08/25/2025
If you're curious about the sick looking lilacs across the state, check out this link from Dr John Ball, our state forester. You can find out what's going on with woody plants in our state and more in his weekly Tree Pest Alert series.https://extension.sdstate.edu/sites/default/files/2025-08/S-00014-169.pdf
Leaf spot diseases, pseudocercospora and septoria
among others, on lilacs are causing increasing concern
in eastern South Dakota. These leaf spot diseases
present as brown spots starting at the margins and
progressing to blotches and eventually defoliation.
Most of the calls and visits are for lilacs used in
windbreaks. I have looked at some windbreaks where
every lilac in a row is bare except for a few leaves at the
tips. These diseases require warm, humid conditions to
flourish, and it has been wet and humid this year.
These leaf spot diseases are not a threat to otherwise
healthy shrubs. Lilacs can withstand a year of
defoliation, so no control is always necessary. If the
lilacs were affected last year and again this year, a
fungicide application of Chlorothalonil (labeled for this
use) can be made just as the leaves are opening next
spring to reduce the severity of the disease.
