Powdery Mildew on Ornamentals

Pest Notes: Introduction

Powdery mildew is a common fungal disease on many types of plants and can be found across California under diverse conditions. This disease can be serious on woody species such as rose, crape myrtle, sycamore, and oak where it attacks new growth including buds, shoots, flowers, and leaves. There are many species of powdery mildew fungi, and each species infects a range of different plants.

Identification and Damage

Powdery mildew can be identified by the white, powdery growth that usually forms on leaf surfaces and shoots, and sometimes on flowers and fruits. New plant growth may be dwarfed and distorted. Infected leaves generally die and drop from the plant earlier than healthy leaves. Almost all plants infected with powdery mildew will develop some distortion of plant parts as the disease progresses past the initial infection. On oaks, powdery mildew can produce an abnormal growth known as witches’ broom. Powdery mildews can infect both new and old foliage.

On perennial hosts such as roses, oak trees, sycamore trees and euonymus, powdery mildew survives from one season to the next as vegetative strands in plant buds or as spherical fruiting bodies, called chasmothecia. A chasmothecium holds the spores (ascospores) that will first infect plants in the springtime; they appear as small black dots on leaves or stems.

Wind carries powdery mildew spores to new hosts. Although relative humidity requirements for germination vary, all powdery mildew species can germinate and infect without water on the plant's surface. Water on plant surfaces for extended periods inhibits spore germination and kills the spores of most powdery mildew fungi. Moderate temperatures of 60° to 80°F and shady conditions are most favorable for powdery mildew development. Powdery mildew spores and mycelium are sensitive to extreme heat and sunlight, and temperatures above 95°F may suppress growth of the fungus.

Powdery mildew fungi belong to the Erysiphales order of the Ascomycota phylum. Most powdery mildew fungi grow as thin layers of fungal matter called mycelium on the surface of affected plant parts. All powdery mildew fungi require living plant tissue to grow. Spores (conidiospores), which you can see with a hand lens, are part of the white, powdery appearance of these fungi and are produced in chains on both upper and lower leaf surfaces, on flowers, fruits, or herbaceous stems.

In contrast, downy mildew (another disease that produces visible powdery growth), has spores that grow on branched stalks and look like tiny trees under a hand lens. Downy mildew spores occur mostly on lower leaf surfaces. Environmental conditions that favor the growth of downy mildew are different from those that favor powdery mildew and include low temperatures of 50° to 70°F, a relative humidity of 90% or higher, and free moisture on the plant surface.

Management

The best methods of control for powdery mildew are preventative. Powdery mildew can be limited by planting resistant cultivars when available, placing plants in full sun, and following good plant care practices. Common ornamental plants susceptible to powdery mildew include aster, deciduous azalea, tuberous begonia, California poppy, China aster, chrysanthemum, columbine, coral bells, corn flower, cosmos, crape myrtle, dahlia, delphinium, euonymus, forget-me-not, gaillardia, godetia, hydrangea, London plane tree, lupine, lilac, mint, monarda, oak, pansy, periwinkle, phlox, pot marigold, ranunculus, rose, rhododendron, rudbeckia, salvia, snapdragons, sweet pea, turfgrasses, verbena, and zinnia. Some ornamentals require protection with fungicides if mildew conditions are favorable, especially susceptible varieties of rose, crape myrtle, and euonymus (See Table 1).

Table 1. Powdery mildew fungi, hosts, and control measures.

Fungus species	Hosts	Controls
Golovinomyces cichoracearum	begonia, Composite family (chrysanthemum, dahlia, phlox, sunflower, and zinnia)	spray with water; fungicides if necessary
Erysiphe lagerstroemiae	crape myrtle	resistant cultivars
Erysiphe graminis	turfgrasses	resistant cultivars or species; fungicides; increased sunlight
Sphaerotheca pannosa	rose	resistant cultivars; fungicides if necessary

Resistant Varieties

Cultivars resistant to powdery mildew are available for some susceptible plants including rose, crape myrtle, euonymus, and sycamore (Table 2). Choose resistant varieties to reduce the likelihood of having to apply pesticides.

Table 2. Some Ornamentals with Resistant Cultivars.
This list is not complete.

Susceptible Plant	Resistant Cultivars
Crape myrtle	those with Native American names, e.g., ‘Catawba’, ‘Cherokee’, ‘Hopi’, also 'Bellini', 'Bourbon Street', 'Centennial Spirit', 'Muskogee', 'Tuscarora', etc.
Euonymus	variegated varieties more resistant than non-variegated types
London plane tree	‘Yarwood’, ‘Columbia’, 'Exclamation!', ‘Liberty’
Monarda	‘Marshall’s Delight’, ‘Blaustrumph’, ‘Colrain Red’, 'Jacob Cline'
Phlox	Most annual phlox are powdery mildew resistant: Phlox maculata ‘Natasha’, P. glaberrima ‘MorrisBer’, P. paniculata ‘Robert Poore’, ‘David’, 'Frosted Elegance', 'Pastel Dream'
Rose	'Double Delight', 'Julia Child', 'Iceberg', 'Sunshine Daydream', ‘Simplicity’ and ‘Meidiland’ roses, Rosa rugosa varieties and many more
Rhododendron	R. yakushimanum, R. macrophyllum, R. ‘Nova Zembla’, ‘Palestrina’, 'Golden Lights'
Zinnia	Pulcino and African varieties; Zahara and Profusion series zinnias

Cultural Practices

Cultural practices are things you can do to discourage disease and include good sanitation, removing debris and infested plant material, proper watering and fertilizing, staking or pruning plants to ensure good air circulation, and using disease-resistant plants.

