Spirea Leaf Colour Change

[megfitz]

My Spirea “Anthony Wareter” has grown and flowered very happily for the last several years, usual pruning, mulching care , but this year half the bush although full of flower has leaves that are much paler than on the other half.
No visible sign of damage to the branches, no bugs or leaf damage so I’m stumped, what is the cause, how can I treat it ( I’m organic).
Any advice/ suggestions gratefully received.

[Chipchopper]

It could be a lack of water effecting the shrub. I noticed a lot of plants behaving strangely this year, keep up the watering and mulching

[megfitz]

Thank you Chipchopper, we’re on chalk so any water drains very quickly and we have not had any substantial rain for weeks.
I shall have to do a rain dance as my OH complains about the water bill,!!