Beginner Gardening: Zucchini and Squash -What is killing mine suddenly?, 1 by Kindlekat

Kindlekat wrote:
Something is killing my zucchinis and squash plants, and I've never seen this before!

I'm using self-watering containers, built from the plans here (http://mckarion.wordpress.com/2009/11/29/frugal-gardner-self...)
my soil is a great quality organic potting mix from a local nursery, mixed 50/50 with a local compost, and with some Plant-Tone mixed in.

Had great growth, then saw some powdery mildew. A mix of 2%milk and water seemed to take care of that, and spayed about once a week.

Then, suddenly, the entire plant turns yellow, there is browning on the edges of the lower leaves, and it seems to be dying.

Could it be too much nitrogen? Too much water? Help! Is there anything I can do to help, or is the plant done for? My pepper plants, cucumbers, and a variety of herbs and greens are doing fine, if bolting a bit (the lettuce, that is)

Attached is a photo I took on the 18th.