SoCal Home Gardening

The white powder on your squash and zucchini is likely powdery mildew, a common fungal disease that affects many types of plants, including squash and zucchini. It appears as white, powdery spots on the leaves, stems, and sometimes the fruit.

Steps to Take With Powdery Mildew

Here are some steps you can take to manage and prevent powdery mildew:

Remove Affected Leaves
Carefully remove and dispose of the infected leaves to prevent the spread of the fungus.

Improve Air Circulation
Ensure your plants have enough space between them to allow for good air circulation. Pruning can also help improve airflow.

Watering Practices
Water the plants at the base rather than from above to keep the leaves dry. Watering in the morning allows the foliage to dry out during the day.

What is Powdery Mildew?

Powdery mildew is a fungal disease caused by several different fungal species in the Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca, and Uncinula genera. It appears as a white, powdery coating on the leaves and stems of affected plants.

The disease thrives in warm, humid conditions with poor air circulation. It prefers to infect young, new growth and can eventually cause leaves to turn yellow and die if left untreated.

How to Identify Powdery Mildew

You can identify powdery mildew by the following:
- White, powdery coating on leaves and stems
- Coating can be rubbed off the leaves
- Leaves may turn yellow and die if severely infected


To confirm if it's powdery mildew, gently rub a leaf - if the white coating transfers onto your finger, it is likely powdery mildew spores.

Treating Powdery Mildew

While powdery mildew is difficult to eliminate completely, there are steps you can take to control it:

1. Prune affected leaves and improve air circulation around the plant.
2. Apply neem oil, baking soda, milk, or other organic fungicide treatments to the leaves.
3. Ensure plants are spaced properly and receiving full sun exposure.
4. Water at the soil level, not from overhead.

By employing these cultural controls and organic treatments, you can manage powdery mildew and allow your zucchini plants to continue producing.