It sounds like a common disorder called blossom end rot, which usually affects tomatoes and squash. This is not a disease and doesn’t spread to other plants. It is caused by a lack of calcium in the developing fruit. Blossom end rot can be a sporadic or persistent problem, depending on what is preventing calcium from reaching the developing fruits. Blossom end rot can be due to low soil pH or plant stress from unusually cool or hot weather, drought, or wet soil conditions. Another cause of blossom end rot can be due to applying too much fertilizer that is high in nitrogen. In addition, root damage can lead to poor nutrient absorption also causing blossom end rot. This typically occurs following heavy rainfall or over-irrigation. On the flip side, drought can be a cause of blossom end rot, so it is important to keep soil evenly moist throughout the growing season.
Fruits that show blossom end rot will not develop properly and should be removed from the plant. To determine if blossom end rot is due to low calcium in your soil and/or to find out if your soil is acidic, you can submit a soil sample to the U of M Extension. You can obtain information at: http://soiltest.cfans.umn.edu/testing-services
I knew about blossom end rot in tomatoes, but didn't know it also happens to squash. Thanks so much for the info!
You are welcome!