Lilac - Ask Extension
Lilac purchased 4 years ago on Macinaw Island.
Sandy soil, 6-8 hours of sun.
Knowledgebase
Lilac #921147
Asked October 31, 2025, 8:02 AM EDT
Lilac purchased 4 years ago on Macinaw Island.
Sandy soil, 6-8 hours of sun.
Oakland County Michigan
Expert Response
Hello
During wet spring periods when leaves are beginning to grow fungi can infect leaves. The spots show up later in summer and fall.
If your lawn sprinklers hit the leaves and they stand wet for many hours, that encourages fungi to grow and diseases like septoria or cercospora leaf spots, too. So, check that your sprinklers are run early in the day so that leaves will dry quickly in the sun. These diseases are cosmetic unless they are defoliating your shrub early in the season.
Clear weeds and mulch around the base, keeping mulch pulled back about 6 inches from all trunks. This will preserve moisture longer in the root zone.
When leaves drop, rake them up and discard to reduce overwintering fungi next season.
Improve air circulation by pruning using a renewal/ thinning type pruning( see bulletin link below). You can prune out 20-25 percent of the branches, cutting them back to the ground, if the shrub is very densely branched. When pruning, place a drop cloth under the shrub to catch everything and bag and place it in trash if the material is diseased.
It is too late to apply any preventative fungicide now. You can apply a preventative fungicide in spring and repeat application according to the label until wet spring weather subsides. This reduces leaf spot diseases.
Here are some helpful bulletins:
https://extension.umn.edu/plant-diseases/lilac-issues-and-diseases
https://lancaster.unl.edu/rejuvenating-older-lilacs/
Pruning guide- https://ckdgardens.com/2023/03/08/prune-some-shrubs-now-and-others-later-2/
If you would like an opinion on which fungi affected your leaves, you may email your pictures ( include one far enough back showing the whole shrub) to MSU Plant Diagnostic lab at <personal data hidden>. They may request branch samples, and that analysis has an associated fee.
During wet spring periods when leaves are beginning to grow fungi can infect leaves. The spots show up later in summer and fall.
If your lawn sprinklers hit the leaves and they stand wet for many hours, that encourages fungi to grow and diseases like septoria or cercospora leaf spots, too. So, check that your sprinklers are run early in the day so that leaves will dry quickly in the sun. These diseases are cosmetic unless they are defoliating your shrub early in the season.
Clear weeds and mulch around the base, keeping mulch pulled back about 6 inches from all trunks. This will preserve moisture longer in the root zone.
When leaves drop, rake them up and discard to reduce overwintering fungi next season.
Improve air circulation by pruning using a renewal/ thinning type pruning( see bulletin link below). You can prune out 20-25 percent of the branches, cutting them back to the ground, if the shrub is very densely branched. When pruning, place a drop cloth under the shrub to catch everything and bag and place it in trash if the material is diseased.
It is too late to apply any preventative fungicide now. You can apply a preventative fungicide in spring and repeat application according to the label until wet spring weather subsides. This reduces leaf spot diseases.
Here are some helpful bulletins:
https://extension.umn.edu/plant-diseases/lilac-issues-and-diseases
https://lancaster.unl.edu/rejuvenating-older-lilacs/
Pruning guide- https://ckdgardens.com/2023/03/08/prune-some-shrubs-now-and-others-later-2/
If you would like an opinion on which fungi affected your leaves, you may email your pictures ( include one far enough back showing the whole shrub) to MSU Plant Diagnostic lab at <personal data hidden>. They may request branch samples, and that analysis has an associated fee.
