Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
1/27/03
Contacts:
Mark Gleason, Plant Pathology, (515) 294-0579, mgleason@iastate.edu
Tara Barrett, Plant Pathology, taralu@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning Jan. 31, 2003
White Blotches Are No Threat to Lilac Health
By Tara Barrett
Plant Pathology Student
Iowa State University Extension
Looking out on the ice and snow during the winter months, I find myself dreaming of spring. I count the days until the grass reclaims its green hue and buds break at the tips of tree branches. I appreciate this explosion of colors, but my favorite sensation of spring is the fresh fragrances that accompany these colors. The smell of the first mowed grass of the lawn, of soil being turned over, and best of all, the intoxicating fragrance of lilacs. The clusters of delicate lilac blossoms overpower the senses with their deliciously sweet perfume. The bloom of the lilac is one of the greatest delights of spring.
But as the season progresses, many lilacs become less picturesque. Of course, the violet blooms fade and drop, leaving deep green leaves to carry on through the summer. You have surely noticed something peculiar growing on your lilacs: a white or gray powdery coat gradually covering the leaves. The culprit is a fungus called powdery mildew.
Powdery mildew is the common name of dozens of fungal species that colonize many plant species. The species most often found on lilac is Microsphaera penicillata. Powdery mildew fungi are host-specific parasites, meaning that a single powdery mildew species can colonize just one or a handful of plant species. Therefore, powdery mildew on your lilacs will not spread to other plants in your yard or garden.
Over the winter, powdery mildew survives in a dormant state on plant debris or buds. In the spring and summer, spores are transported by wind to new leaves and the fungus grows into the familiar whitish, powdery blotches on leaves. Although the fungus does not infect wet leaves, high humidity is a prime condition for spread of powdery mildew. The fungus spreads rapidly throughout the growing season on all aerial parts of lilacs- leaves, stems and flowers- but is most conspicuous on the top and bottom sides of the leaves. The fungus can appear in scattered patches or can cover the entire leaf.
The good news is that powdery mildew is only a cosmetic problem. While it may mar the appearance of a lilac, it almost never harms the shrub permanently. Some owners may object to the leaf blotching and wish to rid their lilacs of the fungus. While chemicals often provide poor control, there are a few things you can do when planting lilac to discourage powdery mildew infestation.
Perhaps the most effective management strategy is planting powdery mildew-resistant lilac varieties. Such varieties can be very effective in resisting the disease, but are sometimes not specifically labeled as resistant at the nursery. Japanese tree lilacs possess a high level of resistance and are good for planting in large spaces, because they can grow to 20-30 feet. Preston lilacs are late-blooming shrubs that also show resistance. Unfortunately, the common lilac, the type that most people have planted in their yards, has no cultivars with high levels of resistance to powdery mildew.
If you are planning to plant common lilac shrubs, there are a few cultivars that are fairly resistant. For red-purple blooms, you could try 'Adelaide Dunbar' or 'Charles Joly.' For pink blooms, 'Katherine Havemeyer' shows some resistance. If white blooms are your cup of tea, try 'Jan Van Tol.' For more information on specific varieties of lilacs, try looking under lilacs on the Web site http://www.mpelectric.com/treebook/common.html or the Bailey Nursery encyclopedia at http://www.bailenursery.com/refroom/refentry.asp.
Several cultural strategies can help in preventing powdery mildew. Because the disease favors high humidity, think about how much space your lilac will need when fully grown. Then plant your lilacs far enough apart to allow them sufficient room to grow. Lilacs like abundant sunlight, so avoid planting lilacs where they will be heavily shaded.
Many people are unsure if the lilacs they already have planted are resistant to powdery mildew. There are a few steps you can take to try to manage powdery mildew on already established lilacs. Overwatering can lead to higher humidity levels around your lilacs. This creates a more favorable environment for powdery mildew growth. By watering lilacs only when needed and at the base of the plant (to avoid getting the leaves wet), you can fight powdery mildew growth. High amounts of fertilizers promote a profusion of young, tender leaves that are vulnerable to attack by powdery mildew. You may be able to manage the growth of the fungus by avoiding overfertilization.
It is likely you will notice powdery mildew on your lilacs at some point in the season. Hopefully, knowing that the fungus poses minimal threat to your lilacs' health will put your mind at ease about those strange white blotches.
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Editors: Two color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 316K and the bottom picture's fullsize photo is 322K. Caption: Powdery mildew lilac 1 |
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| Caption: Powdery mildew lilac 2 |  |
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