Garden Pests and Diseases: Cucumber leaves turn yellow, then brown, then dead, 1 by crabbygardener

crabbygardener wrote:
Is it normal for cucumber leaves to start to yellow in spots, turn brown in the same spots, and then shrivel and die off? (See the photos, but note that they are in the reverse order for some reason.) They did this last year, too, but they still kept growing vigorously (as they do now). Is this just a part of the natural aging process, or is there a pathogen at work? Note that nearby tomato plants have early blight (Alternaria), but I don't think that affects cucumbers (please correct me if I am wrong!). The fruits are unaffected (other than by worms that keep getting in), although sometimes they turn yellow (they're supposed to be dark green Burpless).

Thanks!