Zucchini plants that are turning yellow may not be receiving sufficient water. Zucchini plants require 1-2 inches of water weekly. The soil around the plant should be watered enough to moisten the top 6 inches of soil. If the top 1 inch of soil is totally dry, it indicates the plant needs water.
Damage From insects
Insects are capable of disrupting the flow of vital nutrients and water to zucchini plants. Even the tiniest insects can cause much damage. The insects that cause yellowing of the zucchini plant are:
When squash bugs feast on the zucchini plant, they suck the sap out of the plant. Stopping the damage may be as simple as removing squash bug eggs from the underside of leaves. If the squash bugs are present, trapping then killing them is the answer.
Squash Vine Borers
The squash vine borer is perhaps the most dreaded insect affecting zucchini. The larvae of the squash vine borer devour the spongy matter inside the zucchini plant stem, disrupting the flow of crucial nutrients and water. One result of this is the yellowing of the squash plant.
Preventing squash vine borer infestation can be done by using bug barriers such as floating row covers and aluminum foil or pantyhose on lower stems. Growing squash vine borer resistant varieties can also prevent infestation, but sometimes bugs still attack. A destructive pest like this needs natural interventions like these:
- Neem oil
- Hot pepper wax
- Garlic barrier
Aphids and Spider Mites
The 1/8-inch aphid and microscopic-sized spider mite are capable of causing yellowing of the zucchini plant. Their sucking damage can be halted with these controls:
- Blasting them away with water from hose
- Using neem oil, citrus oil, or insecticidal soap
- Placing common green lacewings or ladybugs in garden
- Spraying plants with mild soapy water
Iron availability to the zucchini plant should be suspected if leaves are yellow between green veins. This can be corrected by lowering the soil’s pH, improving compacted soil, reducing phosphorus, and correcting overly wet soil.
Bacterial wilt and mosaic virus, two deadly plant diseases, are spread by cucumber beetles. Bacterial wilt first causes leaves to change to dull green and wilt in the daytime. Eventually, the leaves turn yellow, wither, and die. Mosaic viruses cause yellow and green mosaic patterns on the leaves.
Using natural means of controlling cucumber beetles is the best way to prevent these diseases. Buying seed from a reputable company may also help.