Powdery Mildew on Landscape and Garden Plants

Quick Guide / At-a-Glance

• Affected Plants:
  Azalea, crabapple, dogwood, phlox, euonymus, lilac, snapdragon, dahlia, zinnia, crape myrtle, rose, pyracantha, rhododendron, spirea, wisteria, delphinium, oak, English ivy, photinia, blueberry, pecan, cucumber, squash
• Primary Disease Types:
  Several closely related fungi cause powdery mildew diseases
• Primary Causes:
  Airborne fungal spores (conidia) are produced under favorable temperature and humidity conditions
• Most Important Prevention Strategy:
  Maintain good air circulation, avoid overhead irrigation, remove infected plant material, and select resistant plants when possible
• Chemical Control:
  Fungicides applied at first symptom appearance; rotation of active ingredients recommended to reduce resistance

Powdery Mildew Disease Cause & Cycle

Powdery Mildew Disease Cause

Powdery mildew is the name given to a group of diseases caused by several closely related fungi. Their common symptom is a grayish-white, powdery mat visible on the surface of leaves, stems, and flower petals. There are many hosts, and although this disease is not considered fatal, severe infection can cause plant damage.

Powdery Mildew Disease Cycle

In spring, as daytime temperatures rise above 60°F, the fungi responsible for powdery mildew begin to produce spores (conidia), which are dispersed into the air. Infections occur when a suitable host is contacted, and environmental conditions are favorable.

Initial symptoms are small, circular, powdery, white spots, which expand and eventually join as infections progress. Infections spread as spores produced in these white patches move by wind and splashing rain to other locations on the plant or nearby plants.

The fungus survives the winter attached to plant parts and plant debris, such as fallen leaves. As the weather warms in the spring, the process begins again.

Powdery Mildew Favorable Environmental Conditions

Humidity is an important factor related to the onset and spread of powdery mildew. Unlike most fungi, these do not require free water to germinate; only a high level of relative humidity is needed.

High relative humidity favors spore formation. Low relative humidity favors spore dispersal, which explains why powdery mildew tends to be a problem when the days are cool and the nights are humid. Temperature is also a factor. Although powdery mildew can occur year-round, it is less common during summer heat.

Powdery Mildew Host Plants

Plants Commonly Affected by Powdery Mildew

Several species of fungi cause powdery mildew, and each has a limited host range. In other words, observing powdery mildew on oak leaves should not be cause for concern for nearby zinnias.

Plants that commonly become infected with various powdery mildew fungi include:

• Azalea
• Blueberry
• Crabapple
• Crape myrtle
• Cucumber
• Dahlia
• Delphinium
• Dogwood
• English ivy
• Euonymus
• Lilac
• Oak
• Pecan
• Phlox
• Photinia
• Pyracantha
• Rhododendron
• Rose
• Snapdragon
• Spirea
• Squash
• Wisteria
• Zinnia

Powdery Mildew Symptoms on Landscape and Garden Plants

Early Symptoms of Powdery Mildew

As powdery mildew fungi grow on the plant surface, they develop structures that insert into plant cells, enabling them to extract nutrients necessary for growth and spore production. This results in a general decline in the host’s growth and vigor, as well as common visible symptoms.

Abnormal growth, such as leaf curling, twisting, and discoloration, may be noticed before the white signs of the fungus are visible.

Plant symptoms include:

• Leaf curling
• Leaf twisting
• Discoloration
• General decline in plant growth and vigor

On dogwood, for example, leaves may take on a yellowish or reddish cast in summer or may develop reddish blotches or dead, scorched patches. The white powdery growth is not always apparent.

Visible Powdery Mildew Growth

When visible, the powdery fungal growth appears on the upper leaf surface and often begins on the lower leaves.

Powdery mildew signs include:

• Grayish-white, powdery fungal growth
• Growth typically appears first on lower leaves
• Powdery growth located on the upper leaf surface

As the disease progresses, leaves become dwarfed, curled, and generally distorted.

Advanced Symptoms and Plant Damage

In severe cases, leaves will turn yellow or dry and brown.

Powdery mildew fungi will also infect flowers, causing them to develop abnormally or fail to open.

