The harvest of butternut squash (Curcurbita moschata) is a symbol of the coming of fall in the garden. They are warm-season annuals that ripen in the long, hot days at the end of the growing season and keep fresh in the cupboard through most of the winter. Butternut squash are susceptible to many insect pests, pollination issues, and other problems that could cause young squash to turn yellow and fall off the vine.
Butternut squash is pollinated by bees and requires pollen from another squash plant in order for the fruit to mature and ripen. If pollination is not achieved, small squash will appear as though they are beginning to form, only to turn yellow and fall off after a short time. Besides the lack of other squash plants, many things can prevent pollination from occurring. For example, bees are inactive during cold or rainy weather and may fail to pollinate a crop of butternut squash during periods of consistently cloudy weather.
A sudden lack of any of the basic growing requirements can cause baby butternut squash to turn yellow and fall off. Even if pollination was successful and the young squash is beginning to develop normally, an extended period of drought could cause the fruit to be aborted. Likewise, if the plant does not receive sufficient sunlight, it may begin to produce fruit and then abandon the effort because it cannot produce enough energy to sustain the crop.
Numerous insect pests attack butternut squash and can cause the fruit to turn yellow and die. Some, like squash bugs and cucumber beetles, burrow into the fruit and eat the flesh from the inside out. Others, such as squash vine borers, attack other parts of the plant, causing it to weaken and eventually abort the fruit. In general, the best way to prevent outbreaks of these pests is to reduce their habitat by removing all weeds and debris, such as sticks, rotting wood, overturned pots and the like.
Some of the insect pests that attack butternut squash are also vectors for disease, such as bacterial wilt, anthracnose and various types of mildew. If a plant is under attack by insect pests or disease, it is likely to reject the developing fruit in order to divert energy towards fighting the infection. Fungal infections also can cause the fruit to rot, especially in hot, humid weather. The best preventative for these is to place a small board or brick under the developing squash vine so it is no longer in contact with the soil.