Home gardeners may easily recognize powdery mildew by the patches of white fungus growth on the upper leaf surface. Some more commonly infected woody ornamentals include lilac, dogwood, azalea, sycamore and rose. Many perennials may become infected, including garden phlox, bee-balm, aster, yarrow, coreopsis and Rudbeckia. In spite of the similar appearance of powdery mildew on these hosts, different species may be responsible for infection. For example, powdery mildew on oak will not infect zinnia.

Powdery mildew thrives in the mid-summer to fall because the warm days followed by cool nights favor disease development, but also will develop in late spring. During warm dry days, the powdery mildew spores (reproductive structure of fungi) may be blown to new hosts. The high humidity at night is favorable for spore germination. Unlike most other fungi, powdery mildew spores do not require water on the leaf surface in order to germinate and then infect a leaf.

After spore germination, the powdery mildew fungus grows on the leaf surface and sends suckerlike structures (haustoria) into the plant’s epidermal cells (cells in the outer protective layer of plants) in order to obtain nutrients. Powdery mildew fungi need live tissue to grow and reproduce. The powdery mildew fungus may survive the winter as small black pepper-like spores (cleistothecia) that form on the infected leaves in the autumn. In the following year, new infections may begin from the infected leaf litter. Some species of powdery mildew may overwinter in the buds.

Because powdery mildew occurs so late in the season, it may not reduce the long-term health of trees and shrubs. Infections tend to be unsightly and of cosmetic concern.

Prevention

Adequate plant spacing will help to increase the air circulation around plants to discourage disease development. Gardeners should avoid placing plants in a damp, shady corner with poor air circulation that will promote powdery mildew. The cleanup of infected leaves may help to reduce disease inoculum.

Resistant Varieties

Gardeners should first consider the use of species or varieties that are resistant to powdery mildew. The following is a partial list of varieties reported to be resistant. Please see the reference list for additional information.

Some Ornamentals Resistant to Powdery Mildew

Monitoring

During conditions favorable for disease development, monitor susceptible plants for the first signs of infection. Initial symptoms may include spotting and curling of the foliage followed by white patches on the leaves. As the disease progresses, foliage and shoots may become distorted, buds may fail to open normally, and foliage may turn chlorotic and yellow. Some leaf drop may occur. As soon as the infection is detected, a preventive fungicide program can begin.

Biorational and Biological Fungicides

Biorational materials, such as potassium bicarbonate, sodium bicarbonate (baking soda), superior horticultural oils and neem oil have been shown to be as effective in controlling powdery mildew as conventional fungicides. They have a relatively short residual on plants and have less of an impact on beneficial organisms than the more traditional pesticides. Additional advantages of these materials include low toxicity to applicators, relative safety to non-target sites and the potential of combined insecticidal and fungicidal activity.

Biological fungicides are beneficial micro-organisms that have been formulated to be applied to the plants to kill the disease pathogen. Bacillus subtillis and Trichoderma are two of the common biological materials registered for ornamentals. The success of bio-fungicides is variable and depends on the plant condition as well environmental conditions.

Contact your local Cooperative Extension center for chemical and management options. Or visit the on-line guides of Cornell University and Penn State. This information may not apply to your state.

References

Auon. 1994. Mildew Resistant Zinnias. Sunset (Central West edition) 192:80 May 1994.

Becker, H. 1994. Neem Oil Locks Out Spores. Agricultural Research, June 1994.

Beckerman, J, and B.R. Lerner. 2009. Disease-resistant Annuals and Perennials in the Landscape. Purdue Extension Bulletin # ID-414-W.

Clement, D.L. S. Gill, and W. Potts. 1994. Alternatives for Powdery Mildew Control on Lilac. Journal of Arboriculture, 20(4) 227-230.

Daughtrey, M.L. and M. Semel. 1987. Herbaceous perennials: Diseases and Insect Pests. Cornell Cooperative Extension Information Bulletin 207.

Dutky, E.M. and P. Willow. 1992. Powdery Mildews. Plant Pathology, Mimeo #12. University of MD Cooperative Extension.

Elmer, W. 2008. Biological and biorational fungicides offer control options. Nursery Management & Production, August 2008, pg 63-66.

Hoover, G, G. Moorman and K. Richards. 2011. Woody Ornamental Insect, Mite & Disease Management Guide. Penn State University.

Horst, R.K. 1990. Westcott’s Plant Disease Handbook. 5th edition. NY.

Horst, R.K., S.O. Kawanoto, and L.L. Porter. 1992. Effect of bicarbonate and oils on the control of powdery mildew and black sport of roses. Plant Disease, 76:247-251.

Mmbaga, M., and R.J. Sauve. 2004. Management of powdery mildew in flowering dogwood in the field with biorational and conventional fungicides. Canadian Journal of Plant Science, 84: 837-844.

Smith-Fiola, D. 1995. Pest Resistant Ornamental Plants. Ornamental landscape plant cultivars and varieties documented to be resistant to specific insects and diseases. Available from Rutgers Cooperative Extension, 1623 Whitesville Road, Toms Rivers, NJ.

Written by: Leanne Pundt, Extension Educator, Commercial Horticulture, University of Connecticut

Updated by: Mary Concklin, IPM, University of Connecticut. 2011

The information in this document is for educational purposes only.  The recommendations contained are based on the best available knowledge at the time of publication.  Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.  The University of Connecticut, Cooperative Extension System, College of Agriculture and Natural Resources is an equal opportunity program provider and employer.