Powdery Mildew – Trees & Shrubs
Many woody plants such as rose and lilac are susceptible to powdery mildew.
Brian Hudelson, UW-Madison Plant Pathology
Item number: XHT1005a
What is powdery mildew? Powdery mildew is a disease that occurs on the above-ground parts (especially the leaves) of many deciduous trees and shrubs, as well as herbaceous ornamental plants, indoor houseplants, and many agricultural crops. Conifers are not affected by this disease.
What does powdery mildew look like? The name of this disease is descriptive. The upper and (less frequently) lower surfaces of leaves, as well as stems of infected plants, have a white, powdery appearance. They look as though someone has sprinkled them with talcum powder or powdered sugar.
Where does powdery mildew come from? Powdery mildew is caused by several closely related fungi that survive in plant debris or on infected plants. These fungi are fairly host specific. The powdery mildew fungus that infects one type of plant (e.g., lilac) is not the same powdery mildew fungus that infects another (e.g., phlox). However, if you see powdery mildew on one plant, then weather conditions, usually high humidity, are favorable for development of the disease on a wide range of plants.
How do I save a plant with powdery mildew? DO NOT panic! For many trees and shrubs (e.g., lilac), powdery mildew is a cosmetic, non-lethal disease. For other plants (e.g., rose, ninebark) powdery mildew can cause severe leaf loss and even branch tip dieback. When a highly valued plant has had severe leaf loss due to powdery mildew for several years, you may want to consider using a fungicide for control. Fungicides containing dinocap, dithiocarbamates, myclobutanil, triadimefon, triforine, sulfur or thiophanate methyl are registered for use against powdery mildew. A combination of baking soda (11∕2 tablespoons) and light weight horticultural (e.g., Sunpray®) oil (3 tablespoons) in water (1 gallon) has also been shown to be effective for powdery mildew control. Most products should be applied every seven to 14 days from bud break until humid weather subsides. Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible. Also consider pretesting any product you decide to use on a small number of leaves or plants before treating a larger area to make sure there are no toxic effects, particularly when treating during warmer weather.
How do I avoid problems with powdery mildew in the future? Consider buying plant varieties that are powdery mildew resistant. This won’t guarantee that your plants will be powdery mildew-free every year, but should result in less severe disease when it occurs. Reduce the humidity around your plants by spacing them further apart to increase air flow. Be sure not to over-water as this can lead to higher air humidity as well. Finally, at the end of the growing season, remove and destroy any infected plant debris as this can serve as a source of spores for the next growing season.
For more information on powdery mildew: See UW-Extension Bulletin A2404 or contact your county Extension agent.
Links & Downloads
- Powdery Mildew – Trees & Shrubs (PDF)
- Powdery Mildew of Ornamentals publication
Tags: disease, tree Categories: Tree & Shrub Problems
Controlling Powdery Mildew
Powdery mildew has been seen on a wide variety of woody shrubs in Minnesota this year. Infections range from a light dusting of powdery white spots to leaves completely covered with dense white felt. Ninebark (Physocarpus opulifolius; especially varieties with dark leaves), Arrowwood Viburnum (Viburnum dentatum), and Lilac (Syringa spp.) are just a few of the ornamental shrubs that have been observed with this fungal disease.
Powdery Mildew on a Ninebark Shrub
Powdery mildew is easy to recognize because it looks just like its name. Powdery white fungal mycelia and spores can be found on the surface of infected leaves, stems, flowers and fruit. If the disease starts on mature leaves, the fungus is often only a few spots or a light coating of white on an otherwise healthy looking leaf. These infections are typically considered minor infections, and have little effect on the overall health of the shrub. If the disease comes in on young developing leaves, shoots and flowers, however, leaves may be severely affected. It is not uncommon to find young leaves that are crinkled, cupped upwards or otherwise distorted by powdery mildew infections. These leaves often turn yellow or are covered in white fungal growth. Young flowers and fruit may be completely coated with the white fungus and often fall off prematurely. Young shoots that are severely infected may even be killed.
One way that the powdery mildew fungi survive Minnesota’s harsh winters is by colonizing young tissue within plant buds. In these cases the fungi starts up new infections as soon as the buds open in the spring. A shrub that has a few very severely infected young shoots next to other, completely healthy shoots, most likely had powdery mildew fungi surviving in its buds all winter long. This type of infection can be seen on many ninebark shrubs this year.
Powdery mildew fungi are favored by mild weather (60-80F) and high humidity. They thrive in shade and cause the most severe infections on young, succulent shoots. In addition, most powdery mildew fungi can only cause disease in one genera or one family of plants. The powdery mildew fungi on lilac will therefore not spread to the rose bush in the yard. A different species of powdery mildew fungi must be present to infect the rose bush.
What can be done to control these powdery pests?
First, many powdery mildew infections do not significantly harm the plant and therefore do not need to be controlled. Many lilacs bloom beautifully year after year despite repeated infections with powdery mildew. Mature trees may have powdery mildew on the lowest leaves, but this is such a small proportion of the canopy that the tree’s overall health is not affected.
