Can't tell which without knowing the size of the fruit when the discoloration first appears or at least seeing a picture. But it is most likely inadequate pollination which causes the fruit to abort. OR it is BER (blossom end rot) which is a moisture control issue. There are FAQs here on both conditions that I linked below. If the squash are 3" or less when they change it probably pollination issues - hand pollinating squash is quite common> If 4-6" then BER is likely. Hand pollination will solve the first issue. Stabilizing the soil moisture levels will resolve the second as the plant matures. Neither issue is a disease nor a pest problem. Dave Here is a link that might be useful: Vegetable Gardening FAQs

Your picture looks familiar. Something similar to this happened to my peach tree last year right before it died. It was due to over watering. Apparently I had a raised pond next to the tree and it sprung a leak saturating the soil around the tree during its growing season. It didn't help that I also have clay soil which retains a lot of water and my peach tree was planted in the lowest part of the yard. All these factors worked against the tree's survival. The moisture level was too much for the tree to handle and the roots started to rot, eventually the leaves became yellow and shriveled. The fruits stopped growing in size and slowly became dry and shriveled as well. I stopped watering the plant once I figured out the tree was getting too much water but the damage was already done and there was no way to remove the hundred gallons of water absorbed by the clay soil which came from my leaking fish pond. The tree eventually died. Peach trees are extremely sensitive to over watering. Well drained soil is a must. If you have clay soil like I do, it's probably best to plant your future peach tree on a sloped part of the yard where the water drains away from the tree. Another option is to plant it in a raised bed to help with drainage. You could also mix sand in with your soil to make it less clay-like (which I find to be too much work). Also, keep an eye on the soil moisture especially during the growing season (spring through summer). If the soil is damp, the tree doesn't need to be watered. Good luck. I hope your tree recovers. Mine didn't.

I think you should be fine. Mine is under 300watts, not directly but does get good enough exposure. I would cut the time to maybe 12hrs. They do like to be watered often. I use spray bottle on daily bases to spary the pot and aerial roots. I am still learning myself with potted trees. Browning could be due to cold. Not sure if anything else would cause it.

The label should tell you if you need to wait a certain number of days to harvest. IIRC, Daconil has no wait time to harvest when used on tomatoes. It would be almost impossible to avoid spraying your fruit if you're spraying correctly. Remember that you have to spray all stems, foliage (both top and underside), and preferably also all developing fruit as well. If you can do that and avoid getting anything on mature fruit, you are very talented.