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If your lilacs started looking rough seemingly overnight this summer — leaves turning yellow, developing brown blotchy spots, curling, or dropping well before fall — you are not imagining it and you are not alone. Lilac fungal disease is a real and growing problem across Becker County, and in the right weather conditions it can move through a plant faster than most homeowners expect.
Here is what is actually happening, why it spreads so fast, and why it frequently gets blamed on something it is not.
What Is Causing the Damage?
The primary culprit in Minnesota is a fungal disease called Lilac Leaf Spot, caused by Pseudocercospora or Septoria fungi. It is a relatively new disease in our region that has been spreading across the Midwest and has now established itself as a recurring problem in Minnesota landscapes.
According to the University of Minnesota Extension, symptoms typically appear from July through September. Leaves begin to yellow, then develop brown blocky spots that grow larger and merge together. Affected leaves twist, curl, and drop from the plant well before normal fall leaf drop occurs. In a bad year, a lilac can lose a significant portion of its foliage by midsummer.
There is also Powdery Mildew — a different fungal issue that shows up as a grayish-white dusty coating on leaf surfaces, as if the plant were dusted with flour. Powdery mildew is common on lilacs in Minnesota and is mostly cosmetic, but in a severe outbreak it...
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