- Author: Mark Bolda
The savage heat of the past few weeks has really wreaked havoc on our blackberries. Normally thought of as being fairly stalwart sorts of plants, relatively easy to grow with few disease and insect issues, blackberries seem to be revealing themselves as a bit sensitive to high temperatures.
Not only are we seeing many, many cases of white druplet and subsequent necrosis (first photo), but also leaves of plants, especially at the margins (second photo), are being burned. Close readers of this space will know that white druplet is caused by intense ultraviolet radiation, and that the marginal burn of leaves probably means that the plant is not keeping up in moving sufficient water to its extremities.
Most galling however has been the large amount of red druplet disorder, better known as reversion. As one can see from the third photo below, reversion is a nuanced reddening of pretty well the whole fruit (as opposed to the starkly demarcated reddening caused by redberry mite - see elsewhere in this blog). This occurs post-harvest, meaning the fruit looks just fine at harvest and only reddens when it's been in the cooler for a bit.
It's almost certain that this fruit reddening is being caused by rapid and large changes in temperature. Research has shown that fruit with an internal temperature of over 72.5o F followed by a fairly rapid drop to cold is most likely to experience red druplet disorder. Given that in our pronounced heat spell over the past few weeks, with temperatures in production tunnels up to and even exceeding 110o F (see photo below), and movement of fruit from that environment to the mid 30's of the cooler (a drop of more than 70o F) being the embodiment of a large, rapid change in temperature, then perhaps having a huge amount of reversion shouldn't be so much of a surprise.
Ways to avoid this problem during high ambient temperatures are to avoid getting fruit that hot into the box in the first place. To whatever extent possible, early picking and quick transport to the cooler are clearly best. The other, which has been explored by researchers and may be in effect in some areas, is a staged cooling which gradually brings down the temperature to avoid the rapid shock of what forced air at 35o F imposes.
- Author: Mark Bolda
The following powerpoint was graciously shared with me by Max Edgley, a PhD candidate at the Tasmanian Institute of Agriculture. I think this is a very useful exposition of a common problem in blackberries and I am very grateful for him sharing it with all of us.
The information below is taken from Max's presentation. Max wrote in to stress that we please be aware that this information is from one season across and is as such very preliminary.
Red druplet disorder, or reversion, is the post-harvest reddening of blackberry fruit. While we understand that there is a loss of anthocyanin pigment in the affected druplets, it is still unclear as to what exactly is the cause of this disorder.
Physical damage to the druplets has been implicated, as have rapid changes in temperature – ie from the hot of the field to the sudden cold of the cooler which seems to swell and then shrink the cells walls of the fruit. Fruit which is harvested at temperatures with an internal temperature above 22 C (that's 72.5 F) before cooling tends to show symptoms the most.
In short, the work here investigates the above implications by testing staged cooling of freshly harvested fruit, physical damage, and the effects of different levels of fertilizer nitrogen in mitigating red druplet disorder.
Again, many thanks to Max for sharing this document with all of us.