This is one of the most recognizable and well known of the plant diseases that affect both ornamentals and crops. It’s very widespread and most severe in warm, dry climates.
Powdery Mildew Host Plants
Nearly any plant can be host to this blight. Powdery mildew is actually several species of fungi that affect nearly all plant types. Everything from cereals and grasses to vegetables, flowers, weeds, shrubs, turf grass, vegetables of all kinds, fruit trees and vines, etc. The rot attacks the succulent portions of the plant, which can be the flowers, stems, buds or fruits.
Powdery Mildew Symptoms
Most all powdery mildew create the same basic symptoms, so despite the fungi being of different species, they are grouped together under this common name due to their effect on plants. The first sign is usually small spots or patches of gray or white with a talcum-powder-like look. Very close inspection will show tiny spherical structures that are the spore houses for the fungus.
It most often appears on the upper (skyward) side of the leaf, the outer edge of flowers, and the top portion of fruits – and stems. On plants that were affected the previous year, the over-winter spores (cleistothecia) will be black scaly-appearing clumps on the stem or leaf.
Powdery Mildew Life Cycle
Preferring warm, dry, even severe climates, the powdery mildew will affect plants even without water being present – a deviation from other fungi. There does need to be high relative humidity, however, which is common in the Rocky Mountains and high plains. The spores do require a host plant in order to survive, however, so this mildew does not spread via soil or insect.
The powder will spread quickly once it has infected a host. The spores will quickly produce and often fall to the ground attached to leaves or stems that have been killed. They will overwinter this way and find a new host growing out of the ground or by being blown into one in the spring.
Powdery Mildew Plant Impact
The way this disease affects a plant will depend on the plant type. Trees will often lose leaves and foliage and may lose fruits and buds, but will rarely be killed by this blight. Garden plants and annuals, however, can be devastated by an infestation of powdery mildew.
It will destroy leaves, foliage, fruits, and often new shoots and stems. For a tree, this can mean a setback for the year, but for less robust and long-lived plants, it can mean disaster.
How to Prevent Powdery Mildew
Prevention is not like with other fungi. Some tolerant and resistant strains of popular plants such as roses, Kentucky bluegrass, and vegetables for the garden have been bred. Avoiding late summer applications of nitrogen can help avoid a fast-acting powdery mildew that thrives on excess nitrogen. Removing debris and litter and properly trimming and pruning plants and trees is another good way to keep this fungus in check.
How to Treat Powdery Mildew
Fungicides can often be effective if applied early. Sulfur, neem oil, and potassium bicarbonate are proven remedies and will have varying levels of success, depending on the plant. Try a fungicide, like GreenCure Organic Fungicide, which has an active ingredient of potassium bicarbonate and can treat a wide variety of plants. Removal and destruction of infected plants is also common.
Want to learn more about the fungal disease of powdery mildew?
Check out these resources:
Powdery Mildews from Colorado State University Extension
Powdery Mildews on Ornamental Plants from Ohio State University Extension
Diseases of Landscape Plants: Powdery Mildew from Purdue University Cooperative Extension