Zucchini leaves turning tan/yellow???

Ok as a newbie this year, I've been able to answer most of my questions by just reading other posts, but I'm needing a little expert advice on this one.

First of all, I believe I am a newbie with not-so-great compost. Most things grew, just not that well...slow and small. They perked up when I added fish emulsion which is what makes me think it's a compost issue that I will rectify before fall planting.

I tried to grow zucchini in my MM boxes and the leaves made it to 2-3 inches across and started turning color and dying. New growth was always green,. At first I tried cutting off the affected leaves, but I couldn't totally strip the plant as leaves were turning faster than the new growth was coming. My cukes were slow and had tan leaves similar to this but I moved them to a sunnier location and sprayed with neem solution and they seem to be finally growing better. I sprayed the zukes with neem but no luck. They eventually just gave it up. My pumpkins (located on other side of yard) have split in stems of 2 of 3 - not sure if it's SVB or if it's overwatering (seems like I read that could cause a problem too?) but they are still growing new growth at top? I say this because I'm not sure if the problems are related?

SO...I plunked a couple more seeds in these boxes with some regular potting soil (all I had - plan to replace this with MM for next planting as well) just to see if I could get anything back... I was also able to move these little boxes almost completely OUT of the black walnut dripline (though my extension sources say squash should grow under black walnut).

The plant currently looks like this (ignore the trampled grass - dogs):


The top of a leaf:


The bottom of a leaf (did find a tiny white bug on this one, but only one...):



I am not sure if this is a disease or a bug, watering, nutrients, or something else? I was going to move these back to a sunnier location closer to my cukes, but I hate to jinx them??? I haven't sprayed these with neem yet, but maybe I should? Any ideas?

Those leaves may just be old and petering out. I would clip them off and give the plant a dose of compost tea or other feed.

Thanks for the advice - I'll try that and see what happens!