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Phlox Powdery Mildew


Phlox Powdery Mildew

Usually appear in

Spring to Autumn

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Caused By:-     The fungi Erysiphe cichoracearum and Sphaerotheca fuliginea


Powdery mildews are one of the most widespread plant diseases. They affect virtually all kinds of plants from cereals and grasses, to fruit trees, and broad-leaved shade trees.
The fungi that causes Powdery Mildew on Phlox is Erysiphe cichoracearum and Sphaerotheca fuliginea which are not host specific and can spread to other plants like solanaceae. It appears as disfiguring, powdery white spots on the foliage that merge and can nearly obliterate any remaining green tissue, eventually moving onto the flowers as well. The disease tend occurs first on the lower leaves, and appears to “move up” the plant as spores from the initial infections cause disease on younger leaves. In severe cases, virtually all of the foliage can be infected.
The white fungal threads which penetrate the leaves, usually on the top surface, and absorb nutrients causing leaves to turn yellow and eventually die. Spores are produced on the fungal mycelium early in the season, which are wind or water splash borne to infect other plants or leaves. The effect of this fungus is to reduce the vigour of the plant, which can eventually die.
Spores are produced on the fungal mycelium early in the season, which are wind or water splash borne to infect other plants or leaves.
It is associated with the weather conditions at that time. Dry warm days from this time seem to start it off first on the foliage
When the infected leaf tissues eventually die, it forms resting spores in the dead material which will then contaminate the soil. This disease can not grow in the absence of living plant tissues.
If it is sprayed in the early stages only some of the tissues will die and the leaves will be scarred. The youngest leaves and shoots are especially susceptible to the fungus. The white growth on the leaves are the spores which are carried by the wind or plashes from watering.
The fungus overwinters on fallen leaves or as spores inside a chasmothecia which eventually breaks down to release the spores to infect other plants.
Powdery mildews are usually host specific, which means they do not spread to other types of plants. There are many types of Powdery Mildew fungi and they all produce similar symptoms on stems, leaves and buds. Infected leaves may become distorted, turn yellow with patches of green, and fall prematurely, stems and twigs may collapse and die and infected buds may fail to open.
The disease is usually observed on the upper sides of the leaves with a whitish fungal growth developing on the leaf surfaces in patches which eventually coalesce to cover the leaves, stems or buds. The infected areas dies or may become soggy.
The severity of the disease depends on several factors such as variety of the host plant, age and condition of the plant, and is especially severe in warm, dry climates. This is because the fungus does not need the presence of water on the leaf surface to infect it. However, if the relative humidity is high the problem will be worse, but it does not tend occur when leaf surfaces are wet. Therefore, Crowded plantings where air circulation is poor damp and in shaded areas makes the problem worse. Young growth usually is more susceptible than older plant tissues.
Picture by The Dow Gardens Archive, Dow Gardens,
See also these other Powdery Mildews Apple, Begonia, Hydrangea, Oak, Pansy, Pea, Powdery, Quince, Rose.
See also Downy Mildews Brassica, Downy, Hebe, Lettuce, Onion, Pansy, Pea.
See also other Mildews American Gooseberry, European Gooseberry, Gooseberry.


Maintaining good air circulation will help reduce powdery mildew problems, but the best way to avoid the disease is to choose a disease resistant variety.
Copper based fungicides are best to use against this fungus.
A weekly spray of milk at a concentration of at least 10% (1 part milk to 9 parts water) is said to significantly reduced the severity of powdery mildew infection.
Weekly preventative sprays of baking soda( 1 tablespoon of baking soda a few drops of washing up liquid to 1 Gallon of water) makes an inexpensive control for powdery mildew on plants. Once the infection has taken hold baking soda offers only minimal benefits. Some plants can be burnt by the baking soda therefore test before use on specific plants.
Spraying with a mixture of baking soda(sodium bicarbonate) or potassium bicarbonate combined with a lightweight horticultural oil is said to cure and prevent the problem.

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