i have 4 large zucchini plants. 3 of them are growing beautifully so far this year, however im having issues with 1. all the leaves are dark green, large, and look healthy- except for new growth leaves which are yellowed & deformed. the fruits are not setting, even with hand polination. the flower buds are all small, deformed, and show a blackish-dark brown hue around the edges. some flowers have almost no petals, but most flowers fail to grow at all as they have that same black tipped look- they just shrivel up and die before they get any larger than the size of a pinky fingernail. the one fruit that did grow earlier on, was 15" long, but uunfortunately grew in the shape of a large letter "c". it was fat and bulged at the ends and thin through the middle. i suspect the issue may be due to japanese grub beetles deep in the soil, likely eating the roots, however thats just a guess. any help is appreciated.
You haven't said whether all four zucchini plants are growing in the same area, or whether they're in separate areas of the garden, but, if you can find no evidence of aphids or spider mites (webbing under the leaves, usually), or if the leaves are actually patchy or mottled yellow rather than all yellow (which would suggest viral infection), I recommend you dig up that particular plant and dispose of it away from the growing area. It would be useful to inspect the soil around and beneath its roots once you've dug it out to confirm or deny your suspicion about larvae affecting the roots, but given you've got three healthy zucchini growing, I'm not sure its worth risking those in case what's wrong with this one starts to affect them as well.
More information regarding zucchini problems here