Lilac pests and diseases

I have two lilac bushes in my yard and honestly I don't pay much attention to them except to cut off the fragrant flowers and prune occasionally. Lilacs are extremely hardy, but can be attacked by a few diseases. 

Jay Pscheidt is an extension plant pathologist with Oregon State University. He says one of the most serious diseases is a bacterial blight. 

"You'll see the new shoots starting to grow and in the springtime, they'll start to die back. They'll get a characteristic sort of dark, almost black coloration to them, and the tips will curl down. We call that symptom a shepherd's crook because it looks like the top of a shepherd's cane," says Pscheidt. "It has that hook to it."

Bacterial blight attacks newly planted lilacs and you'll have trouble establishing any bushes that are affected by it. There are bactericides and biologicals that take care of blight as long as you catch it early. 

Powdery mildew is common on lilacs, especially when the weather’s cool and rainy. The leaves appear as if somebody dumped baby powder on them. In terms of managing it, Pscheidt says good air circulation will help a lot.

"As you're pruning out those flowers to put in your bouquet, you might prune a little lower and make things so that bush has a lot more airflow through there. That will help dry it out and you won't have quite as bad of a powdery mildew problem," he says. "It tends to like the shade a lot, so if you're planting your lilacs in a really shady situation, that might also encourage it so we talk about planting them in more fuller sunlight."

It will also help to space and prune lilacs so they're not rubbing against each other. Don't fertilize late in the growing season, and don't over-fertilize young plants, because high nitrogen favors disease development.