Why is There White Powder or White Spots on My Zucchini Leaves?

Zucchini is a true summer garden staple. This is a plant that doesn’t need too much to be happy – plant it in full sun, give it enough water, and fertilize – and it provides an abundance of fruit for our occasional attention.

If properly grown and cared for, your zucchini plant will produce so much fruit that you can eat it with every meal and still have plenty left over for friends and neighbors. But when you see that beautiful zucchini plant in distress, your heart can sink. White spots or white powder on leaves is often an indication that your plant is under attack either by pests or disease.

So, if you’ve been wondering about that discoloration and what you can do to fix or prevent it, this article is for you! Let’s dive into the most common causes of those white spots or white powder on your zucchini leaves this season.

Causes of White Powder or Spots

While there are a few reasons why your zucchini leaves may be showing white spots or powder, the causes are all quite distinct and are very recognizable. The main culprit is a fungal disease known as powdery mildew, which is a common zucchini problem. But there are a few other reasons. Let’s dig a little deeper.

It Can Be Natural

While most cases of white spots or powder are cause for some concern, there is the possibility that what you’re seeing is completely natural. There are some zucchini varieties that have white mottling or variegation in their leaves. In this case, those white markings will be regular and symmetrical. Obviously, this is no cause for concern.

Certain types of zucchini can also develop white or silvery spots on their leaves as they mature. If you see these spots developing, take a closer look. If the discoloration doesn’t rub off and you don’t see signs of pests, this is a naturally occurring phase of your plant’s lifecycle.

Powdery Mildew

Powdery mildew is, by far, the most common cause of white spots and powder on zucchinis.

Powdery mildew is a fungal disease that strikes garden zucchini. It’s commonly found throughout North America. It is caused by several fungal species including Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca, and Uncinula. Your zucchini – along with cucumbers, beans, gourds, tomatoes, and peppers – are especially susceptible to it.

Visible symptoms can appear at any time in the growing season but tend to be more pronounced when there are swings in temperature and humidity.

This disease looks just like it sounds – it appears as a whitish dust that settles on the leaves of a plant. In the early days, it almost looks like a child sprinkled flour on the plant.

As the disease progresses, however, that dusting will turn into larger white blotches, fuzzy stems, and dead leaves. Powdery mildew prefers young plants and newer growth. You won’t often find it on very mature leaves.

Causes

There are a number of reasons powdery mildew spreads in your garden. But know that even if you do everything right, you cannot completely protect your zucchini from the disease.

Fungal spores move pervasively through the air, on pollinators, and in water and it’s unlikely you can prevent a few from finding your plants. This fungal disease is just a part of the garden landscape that we need to learn to manage.

With that said, there are reasons your plants may be more susceptible to the disease this year. Prime factors include poor air circulation, not enough direct sunlight, water splash back (either from watering or rain), lack of mulch, using too much zucchini fertilizer, or infected compost material.

Prevention

Taking preventative measures will go a long way in helping protect from this annoying fungal disease.

First, ensure your plants are spaced far enough apart. Zucchini plants, in particular, have very large leaves and can take up a considerable amount of space. Providing enough space between plants and other vegetables, allows them to develop fully and allows for proper air circulation.

Second, ensure your plants receive enough sunlight. Zucchinis need at least 6 hours of direct sun each day, but prefer 8-10. If they receive less than this, those excessively shaded leaves will be far more susceptible to fungal disease.

Third, ensure you are practicing proper watering techniques. Water in the mornings at the base of the plant (not the leaves) and do so slowly and deeply. Soaker hoses or a drip system are the ideal way of getting the right amount of water to your plants.

Ensure you have a thick 2-3 inch layer of mulch to act as a barrier between the soil and the leaves of your plants. This will protect against water splashing back from rain or overhead watering.

Treatment

Treatment is pretty straightforward. We would normally recommend removing affected leaves and branches, but with zucchini this can be a bit complicated.

If you are still in squash vine borer season, cutting off those hollow stems creates a massive opening for the pest. We recommend only pruning off affected leaves after mid-July. Do so at the base of the stem.

Rather than removing leaves, treat them with a vinegar solution, Neem oil, or an organic fungicide. Follow package directions and be mindful of pollinators.

Consider planting disease-resistant zucchini varieties like Black Beauty or Desert to avoid this fungal disease problem altogether.

Mosaic Virus

Zucchini yellow mosaic virus is an aphid-borne disease that will eventually be fatal to your plant. While white mottled spots on zucchini leaves are a symptom, you will likely see others as well.

The most common symptoms are raised, war-like regions on the skin, bumps on the fruit, or misshapen zucchini. Mosaic virus is spread through infected seeds, aphids, or gardening tools that have come into contact with diseased plants.

Prevention and Treatment

Unfortunately, you cannot treat the mosaic virus and infected plants need to be removed immediately. If you have dealt with this disease before, consider covering your vegetables with row covers this year and hand pollinating flowers. Be sure to sanitize all tools and gloves after handling any infected plants.

Scarring

Finally, your zucchini leaves could be showing signs of scarring from sap-sucking insects feeding on the plant. Insects such as thrips, aphids, spider mites, and whiteflies pierce the skin of zucchini leaves to get access to the juices within. These scars can look pale green or white, but won’t hurt the plant unless the pest infestation is extreme.

Prevention and Treatment

Predatory insects will be your best friends when dealing with pest infestations like this. By companion planting certain herbs and flowers nearby, you can invite ladybugs, lacewings, and parasitic wasps to help keep pest populations down.

While you can use sprays like Neem oil, insecticidal soaps, or horticultural oil for pest management, this particular group can be well managed by spraying a hard stream of water at them.

If you spray these little sap suckers off leaves, they have a very hard time finding their way back to the plant. Just make sure to check the undersides of the leaves – they love to hide there.