It's hard to tell the cause from the photo, but commonly leaf curling/cupping like this is caused by either aphid, psyllid, or leafhopper feeding while the affected leaves were still expanding. Given that, the insects might not be present now, but you can still check on the undersides of those deformed leaves to see if you notice any evidence of insects. (Both leafhoppers and aphids might leave behind white, shriveled shed skins, even if the insects themselves have since been eaten by predators or moved on.) For Blackberry Psyllids, which look a bit like leafhoppers, they have one generation per year, with adults spending the winter on a different plant host (certain needled evergreens). You can learn more about the life cycle on the linked page (though the page's photos, which are microscope images, are poor but not needed in this case), though we don't recommend its suggested pesticide remedy because it is one of the harsher ingredients with regards to potential impacts on other insects.

Although psyllid damage is our primary suspect, viral infections can also deform leaves, and viruses can unfortunately be quite common in bramble plants. Fortunately, the photos don't appear to show classic symptoms of infection. Exposure to certain types of herbicides can also deform growth, and while the symptoms pictured similarly don't quite seem to match with typical herbicide injury, we can't rule it out. Were any weed killer products used near the shrubs in the past few weeks, or compost applied as a top-dressing that might have been contaminated? (Composted manure, for example, sometimes contains persistent herbicide residues because they can pass through the animal's digestive tract without degradation.)

If psyllids are the culprit, you might not need to take action at this point, but we're checking with a specialist to see what they recommend. We'll get back to you as soon as we have their feedback.

Miri

Hello Miri,

We received feedback about the suspected psyllids, and there isn't much that can be done except to prune out the damaged cane sections. It might be that psyllids which would normally stick to wild blackberry plants (fairly common shrubs found in fields and woodland edges, as several species occur in Maryland) moved into more cultivated areas in years like this one with wet spring weather that may have boosted their populations buy suppressing predator activity which would normally keep their numbers lower. We found a mention of this in our commercial IPM report (a weekly publication for nurseries, farms, landscapers, and arborists) back at the end of May, where the entomologist was mentioning that their team was receiving reports of blackberry psyllid damage and was theorizing that the wet weather earlier this year may have played a role. He also recommended pruning to remove the damage rather than attempting to use insecticide, as plants will regrow readily (though perhaps more so around late spring than closer to mid-/late summer).

Miri