Since shade and moderate temperatures encourage the growth of most powdery mildews, locate plants in sunny areas as much as possible. Provide good air circulation and avoid excess fertilizing or use a slow-release fertilizer. Overhead sprinkling can reduce the spread of powdery mildew because it washes spores off the plant. Also, if spores land in water, they are unlikely to survive. The best time to irrigate is mid-morning, so plants dry rapidly, reducing the likelihood of infections by other fungi, such as those that cause rust or black spot on roses. As new shoots form on perennial plants, watch closely for signs of powdery mildew.

Chemical Control

In some situations, especially when growing roses or other susceptible plants, you may find it necessary to use fungicides as protectants, eradicants, or both. A protectant fungicide prevents new infections from occurring, while an eradicant can stop an existing infection. Apply protectant fungicides to highly susceptible plants before the disease appears. Use eradicants at the earliest signs of the disease. Eradicant fungicides can cause phytotoxicity because powdery mildew fungi survive in the epidermal cells of plant hosts. When the fungi are killed, plant cells also die. Use eradicant fungicides early in the disease cycle to minimize phytotoxicity. Once mildew growth is extensive, controlling the situation with any fungicide becomes more difficult.

Fungicides. Several less toxic fungicides are available, including those with active ingredients such as horticultural oils, neem oil, sulfur, potassium bicarbonate, phosphonates, and biological materials. Many of these are labeled as organic because they are derived from natural sources. Synthetic fungicides are also available for use against powdery mildew. Except for the oils, these fungicides are primarily preventive, although potassium bicarbonate has some eradicant activity. Oils work best as eradicants but also have limited protectant activity.

Horticultural Oils. To eradicate powdery mildew infections, use a petroleum based horticultural oil such as JMS Stylet Oil, Pest Fighter Year-Round Spray Oil, Sunspray Ultra-Fine Spray Oil, Monterey horticultural oil, or one of the plant-based oils such as neem oil (e.g., EcoWorks EC). Never apply an oil within 2 weeks of a sulfur application, or it may injure plants. Also, do not apply oils when temperatures are above 90°F or on water-stressed plants. Some plants may be more sensitive than others, and the interval needed between sulfur and oil sprays may need to be even longer. Always consult the fungicide label for any special precautions. JMS Stylet Oil is the most highly refined and therefore the least likely to damage plants, but this product is marketed to licensed pesticide applicators.

Sulfur. Sulfur products have been used to manage powdery mildew for centuries but are effective only when applied before the disease appears. The best sulfur products to use for powdery mildew control in gardens are those that are specially formulated with surfactants (e.g., Safer Garden Fungicide) that help the active ingredient spread on the leaf surface. Sulfur sprays can damage some ornamental cultivars. To avoid injuring plants, do not apply sulfur when the temperature is near or higher than 90°F, and do not apply it within 2 weeks of an oil spray. Other sulfur products, such as sulfur dust, are much more difficult to use, irritate skin and eyes, and are limited in the type of plants you can safely use them on.

Bicarbonates. Several products containing potassium bicarbonate such as MilStop effectively eradicate powdery mildew fungi. Products containing potassium bicarbonate can injure powdery mildew infected plants, so use them with caution. Do not use bicarbonate on advanced stages of powdery mildew disease as phytotoxicity will be severe.

Phosphonates. These fungicides are made from the salts and esters of phosphorous acid. Phosphonates are systemic fungicides, which means they move through the plant. Active ingredients in this group are usually phosphorous acid or phosphite (e.g., Monterey Garden Phos).

Biological Fungicides. There are some biological fungicides (e.g., Serenade Garden Disease Control) commercially available as beneficial microorganisms formulated into products that, when sprayed on the plant, destroy fungal pathogens. The active ingredient Bacillus subtilis helps prevent the powdery mildew fungus from infecting the plant. Products containing this active ingredient have some effect in killing the powdery mildew organism but are not as effective as the oils or sulfur in controlling it.

Synthetic Fungicides. Products containing myclobutanil include Rally 40WSP for professionals and Monterey Fungi-Max for home gardeners. Myclobutanil functions as an eradicant and protectant against both powdery mildew and rust.

How to Use Fungicides. Apply protectant fungicides to susceptible plants before or in the earliest stages of disease development according to label directions. Once mildew growth is mild to moderate, it generally is too late for effective control with fungicides. These are effective only on contact, so applications must thoroughly cover all susceptible plant parts. As plants grow and produce new tissue, more applications may be necessary at 7- to 10-day intervals (or intervals specified on the product label) if conditions favor disease growth.

References

Dreistadt SH, Clark JK, Martin TA, Flint ML. 2016. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, 3rd Ed. UC ANR Publication 3359. Oakland, CA.

Bettiga LA. 2013. Powdery Mildew. In Grape Pest Management. Oakland: UC ANR Publication 3343. Oakland, CA.

McCain AH. 1994. Powdery Mildew. HortScript No. 3, Univ. Calif. Coop. Ext. Marin County.

Revised based on a previous version authored by WD Gubler and ST Koike.

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