Plant symptoms include:

• Yellowing leaves
• Browning or dried leaves
• Flowers that fail to open
• Abnormally developed flowers

On azaleas and rhododendrons, small areas of dead tissue are often seen.

Long-Term Effects of Powdery Mildew Infection

Powdery mildew creates other effects that are not readily visible, including:

• Reduced winter hardiness in severely infected plants
• Trees may leaf out later in the spring after infection the previous season

Powdery Mildew Prevention Through Cultural Controls

General Cultural Practices to Prevent Powdery Mildew

As with all diseases, optimum plant health is the first line of defense. This begins with selecting healthy plants, planting them correctly and in the correct location, and paying attention to their requirements for light, soil, and moisture.

Space them so they can grow without being crowded, and water thoroughly during establishment and during dry periods. Avoid overhead irrigation, which raises relative humidity within the plant canopy.

Key prevention practices include:

• Selecting healthy plants
• Planting in the proper location
• Meeting light, soil, and moisture requirements
• Providing adequate spacing to reduce crowding
• Watering thoroughly during establishment and dry periods
• Avoiding overhead irrigation

Sanitation and Pruning for Powdery Mildew Management

If powdery mildew is noticed on a few leaves, simply removing them will help with control.

At the end of the growing season, prune out infected stems and remove fallen leaves, which can serve as a source of further infection.

Suckers are common on crape myrtle, dogwood, and other plants. These should be pruned off as they develop because they are especially susceptible, and the disease will spread from them upwards to other parts of the plant.

Sanitation and pruning practices include:

• Removing infected leaves when symptoms are first noticed
• Pruning out infected stems at the end of the growing season
• Removing fallen leaves
• Pruning suckers as they develop

Fertilization and Monitoring

Fertilize to optimize plant health, but avoid over-fertilization with nitrogen as it stimulates young, succulent growth, which is more susceptible to infection.

Plants with severe infections should be monitored the following spring so that, if infections recur, they can be treated early.

To avoid powdery mildew:

• Avoid excessive nitrogen fertilization
• Monitor previously infected plants early the following spring

Plant Selection for Disease Resistance

When possible, select plants that show resistance to the disease (see Table 1).

Chemical Control of Powdery Mildew on Landscape and Garden Plants

When Chemical Control Is Needed

For fungicides to be effective, they must be applied as soon as symptoms are noticed.

Product labels will provide information on how often to spray. When ranges are given, use the shorter interval during cool, damp weather.

Be sure to cover both the upper and lower surfaces of the leaves.

Chemical Control for Powdery Mildew on Ornamental Plants

Table 2 lists fungicides labeled for ornamental plants.

Myclobutanil, propiconazole, and thiophanate-methyl have systemic properties and can be sprayed less often than chlorothalonil, sulfur, or copper-based fungicides.

When powdery mildew persists, and sprays are repeated, it is recommended to rotate (alternate) fungicides to reduce the risk of fungi developing resistance.

Key application considerations include:

• Apply fungicides as soon as symptoms appear
• Follow label directions for spray intervals
• Use shorter spray intervals during cool, damp weather
• Thoroughly cover upper and lower leaf surfaces
• Rotate fungicides when repeat applications are necessary

Seasonal Considerations for Deciduous Plants

When deciduous plants are infected, consider the season.

Generally, foliar diseases occurring in late summer do little damage. The leaves have already produced food for the plant and will fall off soon anyway.

Just be sure to rake and dispose of them as they fall.

Chemical Control Cautions and Label Guidance

As with any pesticide, read the label and heed all precautions. Sulfur, for example, can damage plants when applied during high temperatures and humidity.

Powdery Mildew Control on Vegetable Plants

For information on vegetable crop disease controls and tolerant varieties, see HGIC 2206, Cucumber, Squash, Melon, and Other Cucurbit Diseases, HGIC 2200, Bean and Southern Pea Diseases, and HGIC 2202, Cabbage, Broccoli, and Other Cole Crop Diseases.

Plants with Resistance to Powdery Mildew (Table 1)

Powdery Mildew–Resistant Plants and Cultivars

When possible, select plants that show resistance to powdery mildew. The cultivars listed below have demonstrated resistance or tolerance to powdery mildew under typical landscape conditions.

