I have successfully grown summer squash for many years, but this year the newer leaves on the plants (sunburst hybrid) are wilting and dying, despite (or because of?) the cool wet June weather we are experiencing. The more mature leaves seem to be okay. I cut one of the leaves off at its base and didn't see any evidence of insect infestation in the stem or on the leaf. The base of the plant also appears to be free of insect infestation. Do you know what might be causing the problem, can it be treated, or do I need to pull up the plants, and if so, can I put new plants in the same location (the rest of the garden space is already filled)?