Hi Peter
I'm absolutely no expert, but I did find info only today on yellowed, soft zucchinis meaning the plant needs help with pollination. It might not be the issue, but have a search for pollinating zucchini plants and see. There's many helpful videos and pollinating seems easy to do.
Good luck,
~Mel
I was watching Phil Dudman on this exact issue just now. Sound s like the pollination problem.
Google his name and zucchini or corgette and there is a youtube clip. Unfortunately you can't post the link here.
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