Calcium

• A lack of calcium can cause your yellow squash to develop blossom-end rot, which is a dark-colored dry rot that occurs at the blossom end of the squash. Test your soil to see if you need to add lime before you plant. Mulch with grass clippings, pine straw and leaves. Mulch should be 2 to 3 inches thick to prevent the soil from drying out. This allows the roots to absorb calcium better. Adding organic matter to the soil also helps squash take in more calcium. Be careful not to provide too much nitrogen or potash since they retard calcium absorption.
  
  

Diseases

• Yellow squash is susceptible to several fungal diseases including powdery mildew, leaf spots, belly rot and blossom blight. Fungal diseases are worse on the fall crop, and use fungicide sprays as soon as symptoms appear. To avoid spreading fungal diseases, don't handle the leaves or vines when they're wet. Mulch squash to prevent contact with the ground. This reduces its chances of getting belly rot, which is present in the soil. Bacterial wilt, which is spread by cucumber beetles, can also infect yellow squash. There is no way to control bacterial wilt, and infected plants must be destroyed. Plants infected with mosaic viruses have mottled leaves and the fruit develops green spots and warts. Reducing aphid populations helps prevent this disease since aphids are responsible for spreading it.
  
  

Insects

• Several insects attack and kill yellow squash, including cucumber beetles and pickleworms. Squash bugs suck the sap out of the leaves, disrupting the plant's ability to absorb water and nutrients. Although older plants can tolerate some feeding damage, it may kill young plants. Remove adults and nymphs from your plants and kill them in a pail of soapy water. Vine borers damage plants by boring holes near the base of the plant. The plant wilts and the base may rot away. It is difficult to treat borers unless you can prevent the larvae from entering the plant.
  
  

Fertilizer and Water

• Squash plants prefer slightly acidic soil with a pH range of 6.0 to 6.5. Providing squash plants with adequate water and the right type and amount of fertilizer helps prevent some insect problems. Have your soil tested to determine which type of fertilizer to use. Adding well-rotted manure or compost to the soil before planting can also help, but using too much manure compost can result in phosphate runoff, which pollutes lakes and rivers. Yellow squash plants need at least 1 inch of water every week during the growing season. When watering, be sure to soak the soil completely.