How To Prevent And Treat Powdery Mildew On Plants

Powdery mildew is among the most widespread plant diseases and is easily recognized by its white growth that covers foliage like a dusting of flour. This group of fungal pathogens affects hundreds of plants, from important food crops to trees, shrubs, and flowers. Even some turf grasses are susceptible to powdery mildew. Gardeners find infected plants unsightly, and severe infections can cause foliage to turn yellow and drop prematurely. Heavily infected plants experience reduced growth, flower production, and fruiting, which is particularly problematic for food crops. 

Powdery mildew is difficult to control once plants are infected. As such, management strategies for the home garden focus on understanding the disease cycle and adopting cultural methods to prevent disease incidence. It is also important to learn to identify the symptoms of powdery mildew so you can catch infections early and treat them before they have become widespread. Treatment options include organic and chemical pesticides, which are most effective when used in combination with integrated pest management strategies for long-term control.   

Causes And Symptoms Of Powdery Mildew

Powdery mildew is a disease caused by several different fungal species that all produce similar symptoms on plants. Powdery mildew appears as white to gray spots or patches on leaves, stems, fruits, or flowers. The spots have a powdery-looking appearance, giving the disease complex its
name. These patches grow and spread over plant surfaces and produce root-like structures that penetrate plant cells to extract nutrients.

Symptoms are most commonly observed on the upper surface of leaves, but also affect the underside of foliage and other plant parts. As the disease progresses, plant growth may be stunted and infected plant parts become distorted. Severe infections can lead to premature leaf drop, reduced flowering, and smaller yields.

The right environmental conditions must exist for infection to occur. Powdery mildews produce spores in spring that move to susceptible host tissue by insect or human activity, wind, or splashing raindrops. Although these fungi thrive in hot, dry conditions, they do require high humidity for spore germination. As such, infections are highest in damp, shady conditions, among overcrowded plants, and in other areas where air circulation is poor. Warm and humid spring weather favors infection.  

Identifying Plants Susceptible To Powdery Mildew

The many different powdery mildew fungi affect thousands of plants species, including trees, shrubs, flowers, vines, vegetable crops, fruits, grains, and grasses. Succulent plant tissues are more susceptible to infection, including new growth of young plants.

A few common landscape plants susceptible to powdery mildew include:

Trees:

• Ash
• Dogwood
• Oak
• Pecan

Shrubs:

• Azalea
• Crepemyrtle
• Lilac
• Roses
• Spirea 

Vines:

• Grape
• Virginia Creeper
• English Ivy
• Wisteria

Fruits:

• Apple
• Blueberry
• Pear

Vegetables:

• Cucumber
• Peas
• Squash

Flowers:

• Beebalm
• Dahlia
• Delphinium
• Peony
• Phlox
• Snapdragon
• Zinnia

Prevention And Cultural Control Measures

Understanding the environmental factors that contribute to disease development helps us identify effective preventive measures. Because spores require high humidity to germinate, one way to prevent infection is to reduce humidity in the leaf canopy by increasing air circulation. This is accomplished through adequate plant spacing and selective pruning. Avoid overcrowding plants, particularly in sheltered locations. Use the expected mature size as a spacing guide when installing new plants.

Selective pruning can also be used to increase air circulation in garden beds and within the leaf canopy of individual plants. Thin shoots and branches of susceptible trees and shrubs during the winter months to promote better air flow to the interior of plants. Dense hedges can also be pruned to enhance air circulation in adjacent planting beds. Stake herbaceous plants as needed and remove weeds to keep the air moving. Sterilize garden tools after pruning infected plants to prevent spore transfer to healthy plants.

Another way to reduce humidity in the plant canopy is by watering through drip irrigation or soaker hoses. This keeps moisture at the ground level, where it is needed. Be judicious with fertilizer. Excess nitrogen encourages plants to produce new growth, which is more susceptible to infection.

Environmental conditions are just one part of the diseases cycle. Powder mildew spores must also encounter a susceptible host for disease to develop. Planting mildew-resistant cultivars is a great way to stop the disease cycle. Resistant varieties are available for roses, turfgrasses, and
many shrubs, flowers, and fruit and vegetable crops.

Finally, it is important to practice good sanitation to reduce return infections from one year to the next. Remove and destroy all infected plant parts and debris from infected vegetables, annuals, and perennials when cleaning up as early in fall as possible. Likewise, rake and dispose of fallen leaves from infected trees and shrubs. Throughout the season, remove any infected tissues that fall to the ground and cut out heavily infected stems as practical. Burn infected plant material or dispose of it in a closed container. Do not add infected plant material to the compost pile.  

Treatment Options For Powdery Mildew

Because powdery mildew does not significantly affect the health of most plants, treatment is seldom necessary or practical in the home garden. Many gardeners learn to live with the appearance of infected trees and shrubs, and adoption of preventative measures limits outbreaks in most years.
On the other hand, crop yields can be negatively impacted, often making control among fruit and vegetable crops desirable. Likewise, gardeners may choose to treat valuable species.

Both organic and inorganic fungicides can be used to treat powdery mildew. These are most effective when combined with the cultural controls described above. When using fungicides, timing is critical. They must be applied early in the growing season to protect young green tissue against
infection—they will not cure already infected plants. Inspect susceptible plants weekly for signs of infection. When the first white spot appears, pinch off the infected foliage and begin fungicide applications. The fungicide must thoroughly cover all plant tissues to prevent disease.

The most effective organic controls for managing powdery mildew include neem and other horticultural oils, as well as sulfur-based fungicides. Neem and other horticultural oils are well-known organic pesticides used to manage plant diseases and insect pests. These oils are effective as a preventative treatment against powdery mildew, but it is important to remember they are also toxic to the insects they contact, including bees and other beneficial insects. Use oils sparingly and cautiously, making applications in the evening hours when bees and other insects are less active. Do not use neem or other horticultural oils under excessive heat or drought conditions, as phytotoxicity (damage to plant tissue) can occur.

Sulfur is the oldest recorded fungicide and has been used for more than 2,000 years to manage powdery mildew, rusts, and other fungal diseases. Sulfur prevents fungal spores from germinating and must be applied as a preventive measure before the disease develops for effective results. Some plants do not tolerate sulfur applications. Check product labels to be sure the plant you wish to treat is labeled for the product.  

Home gardeners swear by DIY organic treatments for managing powdery mildew such as applying baking soda mixes or milk sprays. While scientific research is limited regarding the effectiveness of these strategies on various crops, early studies show some potential. Milk sprays have shown promise in vegetable crops, but little is known about their use on ornamental plants. As for baking soda, the greatest success was observed when mixing diluted baking soda with horticulture oil.

Several inorganic chemical fungicides can also be used as a preventative measure against powdery mildew. Follow the same scouting recommendations outlined above to determine when to begin applications. Fungicides labeled for powdery mildew include products containing the active ingredients chlorothalonil, myclobutanil, and propiconazole. These are available at garden centers under a range of trade names. Make applications in the evening to limit impact on pollinators and other beneficial insects.

For both organic and inorganic compounds, repeat applications may be required to provide continuous control throughout the growing season. Always follow label recommendations for the proper timing, application rate, personal protective gear, and method of application to avoid harm to
plants, the environment, and yourself.