Note: Resistance does not mean immunity. Disease severity may still vary based on environmental conditions and plant care.

Table 1. Plants with Resistance to Powdery Mildew

Species	Cultivars
Kousa dogwood, Cornus kousa	‘Milky Way’, ‘Milky Way Select’, ‘National’
Cornus florida x kousa hybrids	‘Aurora’, ‘Constellation’, ‘Celestial’, ‘Stellar Pink’
Flowering dogwood, Cornus florida	‘Cherokee Brave’, ‘Springtime’, ‘Pygmy’, ‘Jean’s Appalachian Snow’, ‘Karen’s Appalachian Blush’, ‘Kay’s Appalachian Mist’
Crepe Myrtle: The Lagerstroemia indica x faurieri hybrids	‘Apalachee’, ‘Fantasy’, ‘Hopi’, ‘Miami’, ‘Osage’, ‘Tonto’, ‘Tuscarora’, ‘Tuskegee’, ‘Wichita’, ‘Acoma’, ‘Sioux’, ‘Natchez’
Phlox	‘David’, ‘Delta Snow’, ‘Natascha’, ‘Robert Poore’, ‘Shortwood’, ‘Katherine’, ‘Glamour Girl’
Zinnia	Pulcino and African varieties, Zinnia angustifolia, Profusion Cherry, Profusion Orange
Hybrid Tea Rose	‘Duet’, ‘Eiffel Tower’, ‘Grand Slam’, ‘Mister Lincoln’, ‘Tiffany’, ‘Jamaica’, ‘Matterhorn’
Floribunda Rose	‘Golden Slipper’
Grandiflora Rose	‘Camelot’, ‘Queen Elizabeth’, ‘John S. Armstrong’, ‘Pink Parfait’
Rugosa Rose	‘Rugosa Alba’, ‘Blanc Double de Coubert’, ‘Fru Dagmar Hastrup’, ‘Topez Jewel’, ‘Alba’, ‘Alba Semi-Plena’
Monarda	‘Marshall’s Delight’, ‘Cambridge Scarlet’

Fungicides for Powdery Mildew Control (Table 2)

Fungicides Labeled for Powdery Mildew Control on Ornamental Plants

The fungicides listed below are labeled for use on ornamental plants for powdery mildew control. Active ingredients are listed separately from example product names.

Note: Active ingredients are listed in approximate order from most efficacious (best control) to least, depending upon the plant and species of powdery mildew fungus.

