Is it possible that this plant got hit with something? Hot water from a sun-heated hose, house cleaning products, liquid fertilizer or other garden products? This looks most like contact-irritant damage of some sort. See if you can figure out if something splashed on the leaves.
There is a blight that causes these filberts to dieback. It usually causes die-back branch at a time. It’s possible that your plant has this. The tell-tale sign of this is small, black dashes of “fruiting bodies” on the stems. More about this here: http://hyg.ipm.illinois.edu/pastpest/200811a.html
General recommendations for a plant showing leaf browning are as follows:
1. Clip off the worst leaves so you can better monitor if the damage is continuing or not. If the damage has stopped, whatever the problem was has come and gone.
2. Water the plant deeply using a sprinkler or soaker hoses, not hand-watering. Hand-watering is never deep enough. It has been a pretty dry summer in your area and it’s possible that this plant just has leaf scorch from being so dry.
3. Do not fertilize until next spring.
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