In my observation, change of leaf color this time of year is always a sign of stress, without exception.

In my area we're in a prolonged dry spell, with no significant rain for weeks, and I'm beginning to see stress signs in some of the more moisture-sensitive trees. If we hadn't had such an epic drought last year they'd wouldn't be vulnerable. But they are still weakened from '07's double whammy of killer freeze/killer drought, and I'm sad to say we'll be seeing trees dying for several years due to 2007's weather, as any further stress will simply be more than some can handle.

Marty