Dear Darla,

Thank you for contacting Ask Extension about your squash crop. You may notice that the browning begins at the bottom of the fruits. This is most likely what is commonly called blossom end rot. One article from Extension explains the various causes and how to avoid the problem:

Blossom end rot most commonly affects tomatoes and squash but can also occur on peppers and watermelons. This problem is not a disease and does not spread from one plant to another. Instead, it is classified as a physiological disorder and is caused by a lack of calcium in the developing fruit. Anytime the absorption or movement of calcium into the plant is slowed, blossom end rot will likely develop. Blossom end rot can be a sporadic or persistent problem, depending on what is preventing calcium from reaching the developing fruits.

While plants absorb calcium from the soil, low soil calcium levels are rarely a cause of blossom end rot... Instead, blossom end rot is most often caused by low soil pH or plant stress due to unusually cool or hot weather, drought, or wet soil conditions. In acidic soils, where the pH is below 5.5, some nutrients are locked up chemically so plants cannot absorb them even if they are present in plentiful amounts. Calcium is one of the nutrients that become less available to plants under acidic conditions. Where low soil pH is the cause of blossom end rot, the problem is usually persistent throughout the entire growing season.

Over applying fertilizers that are high in nitrogen is another common cause of blossom end rot. Nitrogen promotes rapid, dark green, leafy growth. Plants that are growing very quickly often cannot move enough calcium into fruits to support proper development, leading to blossom end rot. To avoid blossom end rot, be careful not to overdo it with high nitrogen fertilizers like MiracleGro or soda. For season long feeding, rely on slow release fertilizers...

In addition to over-fertilizing, any conditions that cause root damage can lead to poor nutrient absorption and blossom end rot. The most common causes of root damage in vegetable gardens are wet soils following heavy rainfall or over-irrigation. Planting in containers or raised beds will promote good soil drainage and reduce blossom end rot in areas with heavy or clay soils. 

Drought is another leading cause of blossom end rot because roots cannot absorb nutrients from dry soils. In fact, the single most important thing you can do to prevent blossom end rot is to keep soils evenly moist by watering during dry weather. Vegetables require between an inch and an inch and a half of water each week from rainfall or irrigation to grow well. When rainfall is lacking, water plants once or twice a week. Soaker hoses are a great way to water vegetables and other plants because they apply water directly to the ground, instead of wetting plant leaves, which can increase disease problems. 

In addition to the suggestions above, we recommend that you keep the soil mulched around your plants. A couple of inches of clean straw works well, but almost any mulch will retain help soil moisture and mitigate temperature extremes.

The effects of blossom end rot can be arrested by correcting the causes. Remove any damaged fruits from the vine, and there may still be time for unaffected fruits to ripen. 

I hope you find this information helpful. Please write again if you have additional questions.