Table 2. Fungicides for Powdery Mildew Control on Ornamental Plants

Active Ingredient	Examples of Brand Names & Products
Bacillus subtilis	BioWorks Cease Biofungicide Serenade Garden Disease Control
Chlorothalonil	Bonide Fung-onil Multi-purpose Fungicide Concentrate Bonide Fung-onil Multi-purpose Fungicide Ready to Use Ferti-lome Broad Spectrum Landscape & Garden Fungicide GardenTech Daconil Fungicide Concentrate GardenTech Daconil Fungicide Ready-to-Use Hi-Yield Vegetable, Flower, Fruit And Ornamental Fungicide Ortho MAX Garden Disease Control Concentrate Southern Ag Liquid Ornamental and Vegetable Flowable Fungicide Tiger Brand Daconil
Copper-based Fungicides	Bonide Captain Jack’s Liquid Copper Fungicide Bonide Copper Fungicide Spray or Dust Ready to Use Camelot O Fungicide/Bactericide Monterey Copper Fungicide Garden Spray Liqui-Cop Natural Guard by ferti-lome Copper Soap Fungicide Concentrate Natural Guard by ferti-lome Copper Soap Fungicide Ready to Use Southern Ag Liquid Copper Fungicide
Horticultural Oil2	Bonide All Seasons Horticulture Spray Oil Ready-to-Use Bonide All Seasons Horticulture & Dormant Spray Oil Ready-to-Spray Ferti-lome Horticultural Oil Spray Monterey Horticultural Oil Concentrate Monterey Horticultural Oil RTU Safer Brand Horticultural & Dormant Spray Oil Concentrate Southern Ag Parafine Horticultural Oil Summit Year-Round Spray Oil
Myclobutanil	Ferti-lome F-Stop Lawn & Garden Fungicide Monterey Fungi-Max Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate For Gardens
Neem Oil Extract	Bonide Captain Jack’s Neem Oil Concentrate Bonide Captain Jack’s Neem Oil Ready to Use Bonide Rose Rx 3-in-1 Concentrate Bonide Rose Rx 3-in-1 Ready to Use Espoma Organic Neem Oil 3n1 Fungicide/Miticide/Insecticide Garden Safe Fungicide 3 Garden Safe Fungicide 3 Concentrate Garden Safe Neem Oil Extract Concentrate Monterey 70% Neem Oil Fungicide/Insecticide/Miticide Natria Neem Oil Concentrate Natria Neem Oil Ready-to-Use Natural Guard Brand by ferti-lome Neem Safer Brand Neem Oil Concentrate Fungicide/Miticide/Insecticide Safer Brand Neem Oil RTU Fungicide/Miticide/Insecticide Southern Ag Triple Action Neem Oil
Potassium Bicarbonate	BioWorks Milstop SP Foliar Fungicide
Propiconazole	Banner Maxx II Fungicide Bonide Infuse Systemic Disease Control Bonide Infuse Systemic Disease Control Lawn & Landscape Ready to Spray Ferti-lome Liquid Systemic Fungicide II Quali-Pro Propiconazole 14.3 Fungicide
Sulfur1	Bonide Sulfur Plant Fungicide Micronized Spray or Dust Ready to Use Hi-Yield Snake Eyes Brand Dusting Wettable Sulfur Safer Brand Garden Fungicide For Flowers, Fruit And Vegetables Concentrate Safer Brand Garden Fungicide II For Flowers, Fruit And Vegetables Southern Ag Wettable or Dusting Sulfur
Tebuconazole	BioAdvanced Science-Based Solutions Disease Control for Roses, Flowers & Shrubs Concentrate
Thiophanate-methyl	Southern Ag Thiomyl Systemic Fungicide
Definitions RTU: Small, pre-mixed bottle RTS: Ready-To-Spray (hose-end sprayer) Use Precautions Note1 Do not apply sulfur when temperatures exceed 90°F or to drought-stressed plants. Do not apply sulfur within two weeks before or after horticultural oil treatments. Note2 Do not apply horticultural oil if the temperature is greater than 90°F. Horticultural oil may injure: Cedars Cryptomeria Hickories Japanese, armur, and red maples Junipers Redbud Smoke tree Add 3 tablespoons of horticultural oil and 3 tablespoons of potassium bicarbonate or baking soda (sodium bicarbonate) to one gallon of water for better powdery mildew control.

Pollinator Protection and Pesticide Use Cautions

Protecting Pollinating Insects

Pollinating insects, such as honey bees and bumble bees, can be adversely affected by pesticides.

Avoid using spray pesticides (both insecticides and fungicides) and soil-applied systemic insecticides unless necessary.

If spraying is required, always do so late in the evening to reduce direct impact on pollinating insects.

Cultural Practices to Reduce Disease and Pest Pressure

Cultural practices to prevent or reduce plant diseases include:

• Pre-plant soil improvement
• Proper plant spacing
• Crop rotation
• Applying mulch
• Applying lime and fertilizer based on soil test results
• Avoiding overhead irrigation
• Avoid frequent watering of established plants

Low-Toxicity Fungicide Options and Label Compliance

Less-toxic spray fungicides containing sulfur or copper soap, as well as biological control sprays for plant diseases containing Bacillus subtilis, are available.

However, it is imperative to always read and follow the label directions for each product.

For more information, contact the Clemson Extension Home & Garden Information Center.

Resources

• Powdery Mildew, Burke County Center, North Carolina Cooperative Extension
• What Do I Do About Powdery Mildew in My Garden?, Forsyth County Center, North Carolina Cooperative Extension
• Powdery Mildew on Ornamentals, Dougherty County Horticulture Topics, University of Georgia Cooperative Extension

Pesticides are updated annually. The last updates were made on 2/26 by N. Jordan Franklin.

Originally published 09/05