How to Control Powdery Mildew on Cucumbers, Squash, and Beans

Powdery mildew is one of those garden problems that seems to arrive overnight. One day you’re admiring lush green growth, and the next you’re facing a dusting of white fungal spores creeping across your plants. It’s especially common on cucumbers, squash, pumpkins, and beans, but the good news is: it doesn’t have to take over. Here’s how we control powdery mildew and keep our cucurbits and legumes going strong.

What Is Powdery Mildew?

Powdery mildew is a fungal disease caused by several different species, depending on the plant affected. It appears as white or grey powder-like spots on the leaves, stems, and sometimes fruit. Left unchecked, it can:

• Reduce photosynthesis, weakening your plants
  
• Cause leaves to yellow, curl, and drop early
  
• Lead to smaller harvests or sun-damaged fruit

It loves warm days and cool nights, classic late summer conditions.

Prioritise Airflow

Poor airflow is powdery mildew’s best friend. Try to grow vertically wherever possible:

• Trellis cucumbers and beans to lift leaves off the ground
  
• Space plants generously to prevent overcrowding
  
• Prune excess foliage, especially on squash and courgettes

Good air circulation helps leaves dry quickly, which discourages fungal growth.

Water at the Roots, Not the Leaves

Moisture on the leaves isn’t always the cause of powdery mildew, but damp conditions make it worse.Always water at the base of the plant and aim for early morning watering so any splashes have time to dry before nightfall.

A layer of mulch also helps keep the soil moist without overwatering the leaves.

Use Organic Sprays at First Sign

As soon as we see those tell-tale white spots, we reach for one of the following:

• Diluted milk spray (1 part milk to 9 parts water) – surprisingly effective
  
• Bicarbonate spray (1 tsp baking soda, a drop of dish soap, and 1L water) – changes leaf pH
  
• Neem oil or sulphur-based fungicides – organic options that slow spread

Apply every 7–10 days, and always on dry foliage.

Remove Infected Leaves Carefully

Don’t wait for the mildew to take over, remove badly infected leaves with clean snips and bin them (never compost). This helps stop the spores from spreading to neighbouring plants.

Disinfect tools between cuts if multiple plants are affected.

Keep Plants Strong

Healthy plants are less susceptible to disease. Feed with seaweed extract, homemade compost teas, or an organic high-potash fertiliser to keep growth balanced. Avoid overfeeding with nitrogen, it encourages leafy growth that’s more prone to mildew.

Rotate Crops and Clean Up at Season’s End

Powdery mildew spores can overwinter on plant debris. At the end of the season, remove and destroy all affected foliage, clean trellises, and rotate crops to a new spot the following year.

Good hygiene makes a big difference.

Powdery mildew on cucumbers, squash, and beans is almost inevitable at some point in the season but it doesn’t have to ruin your harvest. With a mix of good airflow and early treatment, you can keep it in check and your crops producing right through to autumn.