Lilac Problems: Powdery Mildew, Borers, Bacterial Blight and Scale

Powdery mildew: ugly, and almost always harmless

By late summer, a great many lilacs look as though someone dusted the leaves with flour. This is powdery mildew, and it is the single most common complaint about lilacs. It is also, on an established shrub, essentially cosmetic. It arrives after the plant has already done the year's work of growing and setting flower buds, it does not kill wood, and it does not reduce next spring's flowering in any way you will notice.

The important thing is to recognize that this fungus behaves differently from most. It does not need leaf wetness to infect. It thrives in warm days with cool humid nights, in still air and in crowded shady positions, which is why a lilac squeezed against a fence gets it every year and a specimen standing in open air with good spacing often barely does. Watering habits are not the cause, and spraying water on the leaves does not help.

So the practical response is to do nothing about the fungus itself and instead fix the conditions. Thin the center of the shrub when you prune after flowering. Clear crowding plants away from the base. Rake up and remove the fallen leaves at the end of the season to reduce the spore load carried over. Fungicides work but are not worth the effort on a shrub that is already fine, and on a large lilac full coverage is impractical anyway.

If a lilac gets heavy mildew every single year from early summer rather than late, that is a signal about the site rather than the disease. Read it as too little air movement or too much shade, and treat it as a pruning and spacing problem.

The problems that actually damage the plant

A small number of lilac problems do real harm, and all of them attack wood rather than leaves. Borers tunnel into stems and kill them from the inside. Bacterial blight kills shoots and flower trusses outright in wet springs. Scale insects weaken whole branches over several seasons. Learning to tell these from leaf spotting saves you from both panic and complacency.

The general rule is that leaf symptoms are usually cosmetic and stem symptoms are usually serious. Blotchy, spotted, powdery or discolored leaves on an otherwise vigorous shrub can wait. Wilting shoots, oozing bark, sawdust at the base of a stem, or a whole branch failing to leaf out in spring all need attention now, because by the time you notice them the damage inside the wood is already advanced.

Borers, and why old stressed stems are the target

Lilac borers lay eggs on the bark near the base of the shrub, and the larvae tunnel into the wood. The visible signs are neat round exit holes an eighth of an inch (3 mm) across, coarse sawdust caught in the bark crevices below them, and stems whose leaves wilt or whose tips die back while adjacent stems look fine. In a bad case a stem snaps off in wind at the tunneled point.

Borers overwhelmingly go for old, thick, stressed stems rather than young vigorous ones. This is why a shrub that is renovated on a regular cycle, with the oldest stems taken out every few years, rarely has a serious borer problem, while a neglected thicket of thirty-year-old wood attracts them repeatedly. Routine renewal pruning is the real control.

Once a stem is tunneled there is no practical way to save it. Cut it out at ground level and burn or dispose of it away from the garden rather than leaving it in a brush pile, where the larvae complete their life cycle and emerge to reinfest the shrub. Avoid wounding the bark at the base with mowers and string trimmers, because those wounds are exactly where the adults prefer to lay.

1. Inspect the lower stems each spring Before the leaves are fully out, run your eye and hand over the bottom two feet (60 cm) of every main stem, looking for holes, sawdust and cracked or oozing bark.
2. Remove and destroy infested stems Cut affected stems flush to the ground. Do not compost or stack them; take them off site or burn them so the larvae inside do not emerge next season.
3. Keep the base free of mechanical damage Maintain a clear mulched circle around the shrub so no mower or string trimmer ever touches the bark. Every bark wound is a potential entry point.
4. Renew old wood on a cycle Take out one or two of the oldest stems each year after flowering. Young vigorous wood is far less attractive to borers than aged stems.

Bacterial blight in a wet spring

In a cold wet spring, shoots blacken from the tip downward, young leaves develop brown blotches with yellow margins, and whole flower trusses go brown and limp before opening. This is bacterial blight, and it moves fast. It is worst on soft, sappy growth, which is another reason not to feed a lilac heavily with nitrogen.

Cut affected shoots back well past the visible damage, taking at least six inches (15 cm) of apparently healthy wood beyond the blackened part, because the bacteria have moved further than the symptoms show. Disinfect your blades between every cut, since the tool is the most efficient way to spread it through the shrub. Bin or burn what you remove.

After a blight spring, improve air movement through the shrub and hold off on any feeding for a season. The plant almost always recovers fully once the weather turns dry and warm; the damage is to that year's flowers and shoots rather than to the shrub's long-term health.

Record what appeared and when

Note the date you first see mildew each year, not just that you saw it. A shrub where it appears in late summer needs nothing; one where the date creeps steadily earlier each year is telling you that surrounding plants have grown and the site has closed in. That comparison only exists if you wrote down the dates.

For the serious problems, record which stem was affected and its position around the shrub. Borers and blight both tend to recur in the same part of a plant, usually the shadiest or most crowded side, and a two-line note each season turns an annual inspection into a targeted check of the three places that have caused trouble before.