Squash plants need at least two inches (5.1 cm) of water per week. If enough water is not falling from the sky, supplement with water from your hose or sprinkler. Some gardeners recommend watering at the base of the plant to discourage plant diseases from starting.
Aphids, 1/8 inch (.32 cm) insects, suck the sap out of leaves causing yellowing of leaves on squash plants. Some ways to control the aphid population on your plants are:
- Use hose to blast them away.
- Spray with mild soapy water.
- Release in garden natural predators such as common green lacewings and ladybugs.
- Use citrus oils or insecticidal soap.
Spider mites, only visible with a magnifying glass, can cause squash plant leaves to become pale or yellow. They are controlled in the same manner as aphids. Neem oil can also be used.
Squash Vine Borers
The squash vine borer, the most dreaded squash plant pest, can cause yellowing of leaves on a squash plant. The surest way to rid the plant of this pest is to remove the vine borer.
Downy mildew is primarily caused by wet, humid conditions. It causes yellow spots to appear on the top side of leaves and may eventually cause the entire leaf to turn brown. One of the easiest ways to prevent this disease is to plant squash plants about three feet apart; thus, it allows good airflow and the drying of leaves.
The piercing-sucking mouthparts of the squash bug cause yellow spots to appear on the squash plant. A disruption of the flow of vital nutrients and water is caused by the bugs sucking the sap out of the plant. Tearing off the leaves containing squash bug eggs or trapping then killing the bugs may be all that is needed to prevent yellowing of the leaves of the squash plant.
The squash mosaic and zucchini yellow mosaic viruses can attack squash plants causing a mosaic pattern in shades of yellow and green. Most mosaic viruses are spread by aphids; however, the squash mosaic virus is spread by cucumber beetles. Purchasing seeds from reputable companies, and controlling aphid and beetle populations are the best ways to prevent the spread of mosaic viruses.