Balling is triggered by cool, damp conditions, often in a partially shady site, where water-saturated outer petals fail to dry out before being scorched by the sun. The mushy plant tissue dries to form a stiff straightjacket around the petals, preventing the flower from opening. An invisible soft, slimy layer of mycelium then fuses the petals together. The problem is most acute on roses with a multitude of thin petals.
Apparently healthy rose buds develop and fatten but fail to open. They eventually wither or drop off.
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The only option is to cut off the damaged buds and wait for new ones to develop. Be careful not to trigger the problem by splashing water onto the buds when watering your roses. If damaged buds aren’t removed, there’s a risk that grey mould may develop and cause the stems to die back. If this happens, prune back shoots to healthy growth.