Why do I have white spots on my squash plant leaves?

QUESTION: I am growing winter squash plants, and I have white spots on my squash plant leaves. What’s wrong? And what should I do next? Any ideas? – Jenny R

GARDENING CHANNEL REPLIES: Uh oh! There are a few potential causes of white spots on squash leaves, and I will discuss some of the most common ones below:

1. Powdery Mildew: This is a widespread fungal disease that affects many plants, including winter squash. Powdery mildew appears as white, powdery spots on the leaves, which can eventually spread and cover the entire leaf surface. The disease thrives in warm, humid conditions and can be exacerbated by poor air circulation. To manage powdery mildew, remove any affected leaves, increase air circulation around your plants, avoid overhead watering, and apply a fungicide suitable for treating powdery mildew, such as those containing sulfur or potassium bicarbonate.
   
   If I had to guess, this is mostly likely your problem because it is so common. But here are my other theories of what could be happening.
   
2. Whiteflies: These are small, sap-sucking insects that can cause white spots on the leaves of your squash plants. Whiteflies excrete a substance called honeydew, which can lead to the growth of sooty mold, further harming the plant. To manage whiteflies, use yellow sticky traps to catch adult whiteflies, and apply insecticidal soap or horticultural oil to the undersides of leaves to control their population.
   
3. Leaf Miners: These are the larvae of certain flies, beetles, or moths that tunnel through the leaf tissue, creating white, meandering trails or blotches. To manage leaf miners, remove and destroy affected leaves, use floating row covers to prevent adult insects from laying eggs on the leaves, and introduce beneficial insects, such as parasitic wasps, to help control the leaf miner population.
   
4. Sunscald: Intense sunlight can sometimes cause white or bleached spots on the leaves of squash plants, particularly if the plant has been recently transplanted or has experienced a sudden change in light conditions. To prevent sunscald, provide some shade for your plants during the hottest part of the day, and ensure that they are well-watered and not stressed.

To figure out the exact cause of the white spots on your winter squash leaves, carefully examine the affected leaves and look for any signs of insects, fungal growth, or irregular patterns in the white spots. Once you have figured out which one of these options is the real cause, then you can take the right steps to fix the issue and protect the health of your squash plants. Good luck!

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