Avoiding and treating powdery mildew

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A light velvety white layer, or scattered white spots on the leaves, makes powdery mildew (family Erysiphaceae) unmistakable the moment it settles on rose bushes, apple trees or grapevines. Warm weather with high humidity is what lets it spread, so keep the air moving through your beds and moisture from stagnating, and the curative list stays short.

This mold appears in the form of a light velvety white layer or white spots on leaves, which is what makes it immediately recognizable.

What makes powdery mildew appear and spread?

Powdery mildew spreads most easily when warm weather and high relative humidity occur together.

It usually appears in Spring, in mid-spring or late spring, when temperatures rise and the soil and air are still full of moisture from the wet months of early spring and mid-spring.

It also makes a comeback at the beginning of fall, when nights grow longer and moisture once again increases.

• While it’s difficult to control temperature, you can work towards reducing air moisture.

• Heavy nitrogen feeding pushes out soft sappy shoots that the fungus colonises readily, so go easy on rich fertiliser where mildew is a recurring problem.
• Powdery mildew is often worst on plants growing dry at the foot of a warm wall, so mulch the root zone and water to keep the soil evenly damp rather than parched.

How to treat powdery mildew

Powdery mildew should be treated both through preventive and curative actions.

Preventive actions

These actions will keep powdery mildew from appearing, or at least delay its spread.

• Ensure proper air circulation in your plant beds, to avoid stagnating moisture.

• Spread sprouts apart or thin seedlings out if they sprout too densely. Powdery mildew spreads when plants are tightly packed together…

• Remove infected portions immediately. Burn them or discard them, but don’t compost anything contaminated.

• Avoid overwatering, increase soil drainage – excess soil moisture makes plant “perspire” more. They try to evacuate excess root water through leaves. This is far too welcoming to powdery mildew spores!

Curative actions

Once powdery mildew has appeared, it’s possible to contain and stop it.

• Eventually, spray with fungicides.

• When dealing with vegetables, favor organic treatments instead of chemical ones:

• Milk. Mix ½ quart (½ liter) milk to 4½ quarts (4½ liter) water (total 5 quarts (5 liters)), spray every week until the disease is wiped out! This is a great stay-at-home solution that can help you garden during lockdown.

Powdery mildew agents are usually specific to the plant they infect: powdery mildew on grape vines will not contaminate your rose trees.

Preventive and curative measures compared

Powdery mildew is tackled on two fronts: measures that stop it settling in, and measures that stop it once the white bloom is already there. The table below sums up what the article recommends for each.

Prevention costs nothing but attention, and it is what keeps the curative list short.

Plants vulnerable to powdery mildew

Trees and shrubs most often infected with powdery mildew are:
oak trees, maples, maule’s quince, apple trees, pear trees, rose bushes, peach trees, hawthorn, red currant bushes, and grapevines.

In the vegetable patch, powdery mildew often spreads on the following plants:
radish, leek, corn, strawberry plants, zucchini, pickles, carrots, eggplants, red beets, endives, cucumbers and potatoes.

There are even herb spices that can contract powdery mildew:

rosemary

This picture shows powdery mildew on grape leaves and fruits.

See also:

Fighting aphids

Organic caterpillar control

Fighting scale insects

Fighting fruit flies

Fighting cherry flies