion. 

See also blog post on red drupe reversion here: https://teamrubus.blogspot.com/search?q=white+drupelet


Before harvest

Redberry mite. (Redberry disease).

Symptoms. Fruit that are infected with Redberry mites have drupes never turn black. Red drupes are hard.

What is known. The mites inject a toxin into the base of the drupelets and as a result, the drupelets fail to develop normally.  Redberry mites are uncommon in the eastern United States. If you suspect they are the cause of red drupelets, samples should be submitted to your local plant disease and insect clinic for diagnosis.

Here is more information from:

Utah State University.

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2944&context=extension_curall
University of California

http://ipm.ucanr.edu/PMG/r71400111.html 

Excessive rain.

Symptoms. Red drupes are soft and never turn black.

What is known. There have been reports of significant red drupelet in parts of NC this year in fruit that are otherwise ripe and ready to harvest. Many, but not all of the fields have tunnels over them. The cultivar Prime-Ark 45 is seeing most of this disorder this season on the floricane crop. We cannot confirm that the rain is causing the red drupes, it is just our best guess at this time as the above (reversion and redberry mite) have been ruled out.

Viruses?

Red drupes have not been identified as symptoms associated with viruses to the best of our knowledge. 


Authors: Gina Fernandez, Hannah Burrack and Penny Perkins-Veazie and in consultation with multiple experts around the world.