Powdery mildew is one of the most common diseases in greenhouse production. Some greenhouse crops prone to infection include African violet, begonia, dahlia, gerbera daisy, hydrangea, verbena, roses, kalanchoe and poinsettia. Many herbaceous perennials such as aster, centaurea, coreopsis, delphinum, monarda, phlox, rudbeckia and sedum may become infected. Herbs such as rosemary, sage, St. Johnswort, and mint as well as greenhouse tomatoes and cucumbers may become infected with this disease. Although powdery mildews rarely kill a plant, they reduce the aesthetic value and salability of the diseased plants.
Powdery mildew is easily recognized by its white talcum-like growth. Symptoms may appear first on the upper leaf surface, but they can also develop on the lower leaves. When symptoms develop on the more mature leaves, powdery mildew is harder to detect and seems to occur overnight”, catching many growers unaware. As soon as favorable environmental conditions develop, powdery mildew develops into an epidemic as more leaves become infected.
Causal Organisms and Disease Development
Powdery mildews generally look alike so it is a common misconception that they are all caused by the same fungus. But, different types of fungi such as Erysiphe, Leveillula, Microsphaera and Spaerotheca may occur in the greenhouse. All of these fungi are obligate parasites that need a living plant host in order to complete their life cycle. They usually survive in the greenhouse on crop or weed hosts.
Powdery mildews can attack healthy, vigorously growing plants. Erysiphe has a broad host range and attacks many members of the Aster family. Sometimes, mildews are relatively host specific. For example, Sphaerotheca violae only attacks Viola. If you are growing a diverse mix of herbaceous perennials in the greenhouse, it is helpful to know the type of powdery mildew so you can better determine the potential spread of the disease to your crops. This will make scouting easier.
Powdery mildew, unlike many foliar diseases, does not need free moisture on the leaf to thrive. Favorable environmental conditions include high relative humidity (greater than 95%), moderate temperatures of between 68o to 86o F and relatively low light levels. Infections may be more common in the spring and fall when changes between the day and night temperatures encourage high relative humidity levels, especially at night.
Spores (conidia) are produced in chains. Air currents and water splash in the greenhouse easily move these spores. The spores germinate and thread-like strands (hyphae) grow along the leaf tissue. Powdery mildews obtain plant nutrients by sending feeding organs (haustoria) into the epidermis. Once a spore lands on a plant, it may take as little as 3 days but more often five to 7 days for infection to develop.
Maintain proper plant spacing to reduce relative humidity levels within the plant canopy. (This will also help you gain better spray coverage). Keep relative humidity levels below 93 percent in the greenhouse. Heat and ventilate in the late afternoon and early morning to reduce high relatively humidity at night. Clean your greenhouse thoroughly between crops, removing all weeds that could be potential hosts.
Most ornamental crops are not selected for pest resistance. However, some resistant cultivars are available. Resistant pansy cultivars are Delta Pure Rose and Bingo Deep Purple. The Profusion series of zinnia is also resistant to powdery mildew. Many different cultivars of Monarda are resistant such as 'Jacob Cline’, ‘Little Siberia’, and ‘Marshalls Delight’. The Phlox cultivars 'David', 'EcoPastel Dream', ‘Frosted Elegance’, ‘Miss Lingard’, ‘Norah Leight’ and ‘Speed Limit 45’ are also resistant. Certain cultivars of gerbera daisy including ‘Terrafame’ tend to be more resistant. Verbena cultivars also vary greatly in their susceptibility to powdery mildew. (See the article, Verbenas and Powdery Mildew, by Margery Daughtrey and Mary Hausbeck, listed with a web link in the reference list for more information.)
Begin scouting early and as often as you can, at least once a week and more often, every two to three days, if possible. Look for the fluffy, talcum-like, powdery colonies especially on the upper surfaces of leaves. Stems and flowers may also be attacked. On kalenchoe and sedum, brown scab-like lesions develop with little powdery growth. From a distance, it looks like a leaf spot disease or perhaps spray injury. On greenhouse tomatoes, fungal growth is very sparse and easily overlooked. Use a 10x-hand lens to look for whitish threads radiating out from a central point or for chains of spores. Spray residue does not appear as fluffy and tends to have more of a droplet like outline. If powdery mildew develops on the lower surface, you may see a small, yellow spot on the upper surface on poinsettia and other crops.
Powdery mildew may first be detected in locations with more changes between day and night temperatures. Hanging baskets or plants near the vents may first develop powdery mildew. Flag the affected area so you can easily revisit the plants after sprays have been applied. If only a low level of disease is detected, remove infected leaves or plants. Because the spores are so easily airborne, carry a plastic bag and carefully place the infected material into the bag.
Powdery mildews only colonize the upper layer of cells, so chemical eradication is possible. You do not need to spray preventatively for powdery mildew, but you do need to spray as soon as the disease is detected. Rotate among fungicide classes to discourage development of resistance. See the most recent edition of the New England Greenhouse Floriculture Guide: A Management Guide for Insects, Diseases, Weeds and Growth Regulators for more specific up-to-date recommendations.
Extension Educator, University of Connecticut
Photos by Leanne Pundt, used with permission
Daughtrey, M.L., R.L. Wick and J.L. Peterson. 1995. Compendium of Flowering Potted Plant Diseases. APS Press (The American Phytopathological Society) St. Paul. MN. 90 pp.
Daughtrey, M. and M. Hausbeck. 2006. Verbenas and Powdery Mildew. Greenhouse Product News. April 2006. 34-37. Available on-line at http://www.gpnmag.com/Pests-Diseases-Verbenas-Powdery-Mildew-article7004
Daughtrey, M., and J. Hall. 1992. Powdery Mildew- A New Threat to Your Poinsettia Crop. GrowerTalks. September 1992. 23-25.
Douglas, S.M. 2000. Powdery Mildew. New England Greenhouse Conference. Worcester. MA. 2 pp.
Hausbeck, M. 2003. Special Research Report #113. Managing Powdery Mildew on Gerbera. The American Floral Endowment. Available on-line at: http://endowment.org/wp-content/dm113.pdf
Perry, L. 1998. Powdery Mildew on Phlox and Monarda. University of Vermont Extension System. Plant and Soil Science Department. COH 39. 2 pp.
Perry, L. 2004. Evaluation of Phlox and Monarda varieties to Powdery Mildew. Biological and Cultural Tests. Vol. 20.0001
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