An illustration of the issues with lilac the past few years with bacterial blight and fungal leaf spot.

By Jeff Burbrink
Ag & Natural Resources Extension Educator, Purdue Extension LaGrange County

LAGRANGE — A few years ago, we started seeing lilac plants in the area showing signs of decline. By August or September, the plants were often nearly barren of lower leaves. Each year, the plants grew thinner and less desirable.

Jeff Burbrink

This blight has been very distressing to those who love that great lilac smell in the spring. Its not that they are a perfect plant. We all know they get powdery mildew on the leaves every year, but that’s never been a big deterrent to most lilac lovers. The other two issues are called bacterial blight and fungal leaf spot, these are much more harmful to the plants than the powdery mildew, which is mostly cosmetic in nature.

Bacterial blight occurs early in the spring after budbreak. The bacteria enter plants through wounds, like frost cracks or mechanical damage from pruning saws or breaking limbs accidentally.

Symptoms associated with bacterial blight include small water-soaked spots that expand from a pinprick to about 1/8th of an inch in size. Multiple spots in an area can combine to create larger blighted areas on the leaf. As the disease spreads into the stems, symptoms can lead to drooping or shepherd’s crook-like growth at the branch tips and black lesions on the stems. The pathogen can overwinter in the affected tissue, so pruning out and raking up affected plant parts will be important to prevent carryover into the next spring.

In late spring and summer, a different foliar disease can be observed, this one being a fungus. The fungi cause irregular shaped leaf spots that will coalesce to create large areas of blighted tissue. Under severe disease pressure, these fungi can lead to significant leaf loss which can affect shrub vigor if it is a chronic problem. Often this leaf loss occurs in the lower two-thirds of the plant. After three years or so of leaf drop, the overall health of the plants declines so much, its nearly impossible to bring them back to a desirable state.

Both leaf spot diseases usually develop in the lower canopy and move upward as the season progresses under humid and wet conditions, which promote more spores to develop, and more disease symptoms.

What to do about these issues? Like almost any plant disease, cleaning up dead branches and infected leaves might help reduce infection the following year, but in big clumps of lilac, getting all those dead leaves cleaned out is a real chore.

People often ask about fungicides. Fungicides, products that control diseases, have they have drawbacks too. The fungicides on the market are preventatives, not cures, so they need to be put in place before major symptoms begin to show up. Since wet weather and humidity play a big role in spreading diseases, that means the plants will need to be re-sprayed multiple times each season, perhaps every 14-21 days, to protect the foliage. That is expensive, time consuming, and well beyond the effort most people want to expend.

There are some varieties of lilac that are less susceptible to these bacterial and fungal infections. It may be wise and less expensive to give up on severally blighted plants and replace them with varieties that avoid the diseases to begin with. Visiting local nurseries this time of the year might be a good way to observe their resistance, but you will want to ask if their lilacs have been sprayed with fungicides to protect their looks.

For more information on lilac, suggested reading can be found at shorturl.at/gxh20 or shorturl.at/X4Vy8