It is June, the month for harvest of blackberries in many areas. Unfortunately, along with ripe fruit we also see other problems crop up. This year I have seen pollination problems due to rain, SWD damage, stink bug damage, cane borers, anthracnose, and white drupelets. So, what causes these white (or tan) colored drupelets? For quite some time no one knew what caused it (and we probably still don’t with 100% certainty). Early theories revolved around insect damage (stink bugs, mites) or sunscald.
There are some very good descriptions and photos on the Team Rubus blog out of NC State and the UC IPM website. I would encourage you to visit those sites. Below is a photo of the problem in case you are not familiar with it.
The University of Arkansas-released cultivars Apache and Kiowa are most often mentioned as having this problem. I would say that Apache has it most prominently. Both the NC State and UC websites talk about the environmental conditions that contribute to this abiotic condition (abrupt increases in temperature, wind, low humidity) in concert with UV-radiation on the drupelet. The descriptions also talk about “tolerance” to the condition. That would imply a genotype x environment interaction. Since certain cultivars exhibit worse symptoms than others there is a genetic component that might be able to be exploited to reduce this problem in future cultivars. But do we understand fully where it comes from (who is the offending progenitor)?
I broke down the parentage for both ‘Apache’ and ‘Kiowa’ to look for common ancestors. The percentages are below:
Thornfree 31.25%, Darrow 31.25%, Brazos 31.25%, Merton Thornless 4.6875%, and Eldorado 1.5625%
Brazos 50%, Thornfree 18.75%, Darrow 12.5%, Wells Beauty 12.5%, and Brainerd 6.25%
Both of these cultivars have ‘Brazos’ in a significant portion of their parentage (>31.25%). ‘Thornfree’ (US 1410 x US 1414), ‘Brazos’ (F2 of ‘Lawton’ x ‘Nessberry’), and ‘Darrow’ (NY 15826 x ‘Hedrick’) are in both cultivars and make up a large portion of their genetic makeup. ‘Eldorado’ also is on both sides, but in a very small percentage (‘Eldorado’ is a parent of ‘Hedrick’ which is a parent of ‘Darrow’).
Now, this doesn’t get us to the answer — more analysis and experimentation needs to be done — but it seems likely that one of these three (or perhaps more than one) has lent genes that result in white drupelet. Looking at the pedigrees along with real world, in-field data, would narrow it down farther yet. We don’t know what the factors are that lead to the condition — skin thickness? pigment stability? These are possible hypotheses to follow up on.
With the warmer earth we are experiencing, it seems likely that this problem will continue to manifest in many growing areas. New selections are being looked at to reduce this problem, but it may not be entirely eliminated.