Here's a closeup of the leaf.

Screaming for some nutrients, especially potassium and magnesium. I would fertilize with a balanced all-purpose fertilizer as well as with epsom salts (1 T per gallon). Also too small a space for one zucchini plant.

Give us a little background history. Have you fertilized ? With what and how often. What is your watering schedule ? Have you checked closely for insects, what kind if any ? Squash take up more than 1 sq ft of space so consider the shading effect on adjacent plants. They'll easily cover/shade 4 sq feet. What is the planting mix in the garden ? etc,etc

The thyme right next to the zucchini needs entirely different conditions i.e. far less water, no fertiliser and plenty of light. The squash leaves will shade it so one or other needs moving.

Thank you slimy_okra for the diagnosis!

Background, yes, that would be helpful, wouldn't it?
The raised bed is filled with 1/3 of Lyngso Veggie Blend (blend of Top Soil, Diestel Structured Turkey Compost and Fine Redwood Amendment) and 2/3 of (Mel's mix for SFG - compost, vermiculite, peat moss).

I just added the Lyngso Veggie Blend in April for the start of this season. The rest was old soil from last fall. I just stirred it all up.

When I first transplanted the Zucchini, they grew like gangbusters, such vigorous growth! I did not add any more fertilizer, hoping that the Lyngso Veggie Blend would take me through the season. I don't see any insects.

I hand water it every day in the morning.

Thanks for the advice on neighboring plants and the shading effect. Looks like I should move the thyme, thank you floral_uk!

I will add another possibility, not to be contrary, but just for variety (or confusion - sorry).

Since the new leaves at the crown of the plant look green and healthy, you may be getting some root die-off. When roots die, the plant will "prune" itself to restore the balance between roots and foliage.

This can be caused by damping off (a search will get you a lot of information on this.) It can also be caused by a plant that is root-bound.

Unfortunately, about the only thing you can do about a root-bound plant is to dig it up carefully, break up the root ball, cut away any dead roots, replant, and hope for the best. This will set your harvest back, but it would be better than a plant that is perpetually stunted, or just dies.

Oh, and if you replant, cut off any discolored or dry leaves and fruit.

Boy, I better take a Xanax for my oscession. One last thing (I promise), if you only have one plant, you may need to hand pollinate. The female flowers have the little zucchini on them. Collect pollen with a q-tip from a male flower, and brush it on the pistil of the female flower.

This post was edited by terry_neoh on Wed, Apr 30, 14 at 22:41

Thanks terry_neoh for all those possible problems, it's really good to know all these. I will definitely look them up for more info and see if I can dig around to see what's down there.

And, the pollination, yes, I've been going out each morning with a tiny paintbrush to pollinate if I have to. It's kind of fun! I've also tried to plant more flowers around to attract the bees.

Thanks art_1! I had high hopes for Mel's Mix, but it was disappointing. The mix was just too light and needed more "substance" as my neighbor told me.

Looks like you may not have selected a suitable compost element for your Mel's mix. I've been using homemade compost in Mel's mix for years and it works well, as advertised and expected. Also, the bed doesn't look raised to me. The idea is to use Mel's mix on top of the native soil, not incorporated into it.