Calcium. We all need it. Our plants need it too, and they let us know very clearly when they are deficient. If you've ever had a zucchini lose its blossom and start to rot, you know that sinking feeling of being out of control. We hate to put all that effort into our garden and then watch it rot.
My Chaos Garden, which grew in my compost pile, is having this problem, and my friend Justin Duncan of Xeriscaping Solutions gave me an interesting solution. We boiled about a dozen eggshells for 20 minutes or so (I'll warn you, don't use your good pan - the residue is really difficult to remove). After letting the water cool we strained it, put it in a spray bottle and sprayed it on the leaves for immediate absorption. The plants seemed to like it.
And this is what the plant looks like now, after spraying twice. The zucchini look fine. You can see the affected fruit in the bottom of picture - it's actually older than the two perfect zucchini you see in the photo, just stunted. This is definitely something I'll add to my gardening arsenal. Thanks Justin!