JARS v56n2 - Powdery Mildew: A Review and Update

Powdery Mildew: A Review and Update
Hank Helm
Bainbridge Island, Washington

Powdery mildew is a disease that has become well known to rhododendron growers around the world. Most of us have it. None of us want it. We would all like to know how to get rid of it. The first record of this disease on azaleas that I found was in 1974 and on rhododendrons in 1975. Both of these references were from North Carolina. The first great outcry about the disease occurred in the early 1980s when it was reported in Scotland and in particular when it was found in some of the great rhododendron gardens on Scotland's west coast. Although there are some reports the disease traveled to the United States from Scotland, the report, noted above, published by Dr. Strider of North Carolina State University calls this into question. Earlier works describing diseases found on rhododendrons and azaleas do not mention mildew, so we can only assume it was not a problem prior to the late 1970s and early 1980s.

Powdery mildew is found throughout the world, and specific species of mildew attack plants of many kinds. One report of 12 genera of mildew states that 256 plant species within 172 genera in 59 families occurring in 28 countries are affected. Another report places the number of species at nearly 300 in up to 19 genera on more than 7,000 hosts. Most genera and species of the disease are very specific to plants of a single genus. The species of mildew most commonly attributed to attacks of rhododendron and azaleas is the fungus Microsphaera azalea . That being said, however, when Ken Gibson of Vancouver Island, Canada, sent a plant of Rhododendron 'Helen Child' to Agriculture Canada in the 1980s, the report back stated that Microsphaera penicillata , M. vaccinii and Erysiphe polygoni also occurred on rhododendrons. In addition, Microsphaera vaccinii has been found on Rhododendron occidentale in the wild. Literature records other incidents of all of these species on rhododendrons and/ or azaleas. All of these species are within the fungi family Erysiphaceae . An interesting fact is that there are biological strains, which although identical in morphological characters have forms that are specialized in their attacks on various plants. An example of this is a mildew species that has forms that appear to be the same; however, one form attacks peaches and another roses! Classification of mildew is changing rapidly as more is learned and the result is more named species. (Splitters! Sound familiar?) What seems possible to state is that this rhododendron disease is caused by several genera and species of the fungal family Erysiphaceae .

Classification of the various mildews depends on the structure of what is called perithecia, or cleisthecium (depending on shape). This structure is an organ that contains asci produced by sexual reproduction. The asci release ascispores. Ascispores are released from the perithecia that then infect other areas. The classification is based on the morphological characteristics of the appendages, which are branched in different ways. Asexual reproduction also takes place and is in fact more frequent. Asexual conidia are spores that are spread by wind and produce new colonies, as do the ascispores. Spores can also be spread by being transported on clothing and from transport of diseased plants from place to place.

The disease is a parasite that feeds on the tissues of the leaves and other parts of the plant. It produces hyphae and columns of spores (conidia) that give the leaves the gray or other spotting appearance. The leaf epidermal cells are penetrated and a structure called haustoria are formed which act as anchors and which absorb water and nutrients from the plant. On azaleas, the distinctive gray powdery white growth normally associated with powdery mildew appears. There are other appearances the disease can take. On deciduous azaleas, the leaves will generally become covered with the fungus in the fall and the leaves will develop the distinctive gray white coloration. The leaves may shrivel and curl. Some or all of the leaves may drop. Leaves are generally not stunted on azaleas as the disease usually develops toward the end of the growing season.

On rhododendrons, the leaves may display yellowish green or purplish brown spots or blotches on the upper surface. Lower leaf surfaces may show brown or purple areas of various shapes and sizes. Extensive leaf drop and even death of very susceptible plants may occur. The indirect effect of the diseas is in predisposing the plant to winter damage and other stresses.

There are some varieties that seem to be able to withstand the effects of the disease and "grow through it". Examples of this are members of the Walloper Group*, such as 'Lem's Monarch', 'Pink Walloper' and 'Red Walloper'*. To a certain extent, clones from the Loderi Group also can withstand attacks of the disease as can 'Anna Rose Whitney', and clones from the Naomi Group. On the other extreme, clones such as 'Virginia Richards', 'Leverette Richards', 'Elizabeth', 'Unique', Lady Chamberlain Group hybrids, 'Seta', R. campylocarpum , R. viscidifolium , R. thomsonii , and orange flowered forms of R. cinnabarinum suffer a great deal. Leaves drop and some plants will completely defoliate and can die. The purple forms of R. cinnabarinum and the forms previously called R. xanthocodon are not usually attacked. Some rhododendrons, such as 'Purple Splendour' and 'Vulcan's Flame' have the white powdery growth on both sides of the leaf usually associated with azaleas. Rhododendron macrophyllum , R. albiflorum , and R. groenlandicum have not been found to have mildew.

