My zucchini get 6-8 hours morning sun on 9 the floor balcony. Planted in a large pot. Started out growing and producing shiny, smooth fruit. At between 2 and 4 inches in length, fruit develops ridges from flower to where fruit joins the stem and turns yellow and shrivels. The leaves have brown edges and some small yellow patches and the odd black spot and whitish spots that look like a mould. The largest leaves are 9” across and 9”” from stem to tip. The leaves also shrivel.
I am sorry to hear that your once healthy zucchini plant is in distress. There are many possibilities for what is ailing your plant, but it sounds very much like a fungal disease. The conditions right now (very hot & humid) are perfect for fungal diseases to thrive. You could have Alternaria leaf blight which causes yellow and brown spots on the leaves causing them to shrivel and die, it also causes legions on the fruit making it inedible. If this is the case, you should cut off all of the infected parts of the plant, throwing all discarded matter into the garbage instead to the compost. Avoid watering from above, and water only the roots of the plant trying to avoid splashing any soil matter up onto the leaves and fruit. If you do not see improvement its best to discard the entire plant including the soil. This last resort also applies if your plant has Mosaic virus. This virus is spread by Aphids. This disease causes leaves to curl downwards and have an oddly coloured mosaic like pattern. It also causes fruit to be stunted and distorted.
Its hard to say exactly what troubles your plant but the photo you sent could indicate blossom end rot. This is not a disease but is caused by a lack of calcium in the soil. Unfortunately, this disorder cannot be cured at this stage, and the plant should also be discarded. You can prevent this happening in the future by ensuring that your soil is healthy and humus rich. Use a fertilizer low in nitrogen and add gypsum to ensure enough calcium for proper fruit production.
I am attaching a link to the University of Minnesota’s website which catalogs diseases of cucurbits (including zucchini). I think this will be helpful to you in diagnosing exactly what your problem is and its best treatment.