Powdery Mildew on Lilac

Powdery Mildew on Syringa x hyacinthiflora ‘Mt. Baker’

It’s the time of year when you’re likely to notice the leaves of your lilac shrubs (Syringa) covered in a powdery, white substance known as powdery mildew. Powdery mildew on lilac is a fungus whose causal organism is Microsphaera penicullata. There are over 1100 separate types of powdery mildew, and each is plant specific. So even if you see powdery mildew on, say, some roses located near your lilac bush, the organism causing the rose mildew is distinct and specific to roses. The roses didn’t acquire the disease from the lilac.

Powdery mildew in Zones 5 and 6 usually makes its appearance in July or August. Suitable conditions for fungus growth include:  humid air, high daytime temperatures, overcast skies, lack of airflow between branches and leaves, insufficient sunlight, and over-fertilization. The fungus spores overwinter on affected leaves and are easily spread by water. Fortunately, since most of the lilac’s required photosynthesis has already taken place by the time the powdery substance begins to coat the leaves, the only real damage to the plant is aesthetic. However, some lilacs, such as the Mt. Baker variety shown in the picture, produce colorful fall foliage, so you may want to treat the lilacs to prevent the fungus from taking hold.

There are many fungicides available to control powdery mildew on lilac; however, if you’d like to try a less expensive, safer approach, consider using monthly sprays of a summer weight horticultural oil, such as Bonide® All Seasons Horticultural Spray Oil. Although horticultural oil is generally confined to treating plants for insects rather than disease, researchers at the University of Maryland Cooperative Extension, testing horticultural oils against other fungicides for control of lilac powdery mildew, discovered that horticultural oil was equal to or more effective than other recommend treatments. During the first year of the trial, the researchers sprayed with horticultural oil every two weeks, beginning in mid-July. The second year of the trial, the researchers sprayed with horticultural oil only once a month, and they concluded that monthly sprays offered the same protection as using the spray every other week.

To make a preventative spray for your lilacs, use the horticultural oil in either a 2% or 3% solution. To make a 2% solution, pour 1/3 cup of oil into a spray container, add 1 gallon of water and mix thoroughly before spraying. To make a 3% solution, use 1/2 cup oil to 1 gallon of water and, again, mix thoroughly before spraying. Wait until annual flowering is completely finished before beginning your spray program, and then spray late in the day when no rain is forecast for 24 hours. Also, be sure to rake up all lilac leaves in the fall and burn them, bury them, or send them off to your municipal garden waste collection site. Do not compost infected leaves.

All common lilacs (Syringa vulgaris) are subject to powdery mildew, as are most Syringa x hyacinthiflora varieties. In my experience, the most classically fragrant lilacs are those within these two species. However, there are lilacs of many colors, of many species and varieties, and for many uses, including those specifically bred to be mildew resistant. So if you are looking for appearance more than fragrance, you might want to check out the mildew resistant varieties listed by the Virginia Cooperative Extension here. For more information on lilacs, generally, the University of Minnesota Cooperative Extension has a comprehensive listing here. Finally, for online sources of lilacs, including photos of numerous varieties, take a look at these websites, here and here.

Update, September 1, 2014: Unfortunately, the University of Minnesota Cooperative Extension article is no longer available.  An excellent alternative is available here from the University of Wisconsin Cooperative Extension.

Update, September 1, 2015: The Virginia Cooperative Extension listing is no longer available. I will try to find another listing or develop one from available data.

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9 Responses to Powdery Mildew on Lilac

  1. HT says:

    I’m still reading and enjoying your posts – matter of fact I read them multiple times – have to drum them into my tired old brain. Thanks for the info on powdery mildew. I was a bit concerned because one of my plants developed it and it was planted directly beside my dwarf lilac tree. Now I know I don’t have to worry!

    • grayslady says:

      Thanks, HT. I’m a little behind on posts because I was in Missouri on vacation, but I will be catching up in the next couple of days, including some posts on Missouri gardens.

  2. HT says:

    Perfect! Saw you over at Dakinikat’s and was elated. Also saw a whole bunch of my other favorite commenters over there as well, so all is well in my world.

    We are finally getting around to the colder weather here, so it’s time to clean out the garden and pond and plant bulbs…not my favorite time, but I keep thinking about how the garden will look in the spring which makes the effort worthwhile.

  3. Hi Grayslady,

    I’m so glad I found your blog again after reading about it at Dak’s place! I’ve been trying to stumble back here and could NEVER remember the name. I’m replanting/landscaping my small front yard and have been looking for good gardening blogs for weeks. I’m really looking forward to reading through your posts and checking out your blogroll.


    • grayslady says:

      Thanks, Darragh. Good to see you here. I haven’t developed an official blogroll yet, but I will. Meantime, hope you find some useful information.

  4. HT says:

    Actually (somewhat sheepishly here) any suggestions about how to plant bulbs? Should they be in clusters – like with like, or sprinkled through the early plants (such as phlox). I like the cluster scenario, but would appreciate your input.

    • grayslady says:

      I agree with clusters of similar bulbs–makes more of a visual impact when they emerge. Stay with odd numbers–3 bulbs, 5 bulbs, etc.–for your groupings. However, you can still tuck the bulbs in-between plants that will bloom later; that was a favorite trick of Rosemary Verey.

      • HT says:

        Thanks. I’m busily checking out the growing times of bulbs so I can plant accordingly. For example I know hyacinths bloom earlier than tulips which are roughly the same schedule as Nascissus. I wondered when I tore up my lawn to replace it with a garden whether I’d regret it. I’m going through growing pains right now, however the end of October, I can walk out my front door and the Sedum and Asters make me smile (the blue hydrangea is also in bloom – which is odd, but I love it). The wisteria seed pods are hanging down, the leaves are just turning (which is funny because most of the trees lost their leaves a few weeks ago). The squirrels love the wisteria seeds – at least I assume it’s squirrels, cause those seed pods would be difficult for a bird to crack. Anyway, all is well in my world, and I hope it’s well for you too!

      • Sophie says:

        It doesn’t matter how I plant them. The squirrels dig them up and move them. They tend to put lots together–no eye for aesthetics.

        But, they gave me a great idea: a few years back, I saw a Daffodil in the middle of the woods. It was so beautiful, being the only thing blooming yet. So the following fall, I planted lots of Daffodils in the woods on purpose–and some of them even stayed where I’d planted them!

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