The approaches to control powdery mildew are many. Because the disease is not going to go away, we must all learn how to cope and make the choices as to how we are individually and collectively going to deal with it. Mildew can be controlled. The question is just what method to use.

Cultural Practices

  • Work towards obtaining only top quality, disease-free plants of resistant cultivars and species from reputable nurseries, greenhouse and garden centers. Consult horticulturists and others knowledgeable concerning availability and performance of resistant varieties.
  • Practice good housekeeping. That means raking dead and fallen leaves where there has been or is infection and destroy them. Do not put into compost piles. Prune plants to provide air circulation and if it is practical, remove affected leaves. This obviously cannot be done if all the leaves are infected.
  • Maintain plants in high vigor by planting properly in good well-drained soil and where they get some sun exposure. Space plants to allow air circulation. Keep plants moist and water foliage only when it can dry out and will not unduly raise the humidity in warm weather. (Mildew does not reproduce in water so sprinkling and watering foliage can be used as a control; however, high humidity favors reproduction of the disease.)

Chemical Control
Many fungicides are registered for use on plants; however, many are not available to gardeners as they are restricted or are highly toxic. Often they are difficult to obtain in small quantities. Sources are garden centers, variety stores, and many locations on the World Wide Web. The cardinal rule is the law - no matter what anyone else says, always follow the label directions. Also remember that just because products are listed below, does not make them legal or approved in all areas of the world. Always refer to local regulations and approvals for use of any chemical! Many products work better if used with a surfactant or spreader sticker. It is also important to know and understand the life cycle of mildew, as it is important to understand in using the control you have chosen. It is a good idea to alternate treatment so resistance to a specific chemical does not build up. The products below are listed by their common name followed by trade names. This is undoubtedly an incomplete list, but will offer some choices.

  • Water with an addition of surfactant. Although not technically a chemical, water can be used as a method of control as noted above in cultural control. I would recommend you be careful of the water used, particularly on vireya rhododendrons as they may not be tolerant of chlorine found in some city water supplies.
  • Soaps. M-Pede® and Safer's Insecticidal Soap®. Must be sure to get excellent coverage of leaf area.
  • Bicarbonates. Armicarb 100®, FirstStep®, Kaligreen®. Yep, just plain old baking soda. Not much research data yet but may offer some help. Some of the data available is based on adding in oils, which are effective by themselves. Clint Smith has a mix he has recommended and that I have found to be helpful. 1½ tblsp. baking soda, 1 tblsp. apple or white vinegar, 1 tsp. sticker or surfactant; all in one gallon of water. A formula from Los Angeles adds canola oil and uses dishwater soap rather than a sticker. It is essential to obtain very good coverage on the leaves with the spray.
  • Horticultural Oils. Sunspray®, JMS Stylett Oil®, Neem Oil®. A good eradicant of the fungus if it has gotten away from you. Do not work well in rain, irrigation and dew which is much of the time in the Northwest and United Kingdom.
  • Sulfur. Safer's Garden Fungicide®, Wettable Sulfur®, Flowable Sulfur® and many others. Shorter application intervals are needed with these products. May burn foliage (I have had particular experience with this problem!) Best application is when new leaves are emerging in spring.
  • Lime Sulfur or Calcium Polysulfite (lime and sulfur bonded together). Dormant Disease Control®, Orthorix®, Sulforix®, Sulf-R-Spray®, Polysul®. A foul smelling but effective fungicide. Can burn young foliage. Labeled for several ornamentals. Not used on rhododendrons much as it is usually applied to dormant plants.
  • Triforine. Funginex, Triforine, Nimrod T. A locally systemic fungicide. May not work as well as it used to due to build up of fungi resistant to the chemical.
  • Benomyl. Benlate®, Tersan 1991®. Another locally systemic fungicide. Resistance to this chemical is known also. Use of this chemical for ornamentals was pulled in the early '90s.
  • Thiophanate Methyl. Halt®, Fungo®, Duosan®, Topsin M®, Cleary's 3336®, Domain®. This is a close relative of benomyl and is the next best thing for ornamentals. Can sometimes be found.
  • Piperalin. Pipron®.
  • Myciobutanil. Immunox®, Tally®, Eagle®, Systhane®, Nova®. One of the most effective fungicides in commercial agriculture which is now available in home packaging. May be the best powdery mildew product anywhere.
  • Tropiconazole. Banner®, Tilt®, Orbit®.
  • Triflumizole. Terraguard ® , Procure®.
  • Triadimefon. Bayleton®, Strike®.
  • Strobilurin. Abound®, Flint®, Sovran®.

