White Spots on Zucchini or Melon Leaves: Variegation vs Mildew (Photos)

Every spring as I share our garden online, I get numerous comments and questions about the “white spots” on our zucchini plants. What’s on your leaves? Is that a disease? How do you treat that? So, let’s talk about it!

If you’ve also noticed white spots on the leaves of your zucchini, squash, or melon plants, don’t fret. Read along to learn about white markings on squash foliage (and other members of the cucurbit plant family) including what causes white spots on zucchini leaves, how to tell the difference between natural leaf patterns vs disease, and how to treat powdery mildew.

What causes white spots on zucchini and melon leaves?

White spots on squash and melon foliage are usually caused by one of two things: disease or natural variegation. One can be a problem, while the other is not! 

Fungal diseases like powdery mildew causes white spots on zucchini leaves and other plants. Powdery mildew is very common and may require treatment. On the other hand, some (but not all) cucurbit varieties including zucchini, squash, pumpkins, cucumbers and melons have natural silvery-white spots, markings, or patterns on their leaves that are totally harmless. It’s just part of the leaf! 

For instance, our favorite mildew-resistant zucchini variety “Dunja” has heavily variegated leaves, shown below. I think it’s beautiful! Similarly, “Moon and Stars” watermelon has natural yellow spots all over the leaves AND melon rind.

How to tell the difference between natural variegation and mildew

White spots on zucchini leaves caused by natural variegation are usually fairly uniform, displayed in a mirrored pattern on all leaves and concentrated around leaf veins. The markings are flat (not raised) and only visible on the top side of the leaf. Natural variegation patterns typically appear when the squash or melon plants are still young seedlings.

In contrast, powdery mildew (PM) spots are more irregular and scattered than natural variegation – some big spots, some little spots, and some leaves with none at all. Mildew white spots are usually round, fuzzy, dusty-looking, sometimes slightly raised, and can be found on both the top and bottom of zucchini leaves and stems.

Powdery mildew generally shows up later than variegation, with the oldest leaves becoming most heavily infected. A severe mildew case makes the plant look like it’s been dusted in powdered sugar or flour! The infected leaves may eventually turn yellow and dry out too. Mildew white spots can also be wiped off (or at least appear to temporarily disappear) with a wet cloth or paper towel, whereas the natural leaf pattern would stay as-is. 

Powdery mildew vs downy mildew

Powdery mildew causes fuzzy, slightly raised, irregular white spots on plant leaves. The worse the infection gets, the more thick and “powdery” it becomes. On the other hand, downy mildew creates small yellow spots on leaves that eventually turn brown, thin, and crispy.  Learn more about controlling downy mildew in the garden here.

How to get rid of white spots (mildew) on zucchini plants

Once you’ve confirmed the white spots on your zucchini leaves are indeed a fungal disease (and not just a natural leaf pattern), you can proceed to treat the plant as necessary. The course of action to treat powdery mildew depends on the extent of the infection. Thankfully, powdery mildew isn’t fatal to plants except in extreme cases. Yet mildew does hinder overall plant health and can easily spread to other plants in your garden, so don’t let it go unchecked.

Powdery mildew can be prevented by if you choose mildew-resistant varieties, avoid overhead watering (use drip irrigation instead), sanitize garden tools regularly, and follow recommended plant spacing to reduce crowding and increase circulation.

Treat mild cases of mildew by pruning away infected leaves, apply a dilute neem oil spray after dusk, DIY baking soda spray, or our favorite method: with homemade potassium bicarbonate spray, explained below. I recommend removing and trashing (not composting) severely infected plants to avoid the spread of mildew spores in your garden.

See this article to learn even more about organic powdery mildew management.

Using potassium bicarbonate to treat powdery mildew

Potassium bicarbonate is a natural compound similar to baking soda (sodium bicarbonate) but is stronger and longer-lasting against powdery mildew. It changes the pH on the plant leaves, making conditions more alkaline and less hospitable for the fungus to thrive. It can be used as a preventative spray on young plants, or as treatment once white spots appear on leaves. Potassium bicarbonate is nontoxic to humans, wildlife, and the environment, though you should exercise caution not to inhale the powder or get it in your eyes.

To create a potassium bicarbonate spray for powdery mildew, thoroughly mix 1 tablespoon of powder and 1 Tbsp of liquid castile soap with 1 gallon of water in your favorite garden sprayer. (The soap helps the potassium bicarbonate spread and stick to the leaves better.) Shake well and spray the plant thoroughly, saturating both the tops and bottoms of leaves. Repeat treatment as needed, but no more than once every two weeks. As with all foliar sprays, it’s best to apply in the evening hours once the plants are out of direct sunlight.

And that’s the difference between mildew and variegation on plants.

I hope this helps put your mind at ease if you see white spots on your zucchini or other plants. Whether it’s mildew or just the natural leaf pattern, your plants will be okay. Please let us know if you have any questions in the comments below, and consider sharing this article if you found it useful!

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