It can occur for one of two reasons.

#1 Most commonly if squash gets that large and then rots, it is a fungal disease called choanephora fruit rot, which is most common when lots of moisture is present.

With this form of rot, there usually are very specific symptoms present. The fruit shrivels and starts to decay and often the shriveling/decaying is accompanied by a sort of fungal growth sort of like bread mold on the infected area. These fungal growths often look like whiskers. Most gardeners, unless they are out of town on vacation or something, will find the rotting fruit and remove it before the growth gets that far.

Remove any rotting fruit and throw them away. You can put them on your compost pile if you want but be sure you turn it often and it gets hot enough to kill fungal spores. Just because this fruit has the fruit rot doesn't mean all others will, especially if the rain doesn't fall for a while and the area dries out. In persistently wet weather, though, the fungal disease can persist for a while.

In the future, the best way to prevent it is to raise your plants in raised beds with well drained soil so the area doesn't stay too wet. Also, space the plants well apart so that there is good air flow. You cannot control how much rain falls or how humid the air is, but with well-drained beds and wide spacing for good air flow, you can decrease the risk of having your squash develop this form of fruit rot.

This is a fruit rot that develops quickly and moves fast when the conditions are right for it. Using fungicides is not usually effective because it can develop daily on new blossoms that open,and you would have to spray daily on all the new blooms early in the day. The best thing is to just remove infected fruit so the disease will not spread to some of the other veggies (including beans and okra) in your garden. Luckily, except in the wettest parts of the wettest years, this disease is not common here. Most parts of OK are too hot and too dry most of the summer for it to develop unless a garden is being overwatered, or unless you have just endured several weeks of nonstop rainfall like we have had here in much of OK in the last few weeks.

#2 If squash is not properly pollinated, it may try to develop but then cannot proceed and starts to shrivel and rot.

Dawn

Thanks for your answer. I suppose it could be either one. We have gotten quit a bit of rain recently and everything has been pretty wet.

How can you be sure it is pollinated properly? We have raised beds and have grown squash for years. We usually have to almost pay people to take it. I've been so looking forward to having the first bunch of squash, but don't know if it's ever going to happen.

we have a couple of zucs , yellow summer, white patty pan and yellow patty pan.