Biological Control

  • (Deuteromycetes) Amepelomyces quisqualis AQ10. This is a fungal hyperparasite that has been shown to be effective against powdery mildew. Repeated applications are required and high humidity and rainfall aid in its spreading to developing mildew colonies. It has little effect for seven to ten days while the microparasite spreads. Thereafter, the infected cells of the mildew die rapidly. Studies of control of mildew with A. quisqualis have taken place for many years, but I have seen very little evidence of extensive use. This is not a preventative, as the disease must be present for the hyerparasite to have something to feed upon.
  • Compost Teas, Soilsoup®, Lace® and Blend®. These products are being introduced in the Northwest and are highly touted for a variety of diseases of all kinds of plants. Lace and Blend are trade names for products developed by Dr. Elaine Ingrum who did her research at Oregon State University and is now with a private company, Soil Foodweb Incorporated. Lots of information is available at www.soilfoodweb.com. The products are available from Soil Foodweb Incorporated on the World Wide Web and on Bainbridge Island at Bainbridge Gardens and Bay Hay and Feed. The aerobic mixing devices are available at these locations, SoilSoup, Inc. in Edmonds and others. The brewing devices are expensive. A 30 gallon system can cost $500. Unless a group would be producing the tea, it is probably less expensive to purchase the tea from a garden shop. The cost for five gallons this past summer was $5. The dilution rate for spraying is from 2:1 to 8:1 after initial treatment. Forty gallons goes quite a ways.

These "probiotics" are concentrated solutions of nutrients that support soil biota. These are the bacteria, nematodes, amoebae, etc., that are naturally occurring in soils and plants. The reason for these products according to those who produce and sell them is because of the damage we do and have done with chemicals and treatments to the soils in our gardens. The "good guys" compete with the "bad guys" for space in the soil and plants. The idea is to provide enough "good guys" to displace the "bad guys." The organisms additionally aid plants by helping make it possible for plants to access soil nutrients that are locked up in many cases.

When the products are used as foliar sprays, the empty bacterial spaces in the leaves are filled with beneficial "critters." The claim is that if plants are healthy there will be no chance for pathogens to find a space to grow. In addition, some amebas feed on bacteria.

These probiotics are dormant, much like yeast, which are activated by combining with water and air in a brewing device. They must be used within a few hours of mixing to be effective. Usually that time is within from eight to ten hours. They may be sprayed on plants and soil. They must not be applied with sprayers that have been used for any chemicals.

I was somewhat skeptical about these probiotics until I read an article about the use of compost tea by the University of Washington. The University began brewing their own tea in 2000. They have reduced their reliance on insecticides and avicides from almost 3,500 ounces in 1996 to less than 500 ounces in 2000. In 2000 they applied the tea to the roses around Drumheller fountain and found it useful in preventing powdery mildew. The method is being used by Seattle City Light, Seattle City Parks, Seattle University, Seattle Tilth and Snohomish County.


Keith L. Smith, Associate Vice President for Ag. Adm. And Director, OSU Extension, 'Ohio State University Extension Fact Sheet', OSU web site

'Abound: A Novel, Broad-Spectrum Fungicide for Control of Grape Diseases'. C. Abbott-Tikkanen, D.W. Bartlett and J.R. Godwin. Zeneca Ag Products, Kingsburg, CA 93631; Zeneca Agrochemicals, Jealott's Hill Res. Sta., Bracknell, Berkshire, England RG126EY. Phytopathology 87:S2. Publication No. P-1997-0010-AMA

http://aruba.nysaes.cornell.edu/ent/biocontrol/pathogens/ampelomyces.htm . 'Biological Control': A Guide to Natural Enemies in North America. Weeden, Shelton, and Hoffmann, Editors. Last modified August 10, 1998

Dr. Gary G. Grove, Washington State University, Tree Fruit Research and Extension Center, Wenatchee; 'New tools for managing tree fruit and vine disease' From their Web Site

