Right now our heat wave doesn’t seem to have an end in sight! We have been experiencing a long stretch of hot and dry weather in southern Missouri. Even though we have irrigation systems on all of our plantings, some of the fruit crops are showing symptoms related to the hot and dry conditions.
We have seen several examples of discolored fruit on raspberries and blackberries – tan or white drupes on the berries due to the hot and dry weather. A drupe or drupelet is one of the little fruit balls that make up the clusters that comprise a raspberry or blackberry. Both blackberry and raspberry fruit or berries are aggregates of drupelets. The difference is when you pick a raspberry, the receptacle of the fruit stays on the plant, leaving a “hole” in the raspberry, but when you pick a blackberry fruit, the receptacle of the fruit stays in the fruit leaving no hole.
Now back to the drupelets. White or tan drupelets on ripening raspberry and blackberry fruit are caused by the ultraviolet radiation from the sun that appears when there has been an abrupt increase in temperature along with a drop in humidity. Wind is also a factor, as hot, dry air exacerbates the disorder. Apache and Kiowa blackberries and Caroline primocane-bearing red raspberry have been observed to have this problem more frequently according to the Plant Health Clinic News published by the University of Arkansas Department of Plant Pathology (Issue 15 – Jun 2, 2010). Discolored drupes have also been linked to stinkbug damage to young fruit and feeding damage by red mites – but the pattern of discoloration is in a random pattern rather than on the sun side as is the case with solar injury.
There is not much you can do about this problem. You may be able to shade the plants during hot periods, or if you grow brambles in containers, you can move them out of direct sun temporarily until the heat wave subsides. Keep the plants well irrigated during the hot and dry spell so they remain healthy and cooler. The berries affected with white drupe will taste fine or may be just a bit dry. Once mixed into a cobbler or preserve, no one will be the wiser.