In cases where leaves and shoots are distorted, yellowed, and stunted, action should be taken. If only a few shoots are severely infected, these can be pruned off and removed from the garden. This is true for infected flower clusters as well as leafy shoots. In addition, infections on stems should be removed, since these often turn into bud infections and survive the winter in that form.
Humidity around the plant should be reduced as much as possible. By mulching around the shrub with woodchips or other organic material it will help to keep moisture in the soil. Improve air circulation around the plant by removing any weeds crowding the plant. Place shrubs in areas of the garden where there is good air movement and sun appropriate for the shrub’s needs. Do not over fertilize shrubs suffering from powdery mildew. Fertilizer often results in a flush of new succulent growth that is easily infected by the fungus.
Rose leaves affected by Powdery Mildew
If all cultural control practices are not enough to control the disease, a protective fungicide can be used to control powdery mildew. Copper fungicide can be used to help control powdery mildew, as well as if any of your plants have early or late blight and/or leaf spot. Another product that works great is Fung-onil; it is a great all-purpose fungicide that can be used on any plants from your ninebark to your roses. If you’re looking for an organic option use EcoSmart Organic Garden Fungicide; it’s environmentally safe and works on contact. If your roses are the only plant that seems to be affected by powdery mildew Rose RX 3 in1 is a great multi-purpose product; two way triple action: insecticide – miticide – fungicide. Rose Rx 3 in 1 does contains Neem oil organic insecticide and is approved for organic gardening. And as always please remember to read the labels and follow the instructions as directed when you’re using any chemical.
Information provided from the University of MN Extension Services.
Physocarpus (Ninebark) is a genus of about ten species of flowering plants in the family Rosaceae, native to North America (most of the species) and northeastern Asia (one species).
They are deciduous shrubs growing to 1-3 m tall. The name comes from the appearance of the bark, which is flaky, peeling away in many layers. The leaves are maple-like, palmately lobed, 3-15 cm long and broad, with an irregularly serrated margin. The flowers are white with five petals and numerous stamens, produced in corymbs. The fruit is a cluster of inflated follicles, which turn dry and brown and then split open to release the seeds.
- Physocarpus alternans (Dwarf Ninebark)
- Physocarpus amurensis (Asian Ninebark)
- Physocarpus bracteatus
- Physocarpus capitatus (Pacific Ninebark)
- Physocarpus glabratus
- Physocarpus malvaceus (Mallow Ninebark)
- Physocarpus monogynus (Mountain Ninebark)
- Physocarpus opulifolius (Common Ninebark)
Current use of this plant in its name cultivars (see below) for landscape design is to form a line of uninterrupted, loose informal hedge. Individual specimens may be used for the attractive colors of the foliage. There are little medicinal or food uses of this plant.
Much development has gone into developing attractive foliage and smaller size of this native shrub. Two popular and common cultivars exist. ‘Darts Gold’ is a gold color leaf version of this native plant, while ‘Diablo’, a purple (almost black) leaf cultivar also exists.
Since the new century, much work is continuing to develop hybrids based on the above two cultivars. ‘Coppertina’ a cultivar exhibiting orange-red leaves, is such a development, created when the above two cultivars were crossed.
Little maintenance is required for this native shrub once established. To maintain the shape of the plant, pruning is highly recommended. As the old branches exhibit an exfoliating bark which is for some gardeners its chief attraction, leaving the older branches on the plant is often achieved by only pruning and rejuvenating one third of the older branches on a plant in any one year. If a more youthful looking plant is desired, then hard pruning of all the older branches may also be accomplished. In such a case, pruning should be conducted during the spring.
Like all native shrubs that originate from a basal rosette of stems from a common root, this native North American shrub is easy to propagate.
The shrub after flowering forms many tiny seeds, which are held onto the plant well into winter. Mature seeds may be easily released when the shrub is brushed against or shaken. Seedlings formed from self seeding into cracks and crevices may be carefully removed and transplanted.
Another method of propagation is to divide the basal clump of stems, ensuring each group of stems are attached to a viable root. This is done by cleaving the clump at the basal rosette of stems, right down to the root level, separating the one clump into two or more smaller clumps. Each small clump can then be transplanted. In this initial stage, adequate watering is crucial to ensure the survival of the severed clumps. If a severed clump survives the first winter, it will live on. Severing of the clumps may be done anytime, but typically fall time is best, as the cooler temperatures and increased rainfall ensure less evaporation and demands on the smaller severed root system.
A third method is achieved by layering a long branch. This is done by bending one of the younger more flexible stems to the ground, and pinning it in place. Over the course of one year, tiny rootlets will develop where the stem touches the ground. At that point, the rooted portion may be severed from the mother plant and carefully tended.
Cuttings rooted in rooting hormone has a lower success rate but nonetheless may be a viable option of reproducing this species. However, many state laws prohibit the unauthorized reproduction of any named cultivar without the prior consent of the original nursery that had developed the cultivar.
Pests and Diseases
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