This usually occurs on the older (year or more) leaves and is normal in most cases. Some grown with ideal nutrients don't exibit this as as soon. Eventually these will fade even more and need to be removed when they brown about 50% or more. The leaves don't live forever but you can add more nitrogen in your fetilizing regime next spring. It won't cure the natural process, but the leaves may remain a healthy green longer.
Fertilizing without having seen a soil test can backfire. Phosphorus overdose in particular is common. The next step is a soil test. And if a nutrient does turn out to be deficient it's better put on in fall than spring, when dealing with hardy plants.
