What's wrong with the lilacs? Beloved plants in Minnesota threatened by leaf fungus

The calls started in early August, from homeowners and arborists concerned about what they were seeing on lilac hedges — yellow and dried-up brown leaves and dying branches on mature plants that had never had problems before.

"Master Gardeners are getting bombarded with lilac questions," said Julie Weisenhorn, Extension educator in horticulture at the University of Minnesota.

It's unusual because lilacs, while beloved in spring for their fragrant blooms, are low-maintenance survivors that are easily forgotten the rest of the year.

"They're long-lived and generally ignored," said Grace Anderson, research scientist and diagnostician at the University of Minnesota's Plant Disease Clinic. "Lilacs you kind of count on. They're a sentimental and much loved plant."

The culprit appears to be Pseudocercospora leaf spot, a fungus that is causing leaf blight. The fungus, which lives in soil and plant debris, has been found in samples analyzed by the clinic this year, said Anderson.

While it isn't new, this year's weather created a "perfect storm" for the fungus to wreak havoc, said Weisenhorn.

First, there was a cold snap in May when leaf buds were forming.

"If they freeze, the plant has to reproduce them all over again, at great cost of energy," said Anderson, making the plant more susceptible to fungus and other diseases. At the time, there was no snow cover, which also stressed plant root systems.

Then, July brought a long stretch of steamy days.

"We had a super streak of hot, humid weather with no relief at night, which continued the hothouse effect," further stressing the plants, Weisenhorn said.

So what does all this mean for next year's lilacs? Experts aren't sure.

"Usually, leaf spot diseases don't kill," said Weisenhorn. "But we don't know. We will have to wait and see. Lilacs can live 100 years. They don't die easily."

Lilacs that survive the winter will most likely bloom next spring, she said. "I wouldn't guess it would affect flowering. The fungus is a leaf fungus, not a flower fungus."

But some seriously stricken lilacs may not recover. "If it's stripped bare, with no leaves at this point, I can't promise you it will come back," said Anderson.

To give a sick lilac the best chance of beating the fungus, clean up dropped leaves and other debris at the base, to reduce the risk of reinfection next year. Don't compost the debris; a home composting system won't get hot enough to kill the fungus. Bag and discard the leaves instead.

If your lilac has branches that appear dead, prune them. "Open up the canopy so the air moves through it," said Anderson.

And water generously this fall. "Lay a soaker hose to support the root system," said Anderson. "To get through the winter [ailing lilacs] need a good root system."

Common lilacs seem to be the most affected by the fungus, athough some tree lilacs also have been susceptible. Anderson's lilacs at home, Miss Kim, a Korean dwarf variety, are under duress this year. So are lilacs at the Minnesota Landscape Arboretum. "There's a most spectacular collection of lilacs at the Arb, including some old special varieties, and they're suffering."

Fungus isn't the only thing that can damage lilacs; insects or herbicides also could be a factor. If you want to know definitively what's ailing your lilac, you can submit a sample to the U of M's Plant Disease Clinic and get a diagnosis for $45. For information about how to submit a sample, visit pdc.umn.edu/submit-sample.

Even if your lilacs are looking good this year, they may struggle in years ahead.

"The climate is changing and is more conducive to fungal diseases," said Anderson.