Answer: Normally when I hear about incompletely-formed fruit, I think that pollination might be the problem. But pollination in raspberries is rarely a problem. I have a feeling you may have some insect pest attacking your fruit in the early stages of its development. It's hard to say what it might be. Keep a close eye on your plants this year to see if you find any possible culprit. It's a good idea to keep the raspberry bed as clean as possible, pruning off and burning any dead or diseased canes and keeping weeds down. Often these steps can reduce pest problems. Another possibility is lack of water--raspberries need adequate water, especially at blossoming time.
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