If they are starting to turn yellow and there is no sign of insect damage, they are ripening and you should pick them. It could be that the heat is hastening the ripening process prematurely. At least pick one and cut it open and see what it looks like inside.

I don't think it could be the water if the leaves are not yellow too. They would be the first indicator of overwatering. Is there any sign of anything on the leaves?

Maybe if you could rig something so they are a bit shaded in the heat of the day it might help.

Other than that I don't know. Maybe someone else does.

oilpainter,

I covered the plants with a row cover to try and shade them, i hope it works.I dont see any insects.
Thanks

I am no expert, but usually, when mature fruit like yours rots from the tips, that is blossom end rot. There could be two reasons for this: the squash has some kind of disease (e.g., bacterial wilt, but this is unlikely in your case as you would see symptoms on leaves before seeing fruit rot) or the plant got too much water.

I think in your case, you just need to make sure not to overwater the plant. I wouldn't shade it as squash loves sunlight and depriving it from it could stunt the plant, which wouldn't help it in recovering from whatever ails it.

If this is in fact blossom end rot from overwatering or wet weather, under the right circumstances, the plant will recover and resume producing healthy rot-free fruit (unless the blossom end rot is caused by nasty underlying things like the dreaded squash vine borer, in which case you would need to treat the underlying problem before seeing healthy fruit).

Fruit with simple blossom end rot is okay to eat--just cut off the affected part plus an inch more and eat the rest.

By the way, if it is hot, I don't think you should water more. Instead, try watering more frequently but give your cukes the same overall amount of water. The problem with hot weather is that the water evaporates more quickly. If you divide the daily watering into several smaller doses, more of the water will be absorbed by the roots. If the water evaporates and the soil around the roots dries out some, the roots will more readily absorb the water next time you water. If you only water once a day, the capillary properties of the soil will only allow part of that water to be absorbed by the roots and the rest will evaporate.

Needless to say, avoid getting any water on the fruit. In fact, avoid watering anything but the soil around the stem. Deep watering of the roots only is the way to go.

The best thing you can do to prevent hot weather from stealing water from the plant is mulching. Put a two-inch thick and two-foot wide patch of leaf mulch around the stem of the plant (make sure the mulch doesn't touch the stem). This will greatly help in keeping the water from evaporating.

Hi Ralph, did the shade cloth help? Other than what others already mentioned, it just occurred to me this could also be a pollination problem. If the shade cloth still doesn't help, you can try hand pollinate the female blossoms.