I have two pink hydrangea bushes that must be well over 20 years old, but some stems are black with white spots.
Do you have any idea what this could be? Is it a disease?
This is mysterious. Most fungal diseases found on hydrangeas cause leaf spotting on the foliage rather than the stems, so I suspect that the white spots could be the eggs of a sap-sucking scale insect called Pulvinaria hydrangeae, or hydrangea scale. This limpet-like insect also feeds on acers and cherries.
Scale insects are tricky to eradicate so, rather than recommending a chemical, I would cut the entire bush down to the ground in spring and dispose of all the trimmings. Don’t compost them or leave any on the ground.
However the black stems are troubling, because this suggests a plant that’s dying, rather than a mildew problem caused by dry conditions. The average lifespan of a hydrangea should be 50 years, but the symptoms are worrying.
I suggest cutting them right back to the ground in early spring to give them one more chance. Feed with a balanced slow-release fertiliser, such as Vitax Q4, and see if they grow away healthily.
Always avoid watering from above: this spreads fungal spores. If the severe trim doesn’t work, you will have to bin them. Hydrangeas are susceptible to honey fungus, so replace them with more resistant shrubs. The RHS has a list of plants resistant and susceptible to honey fungus.