In the last month, we have received many inquiries regarding lilacs. On the few leaf samples we have received, we have observed mainly three different fungal pathogens: Pseudocercospora and Septoria causing leaf spots, and powdery mildew. All are contributing to leaf blight (rapid death), and some cases to abundant leaf drop.
This year environmental conditions and dew have proven very conducive for these pathogens. Also, stressors like drought could be exacerbating these problems.
When assessing your lilacs, it is also good to examine the stems and trunk for evidence of any mechanical injury that may be caused by borers: https://hortnews.extension.iastate.edu/lilacash-borer, animals, or mowing equipment.
This is problems, are a good reminders that good care and pruning can help improving airflow on shrubs and tree health in general. Pruning Ornamental Shrubs is a resource to help with that.
Removing and destroying leaf debris and pruning out dead branches may be the best strategy in reducing diseases in general for shrubs and trees.
Fungal leaf spots rarely become severe enough to cause the decline of the plant. However, if repeated severe infections occur, fungicide treatments should not be considered, as it is too late into the summer. Preventative spring to early summer fungicide applications may help prevent diseases (but won't eradicate them).
All care you can provide, so plants thrive can help. You may consider renovating your established lilacs. see the article renovating lilacs.
See our articles on lilac diseases:
Other lilacs problems (not as common)
Lilacs perform well in clay soils, but slow water drainage leads to excess moisture in the root zone, and prone to root rots, with less defined foliar symptoms (no spots nor mildew on top). See https://hortnews.extension.iastate.edu/2017/01/phytophthora-shoot-blight-lilacs
Lilacs are also susceptible to verticillium wilt, where the leaves will look droopy, and water-deprived, change from green to yellow to brown and drop. See the article Verticillium Wilt.
If you need help with your plant problems, the PIDC is happy to help. We are providing digital diagnostics mainly and requesting samples for testing when we deem it critical. We are operating at reduced capacity. We appreciate your patience. Submit your plant problem image assessment request here
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