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. 

Red drupelet disorder

Red drupelets in blackberry fruit harvested in the afternoon. Upper image is showing more than 15% red drupelets. Lower image is a mild case. Upper image PPV, lower image GF.

Now that we are reaching peak harvest and peak summer temperatures, red drupelet disorder is starting to appear in harvested fruit. After the fruit is harvested, individual black drupetlets will revert back to a red color. Red druplet disorder is also called reversion, reddening or red cell. Penny Perkins-Veazie has worked on this problem for several years. Here are some of her thoughts:

- Harvest before 10 am, get to cooler within an hour of harvest. 
- Remove heat quickly or delay field heat development.
- Forced air cool may need to be set 5 F higher than usual to avoid excess coldness at top of pallet.
-Semi truck loads tend to have more red drupe at front end (near rig) and at top due to air movement and nearness to cooling units.
-Least susceptible variety continues to be Navaho.  Those showing problems are Natchez, Tupi.  Ouachita can be problematic if rainfall has been high and harvest is going into afternoon.
-I'm not sure about PrimeArk 45 or Osage.  The new 'crispy' type does not seem to get red drupe, indicating a firmness/cell wall component in color reversion.
-A 15% red drupe (color reversion) in the load is considered the limit to avoid rejection.  
-Trellising or using E-W row orientation helps to keep fruit in shade longer, decreases exposure to light/heating.  (GF: our shift/RCA trellises seem to have less red and white druplets in general)
-We are not yet sure of production practice issues on red drupe although some reports with tunnel grown blackberries indicate keeping nitrogen rates lower and avoiding heat buildup help with the problem.
-Environmental possibilities for exacerbating the problem seem to be water stress (high rainfall within a few days of harvest, or high rainfall in spring followed by very hot temperatures), nitrogen imbalance, and possibly calcium/potassium availability.

Max Edgley, from the University of Tasmania, has an excellent slide show on this disorder. Max attended the Rubus and Ribes Symposium last year and has shared some of his findings here:

http://www.utas.edu.au/__data/assets/pdf_file/0003/792363/Red-drupelet-disorder-Presentation.pdf