Powdery mildew is one of those strange fungal molds that grows on the surface of the leaf but does not penetrate into the leaf tissue. It causes damage by blocking the light from the leaf. There are many different forms of the powdery mildew fungus and there is a specific one for each species of plant that it infects. See the list below for several ornamental plants that are commonly attacked by powdery mildew.

In later summer during the hottest, most humid weather, whitish or gray coloration appears on the leaves of many plants. It looks as if someone took ashes from their fireplace and scattered them over their plants. If the stuff gets thick enough, it blocks the light from getting to the leaf surface and causes them to lose chlorophyll and become yellow. Late in autumn, peppery looking black spots may appear on the infected leaves.

1. Resistance - Probably the best approach to any disease, including powdery mildew, is to avoid it. Of the plants that are susceptible to this disease, plant breeders have been busy trying to find types that are less prone to the problem. Whenever possible, plant cultivars that are known to be resistant to powdery mildew.

2. Proper Placement - Since powdery mildew is encouraged by humid air, susceptible plants should be placed in locations that get plenty of sunlight and air movement. Avoid planting them in low lying areas where heavy, humid air will settle in around them.

3. Irrigation - Avoid watering onto the foliage of susceptible plants especially in the evening when the leaves do not have time to dry out before nightfall.

There are several fungicides labeled for use against powdery mildew. Of course, these are preventatives and need to be applied before infection. This is often difficult to attain in the home garden.

Other approaches include choosing mildew resistant cultivars or species of commonly grown ornamental plants such as garden phlox or lilac shrubs. Placing the plants so that they get plenty of sun and air movement may help reduce the incidence of powdery mildew.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

Listed below are many ornamental plants that are liable to be infested by powdery mildew. The range of susceptibility can be large, however. Some of these species will only develop minor levels of mildew depending on the environmental conditions. Others such as common lilac, monarda, zinnia, salvia, roses, etc. are plants that are extremely susceptible and get mildew just about every year.

Achillea - Yarrow
Aesculus - Horsechestnut
Ajuga - Carpet bugle
Alcea - Hollyhock
Antirrhinum - Snapdragon
Amaranthus - Amaranth
Aquilegia - Columbine
Alnus - Alder
Acer - Maple


Ceanothus - Wild Lilac
Calendula - Pot Marigold
Centaurea - Bachelor Button
Chrysanthemum - Mum
Cornus - Dogwood
Cotinus - Smoke Tree
Crataegus - Hawthorn
Corylus - Hazelnut

Dianthus - Pinks, Sweet William
Erica - Heath
Eschscholsia - California Poppy

Fragaria - Strawberry
Fraxinus - Ash

Gerbera - Transvaal Daisy
Gaillardia - Blanketflower

Heuchera - Coralbells
Helianthus - Sunflower

Iberis - Candytuft

Juglans - Walnut


Lathyrus - Sweet Pea
Lagerstroemia - Crape Myrtle

Ligustrum - Privet
Liriodendron - Tulip Tree
Lobularia - Sweet Alyssum
Lonicera - Honeysuckle

Myosotis - Forget-Me-Not
Malus - Apple, Crabapple
Monarda - Beebalm

Nicotiana - Flowering Tobacco

Philadelphus - Mockorange
Papaver - Poppy
Photinia - Nine Bark
Prunus - Plum, Cherry
Pyracantha - Firethorn
Pyrus - Pear
Potentilla - Cinquefoil
Passiflora - Passionflower
Platanus - Planetree, Sycamore
Populus - Poplar

Quercus - Oak

Ribes - Currant, Gooseberry
Rosa - Rose
Rhus - Sumac
Rudbeckia - Blackeyed Susan
Rhododendron - Rhododendron or Azalea
Robinia - Black Locust
Rubus - Bramble

Salpiglossis - Painted Tongue
Salvia - Sage
Senecio - Cineraria
Symphoricarpos - Snowberry
Syringa - Common Lilac

Vinca - Periwinkle
Viola - Pansy, Violet
Vaccinium - Blueberry



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