Powdery mildew is one of the most common plant diseases, affecting a wide range of plants & is wildly distributed worldwide. It is caused by a fungus of the of the genus Erysiphe, which contains many species. Each species tends to infect a particular plant species, although closely related species can be infected. So for example, a tomato plant affected with powdery mildew couldn't infect a lilac plant.

Powdery mildew prefers warm conditions, shade with closely spaced plants, which creates poor air flow. Spores are spread via the wind & rain & can travel long distances. It takes approximately 7 days from the time the spores land on the plant to the appearance of symptoms.

What are the signs of powdery mildew?

• Powdery mildew begins as small, white circles on the leaf surface, usually on the upper side. It eventually becomes a powdery white coating on the entire leaf surface, most commonly on the upper side of the leaf although the underside & stem can also be affected.
• Leaves may turn yellow to brown, distort & eventually fall off.
• Badly infected plants will eventually die.

How do plants become infected with powdery mildew?

Spores are dislodged by wind from infected leaves & infect other plants in the region. 

Powdery mildew treatment:

• Prune affected parts & dispose of carefully. Remove badly affected plants.

• Apply a fungicide spray as soon as you notice a problem. There are a variety of fungicides that can be used, check with your garden centre for the best one to suit your plant(s). For example, some are not appropriate to use on food plants.

You could also try the following home made remedies which are sprayed onto the plant:

Recipe 1:

1 teaspoon baking soda
1 litre of water
A few drops of liquid soap

Spray as soon as you see evidence of powdery mildew.

Recipe 2:

50ml milk
450ml water

Spray onto plants once a week.

Preventing powdery mildew:

• Keep plants well watered, especially avoiding dryness at the roots & mulch well to prevent evaporation of water.
• Water in the morning to allow plenty of time for the plant to dry out before night time.
• Avoid overhead watering.
• Plant in good sunlight, in a well drained area.
• Don’t overcrowd plants to ensure adequate air flow. There should be approximately 12 inches between plants.
• Look for resistant plants where possible.
• Pick up any leaves that have fallen from the plant & dispose of in a garbage bag. Don't compost.

Also see:

Bulb planting guide, Caring for your fruit trees, Companion Planting, Growing fruit & vegetables in containers, Growing garlic, Growing potatoes, Home composting, Mulching basics, Powdery mildew, Saving tomato seeds, Snail & slug Control, Whitefly