We can't tell from your photo what the issue is. The most common issue Pieris/Japanese Andromeda sometimes has is lace bugs, but we see no signs of that on those leaves.

Pruning it out was the right thing to do. One branch on an otherwise healthy-looking, mature plant isn't likely to be of concern.
Do not spray anything unless you know exactly what you are treating for, especially on hot days which can burn foliage or allow pesticide drift to affect non-target plants and/or organisms.

It's possible that drought/heat stress is involved, which is stressing many landscape plants of late. 
Offering supplemental water to high-value tree or shrub specimens can be helpful.


Christine