Do a search (bottom box down the page) on borers, brown leaves and rot for more info.

In summer the leaf tips begin to brown on mine. If it looks ugly to me I scissor it off. Eventually the old leaves turn completely brown and new foliage replaces them. Not sure if you're referring to old or new foliage.

Drainage, drainage, drainage. We'd love some of your rain but in our heavy soil (adobe prior to amending) it would lead to lots of rot like you found on one.

If this browning is on older foliage (the leaves on the outside of the fans) brown tips are normal this time of year in zone 5. New foliage should be growing from the center of the fans to replace the older leaves, which, as Iris Gal said, will eventually turn completely brown. If you're getting brown tips on new growth, that could be a sign of trouble. It's a bit too early for borer damage to be that severe in zone 5. Rot would be more likely.