Dr. Elaine Engham Newsletter; http://www.soilfoodweb.com 'Rhododendron – Powdery Mildew', Oregon State University, http://plant-disease.orst.edu/disease.cfm?RecordID-963 Content Edited by Jay W. Pscheid on Jan. 1, 1999 (Page edited on: April 9, 2001)

Compilation of papers and information by Fred Minch and distributed by Doug Denkers on May 10, 1992. Articles include: 'Inverewe expects…' by Nigel Price; 'Powdery Mildew – Disaster or Nuisance?' reprint of article by Ken Gibson; 'More than you ever wanted to know' reprint of article by Alec McCarter'; 'The Unknown Garden Intruder', by Ken Gibson. Letter from Ken Gibson to Fred & Jean Minch, 1991.11.19; 'Is the Unknown Garden Intruder the Kiss of Death' reprint of article by Ken Gibson. Letter from Ken Gibson to Fred Minch dated 1992.04.08 with copy of letter from Dr. Coyier, Research Plant Pathologist (retired)

Jay W. Pscheidt, Extension Plant Pathologist, Oregon State University, 'General Powdery Mildew Tactics' unknown date

An online Guide to Plant Disease Control, 'Fungicides for Disease Control in the Home Landscape', Oregon State University, Article updated Jan. 1, 2001, Edited by Jay W. Pscheidt and Cynthia M. Ocamb, Extension Plant Pathology Specialists, Oregon State University.

http://pestdata.ncsu.edu/cropprofiles/docs/warhododendronazalea.html 'Crop Profile for Nursery-grown Rhododendron and Azalea in Washington, December 2000. Database and web development by the NSF Center for Integrated Pest Management located at North Carolina State University. Credit to USDA

http://depts.washington.edu/uweek/archives/2001.03.MAR_08/_article9.html "'Compost tea' allows gardeners to brew greener pastures" by Steve Hill, University Week, the faculty and staff publication of the University of Washington

'Rhododendron Powdery Mildew', Regional Garden Column, June 11, 2000, Area Extension Agent, Washington State University, "Gardening in Western Washington", Presented by WSU Cooperative Extension.

'Powdery Mildew', Regional Garden Column, Oct. 1, 2000, Area Extension Agent, Washington State University, "Gardening in Western Washington", Presented by WSU Cooperative Extension.

Jean L. Williams – Woodward Extension Plant Pathologist – Ornamentals, University of Georgia, Updated 6/97

Ann Lovejoy for Bainbridge Gardens 3-2001, 'Using Probiotices: Blend & Lase'

Ann Lovejoy for Bainbridge Gardens, 'Soilsoup for Gardeners' undated

http://www.soilfoodweb.com , Soil Foodweb Inc., Dr. Ingham's Free Monthly E-Newsletter, 4/13/2001. Answer to question of benefits to paying attention to biological side of soils in addition to chemistry and mineral fertility. 'The benefits to Plant and Soil'

http://www.statlab.iastate.edu/survey/SQI/sqw.html 'What is Soil Quality?', Soil Quality Institute, Natural Resources Conservation Service, United States Department of Agriculture, downloaded 4/16/2001

www.nssc.nrcs.usda.gov USDA Natural Resources Conservation Service, January 1998, "Soil Quality Information Sheet", 'What is soil biodiversity', 'What are the benefits of soil organisms?', 'Management considerations'

http://www.rhs.org.uk/advice/profiles0800/mn_powdery-mildews.asp "Problem profiles", 'Powdery mildews', Royal Horticultural Society, 2001

'Powdery Mildew on Ornamentals and Vegetables', Integrated Pest Management Solutions for the Landscape Professional, "The Green Garden Program" a collaborative effort of Seattle Tilth, Washington Toxics Coalition and WSU Cooperative Extension, King County. Sponsored by the Seattle Public Utilities

Robert Smaus, The Los Angeles Times real estate writer. A homemade pesticide for the cure for powdery mildew. Date unknown

Nancy R. Pataky, Extension Specialist and Director of the Plant Clinic, 'Powdery Mildews of Ornamentals', University of Illinois Extension. RPD No 617, 1987. Department of Crop Sciences, University of Illinois at Urbana-Champaign

Hank Helm, a member of the Kitsap Chapter, recently wrote a review and update on root weevils, ("Root Weevils: Troublesome Rhododendron Pests," in the fall 2001 issue of the Journal).