Help, My Zucchini has Yellow Leaves!
If you notice your zucchini leaves are turning yellow, it may not be too late to save the plants. The likely culprits are either insects or disease, and sometimes, disease caused by insects.
Cucumber Mosaic Virus
One of the more common diseases caused by the presence of insect pests is cucumber mosaic virus which, as its name suggests, also afflicts cucumbers, which are in the same family. The disease manifests itself as yellowing zucchini leaves, usually along the veins. The culprit? Aphids feeding on the undersides of the plants leaves. Cucumber mosaic virus is transmitted by these tiny insects, resulting in stunted growth and poor fruit development. The bad news is that once the plant is infected, there is no cure. You can try to halt the progression of the disease by removing and destroying any infected plant parts. Ideally, you will be monitoring your plants for aphids before it becomes infected. Any sign of aphids should be treated immediately with an insecticidal soap or neem oil.
Spider Mites
Another insect pest, the spider mite, also sucks the sap from the leaves of the plant, resulting in the zucchini leaves turning yellow. Again, treat the plants with an insecticidal soap. Spray the entirety of all the leaves, including the undersides. Also, introduce or encourage ladybugs and lacewings who will feast on the spider mites (and aphids, too).
Fusarium Wilt
Another disease that can result in zucchini plants with yellow leaves is Fusarium wilt. This fungal disease affects the plant’s vascular tissue. The spores live in the soil and can be carried by cucumber beetles that don’t care that this is a zucchini and not a cucumber. Unfortunately, once the plant is infected, fungicides are ineffective. It is best to remove and destroy the infected plants.
Fixing Yellowing Zucchini Leaves
The best bet is to try to prevent yellow leaves on zucchini by planting disease resistant varieties and to properly prepare the bed. Prior to planting, amend the soil with compost and other organic matter. This will improve the overall soil structure. If the soil is dense or has heavy clay, add peat moss and compost to lighten the soil and improve drainage. Also, test the soil before planting to identify any insufficient nutrients and test the pH level. Zucchini likes soil that is slightly acidic or neutral (pH of 6.5-7.0). Zucchini plants are heavy feeders, so any deficiencies in manganese, sulfur, or iron can cause yellowing in younger leaves, gradually progressing and affecting the more mature leaves.
