Lilac leaves coated with powdery mildew. Source: glimpsesofglory-karen.blogspot.com
Question: I have lilacs and every year at the end of the summer leaves are coated with a whitish powder. What can I do to stop this?
Answer: Wear green-tinted sunglasses!
OK, that was a bit flippant, but … that’s pretty much what you should do!
Powdery Mildew Is…
It’s important to understand that this disease, called powdery mildew, is pretty much harmless to lilacs, especially when it appears at the end of the season, which is usually the case. It doesn’t weaken the shrubs in any substantial way nor does it keep the plants from blooming normally the following year.
We call the disease powdery mildew because the foliage seems covered with a white powder, but in fact, it isn’t really powder: what you see are the fruiting bodies of a microscopic mushroom that has set up shop on the upper surface of the leaf. These bodies produce the spores that will eventually be carried by the wind to another lilac.
There are over 1000 species of powdery mildew fungus, but the two most commonly seen on lilacs are Erysiphe syringae (formerly Microsphaera syringae) and Phyllactinia syringae. They are specific to lilacs and a few close relatives (one variety of Erysiphe syringae will attack privets [Ligustrum spp.], for example), but won’t attack other plants, so you don’t have to worry that lilac powdery mildew will damage your other plantings.
On lilacs, as on many plants, powdery mildew is mainly a disease of senescence, that is, of aging. It mainly attacks the leaves towards the end of their useful period (end of August and September). That’s why it doesn’t really harm the shrub: by then, your lilac has already stored up its energy reserves for next year. So, it’s of little consequence other than visually. And I suspect no one else besides yourself would notice your lilacs are ill. From a distance, at least, they just seem to have a grayish cast that most people would mistake for their natural coloration. There are gray-leaved shrubs, after all.
Controlling Powdery Mildew on Lilacs
Actually, you don’t really have to treat powdery mildew on lilac, given its minimal effect on its host. At most, you could try to prevent it with the following actions:
- Choose a less susceptible variety. Powdery mildew is pretty much limited to the common lilac, also called French lilac (Syringa vulgaris). Most other commonly grown lilacs, such as S. meyeri, S. patula and S. x prestoniae, are rarely affected or, if they do catch it, the damage is extremely limited and scarcely noticeable. But not all common lilacs are sensitive to powdery mildew. Here are a few that are resistant to the disease and will be symptomless most years:
- Syringa vulgaris ‘Arch McKean’
- Syringa vulgaris ‘Avalanche’
- Syringa vulgaris ‘Charles Joly’
- Syringa vulgaris ‘Charm’
- Syringa vulgaris ‘Edith Cavell’
- Syringa vulgaris ‘Firmament’
- Syringa vulgaris ‘Henri Robert’
- Syringa vulgaris ‘Katherine Havemeyer’
- Syringa vulgaris ‘Ludwig Spaeth’
- Syringa vulgaris ‘Macrostachys’
- Syringa vulgaris ‘Madame Lemoine’
- Syringa vulgaris ‘Marie Finon’
- Syringa vulgaris ‘Marie Legraye’
- Syringa vulgaris ‘ Harry Bickle’
- Syringa vulgaris ‘Paul Thirion’
- Syringa vulgaris ‘President Lincoln’
- Syringa vulgaris ‘Primrose’
- Syringa vulgaris ‘Sensation’
- Syringa vulgaris ‘Silver King’
- Syringa vulgaris ‘Vestale’
- Give the shrub the best possible conditions, including full sun with plenty of space, good air circulation and well-drained soil, if possible slightly alkaline.
- Prune the shrub to improve air circulation.
- Water the soil at the base of the lilac during drought, because powdery mildew tends to settle on plants stressed by a lack of water.
- Spray the foliage weekly with plain water during hot, humid weather (when the disease usually starts), as this tends to wash the spores off the leaves before they start to germinate. Do this in the morning so that the foliage has time to dry out before dark, otherwise it can leave the shrub susceptible to other diseases.
- Avoid fertilizing lilacs, especially with lawn fertilizers, as nitrogen, especially, tends to stimulate fast but disease-sensitive growth. In any case, the lilacs are very comfortable with very ordinary soil fertility: they’re just not heavy feeders.
- Collect fallen leaves in the autumn and, if you use them as mulch or compost, avoid using the final product at the foot of the lilac, saving it for other plantings instead.
- Spray the foliage monthly with horticultural oil (available in garden centers). Start after the flowers fade in late spring and continue until fall. A light coating of oil can help keep the spores from germinating. Note that spraying with oil will not eliminate powdery mildew once symptoms are visible: it must be used as a preventative. I consider this method to be one of last resort, reserved for lilacs that have repeatedly shown themselves to be susceptible to the disease in the past and that I have not yet got around to tearing out.
Or Do Nothing at All
Personally, the only actions I would take would be points 1 and 2: plant disease-resistant lilacs under appropriate conditions for their healthy growth. Otherwise, why fight a disease that doesn’t harm the health of its host and is not even very noticeable? This is one of those situations where you should simply apply the 15 pace rule: before treating, step back 15 paces. If you can’t see the problem at 15 paces, it’s not worth treating.
Benign neglect in the garden? Yep, I’m all for it!