Zucchini Leaves Turning Yellow: Reasons For Yellow Leaves On Zucchini

Zucchini plants are one of the most prolific and easy crops to grow. They grow so rapidly they can almost overtake the garden with their rambling fruit-heavy vines and their large shading leaves. Fast and easy as they may be, even zucchinis have their problems. A common problem is yellowing zucchini leaves. Yellow leaves on zucchini, a condition also called chlorosis, is a symptom that may be caused by a number of things.

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. The following article delves into some of the causes for zucchini plants developing yellow leaves and what you can do about it. But let’s first take a quick look at the squash itself and what kinds of growing conditions it prefers.

About Zucchini

Zucchini, or “courgette” in some countries, is a thin skinned summer squash variety of the Cucurbita pepo species. Zucchini fruit is generally a green oblong squash between 4-10 inches (10-25 cm), but it can also be picked when it’s smaller and cooked whole.

Zucchini squash comes in a range of shapes and may also be bright yellow in color. It grows from either vining or bush plants, and can be started by seed either indoors, or direct-sown when there is no longer any danger of frost. Plant zucchini in full sun, in well-draining fertile soil and keep the plants consistently watered. Plants should be spaced 3-4 feet (0.9-1.2 m) apart.

Why Are My Zucchini Leaves Turning Yellow?

While zucchini is quite easy to grow it may still present with some problems. Yellowing foliage on a zucchini may be the result of environmental factors, pests, disease, or even a genetic marker.

Sometimes yellowing zucchini leaves are a result of environmental factors including lack of sunlight, over- or underwatering, root damage, nutrient deficiencies, disease, or pests. The first consideration in solving the problem of yellowing zucchini leaves is whether the growing conditions are suitable.

Drought or Waterlogged Soil

Waterlogged or overly dry soils can cause a number of problems with zucchini. Both may lead to yellowing and wilting. Heavy soils that don’t drain readily become deprived of oxygen, which leads to wilting and yellowing foliage along with browning of leaves and stems. Prolonged periods of rain combined with heavy clay soil leads to a lack of drainage followed by the above signs of disease. The solution may be to plant your zucchini squash in raised beds where you have more control over the soil.

Fertilizer/Pesticide Burn

Using fertilizer and pesticides can cause yellowing squash foliage if they’re improperly used or overused. Fertilizer can result in leaf burn or scorch as well as wilting, margin yellowing, and stunting. This occurs most often when conditions are hot and dry, which concentrates fertilizer salts in the soil. Yellowing as a result of fertilizers can be mitigated by using a slow-release, organic fertilizer, or amending the soil with 1-2 inches (2.5-5 cm) of compost. If you’re using granular fertilizer, be sure to keep it off the leaves of the plant and water it in well immediately after application.

Pesticide burn or phytotoxicity causes a number of symptoms which may be confused with disease or pest damage. Pesticide burn occurs when stressed plants are sprayed with pesticides leaving them open to chemical damage. Pesticide damage occurs most often when weather conditions are extreme. Even the more benign insecticidal soaps and horticultural oils can damage zucchini plants in adverse weather.

Pests

Perhaps the most common culprit when it comes to yellow zucchini leaves is a wide range of pests which can all cause yellowing of foliage on squash. These include the squash bug, squash vine borer, spider mite, and aphid.

Cucumber Mosaic Virus

One of the more common zucchini 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 as yellowing zucchini leaves, usually along the leaves’ 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. However, ideally, you will be monitoring your plants for aphids before they become 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 beneficial insects like ladybugs and lacewings, who will feast on the spider mites (and aphids, too).

Fusarium Wilt

Another disease that will result in zucchini plants developing yellow leaves is Fusarium wilt. This fungal disease affects the plant’s vascular tissue. These spores live in the soil and can be carried by cucumber beetles that don’t really 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.

Other Issues

There are a number of diseases which may result in yellowing leaves on zucchini plants. Among them are the bacterial diseases of angular leaf spot and bacterial leaf spot, as well as fungal diseases like alternaria leaf spot, gummy stem blight, phytophthora crown rot, scab, and aster yellows.

With the exception of aster yellows which is actually caused by a phytoplasma, most of these diseases do not turn entire leaves or margins yellow but rather are multi-symptomatic.

How to Treat Yellow Zucchini Leaves

Because yellowing leaves may be the result of such a wide range of problems, it’s important to identify what’s causing the condition before attempting to treat the plants.

If the yellowing is determined to be too little irrigation, set up a regular schedule for watering early in the morning on a regular basis. Water deeply at the base of the plants and avoid getting water on the leaves.

If too much water is a problem, you can always plant in raised beds or amend the existing soil before next season's planting to make it well-draining.

Use water-soluble, organic, slow-release fertilizers when possible or follow the manufacturer’s instructions for use of granular fertilizer. 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. We recommend you 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).

If you are using horticultural oils or insecticidal soaps to combat pest infestations, do not use them on overly hot, dry, windy days. Wait for a cooler, calm day to use any pesticide. Carefully read the manufacturer’s instructions before applying any pesticides. Check for any cautions regarding sensitive plants and only apply on a calm, dry, cool, day with temps below 85 F (29 C).

Preventing Yellowing Zucchini Leaves

Yellow leaves may simply be a sign of normal aging. This is called leaf senescence and it just means that the older leaves turn yellow as they age while the rest of the plant stays a healthy green. Clip these leaves from the plant with a sterile tool.

It could also be genetic. The “B” gene causes leaves to yellow, and also causes the fruit stem to yellow rather than green up. This gene may also cause fruit to ripen earlier and increase the number of female flowers, but otherwise has no ill effect upon the zucchini plant.

The best idea for preventing yellow leaves on zucchini is to plant disease-resistant varieties and to properly prepare the planting 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.

Note: Any recommendations pertaining to the use of chemicals are for informational purposes only. Chemical control should only be used as a last resort, as organic approaches are safer and more environmentally friendly.