Powdery Mildew: Treatment and Control on Plants

Powdery mildew is one of the most commonly occurring plant problems. It is a fungal disease that begins as circles of white powder on plants or gray spots affecting the upper part and undersides of a plant's leaves and stems. In severe cases, powdery mildew can even spread to the buds, flowers, and fruits of plants. Although any plant can get powdery mildew, young plants are most susceptible to the disease, as well as some types of plants, such as crab apples, cucumbers, and all squash, lilacs, phlox, and roses.

The white coating greatly diminishes the appearance of the plant, but the disease is not fatal unless left uncontrolled. However, as it spreads, it stresses and weakens the plant and makes it hard for photosynthesis to occur. Left untreated, powdery mildew can leech nutrients from the plant, cause leaves to yellow, dry out, and wither, exposing the fruit to sunburn. It can even affect the flavor of fruit and reduce blooms on plants. Most importantly, powdery mildew on one plant can quickly spread to other plants, so it's important to prevent its spread.

Powdery mildew typically occurs in late spring/early summer. New foliage is especially susceptible to the fungus, which, unlike other mildews, occurs in moderate temperatures with warm days/cool evenings, low light, and high humidity—but dry foliage.

Powdery Mildew Causes

Powdery mildew is caused by related fungi spores in the Erysiphaceae family that are carried by the wind and travel for hundreds of miles. Once spores land they will germinate and infect the host and its surrounding plants. The spores can also survive through the winter if left untouched.

How to Prevent Powdery Mildew

Prevention of powdery mildew is key because it can be more difficult to eradicate as it spreads. Prevent powdery mildew with the following methods:

• Choose resistant plant varieties: Look for resistant varieties by reading the seed packages or tags at the garden center. There are also powdery mildew-resistant cultivars of flowers available, including ‘Simplicity’ and ‘Meidiland’ roses, and Rosa rugosa varieties.
• Plant in the sun: Planting in sunny areas will reduce the humidity that helps powdery mildew thrive.
• Prune plants: Pruning plants will improve the airflow around foliage and reduce the humidity that attracts some types of powdery mildew.
• Remove dead foliage: Dead or dying foliage will drop to the ground and become a breeding ground for fungi. Remove dead leaves from the plant and from the ground, which will reduce the problem from spreading.
• Space plants: Giving plants significant space helps increase air circulation which will also allow water to better evaporate from leaves. 

Powdery Mildew Treatment

Once powdery mildew has developed, treatment should be performed as soon as possible. Reapply each treatment weekly until the mildew is gone. In the first signs of infection on a plant, remove the leaves with powdery mildew, if there aren't too many infected leaves, then spray the rest of the plant. Spray any susceptible plants located nearby, too. 

The treatments work better as preventative measures applied before powdery mildew has the chance to spread on your plants. Though they are less effective once the fungus has taken hold, these treatments can be helpful to stop the spread. Here are the best treatment options when you spot powdery mildew:

• Baking Soda: To control powdery mildew on plants with a baking soda spray, mix the following in a spray bottle: 1 tablespoon of baking soda, 1/2 teaspoon of liquid non-detergent soap, and 1 gallon of water. Evenly coat the affected plant including the underside of leaves and stems. This recipe is effective, but too much can burn leaves. Water plants well a couple of days before applying, don’t apply it in full sun and try it on an area to test the plant's reaction. Spray weekly and reapply after rain.
• Neem Oil: Neem oil, an organic fungicide, won't cure the problem, but applying it will help prevent other spores from germinating. It will also prevent the penetration of the spore into a plant's leaf tissue.
• Organic Fungicide: Fungicides can't reverse the damage the powdery mildew has done, but they can kill off existing pathogens responsible for spreading the disease. Sulfur is typically a preventive fungicide so instead, use horticultural or plant-based oils (such as neem oil or jojoba oil) after the disease has developed.

Plants Likely to Be Infected by Powdery Mildew

There are a few types of powdery mildew fungi, but not every type attacks every plant. These plants are especially susceptible to one type or another of powdery mildew:

Flowers, Shrubs, and Trees

• Azaleas
• Begonias
• Chrysanthemums
• Coral bells
• Cornflowers
• Cosmos
• Crab apple trees
• Crape myrtles
• Dahlias
• Delphiniums
• Hydrangeas
• Lilacs
• Oaks
• Pansies
• Phlox
• Roses
• Rhododendrons
• Sweet peas
• Vinca
• Zinnias

Vegetables and Fruits

• Artichokes
• Beans
• Beets
• Carrots
• Cucumbers
• Eggplant
• Lettuce
• Melons
• Parsnips
  
• Peas
• Peppers
• Pumpkins
• Radishes
• Squash
• Tomatoes
• Turnips