With summer comes lilac leaf spots; at the plant clinic this year, we have seen the usual fungal pathogen Pseudocercospora causing leaf spots and blight (rapid death). Historically, Septoria leaf spot and powdery mildew also contribute to leaf blight, and in some cases, to abundant leaf drop. Environmental conditions, humidity, and stressors like drought, shaded locations, or lack of proper pruning could be exacerbating these problems.
What management practices are recommended?
Fungal leaf spots alone rarely become severe enough to cause the decline or death of the plant. Removing and destroying leaf debris and pruning out dead branches may be the best strategy in reducing diseases in general for shrubs and trees.
Fungicide treatments should not be considered late into the summer. If repeated severe infections occur, preventative spring to early summer fungicide applications may help prevent diseases (but won't eradicate them). Because of this and the fact that lilacs tend to be large and difficult to effectively spray fungicide on, fungicide applications are rarely warranted.
These problems are good reminders that good care and pruning can help improve shrub health in general. Pruning Ornamental Shrubs is a resource to help with that. Providing good, consistent care by making sure shrubs get at least 6 hours of direct sunlight a day and watering during drought periods can help lilacs thrive. Consider renovating your established lilacs. This will increase air circulation (reducing foliar fungal diseases), improve appearance, and as an added benefit, promote better flowering. See the article renovating lilacs.
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 is prone to root rots. With root rots, shrubs develop less defined foliar symptoms (no spots nor mildew on top) and tip die-back. See Phytophthora shoot blight on lilacs
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 animals, mowing equipment, or borers: https://hortnews.extension.iastate.edu/lilacash-borer.
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. with progressive branch wilting at a slower pace. See the article Verticillium Wilt.
If you need help with your plant problems, the PIDC is happy to help. Submit your plant problem image assessment request at the Image Submission (Plant Problems- Diseases) form
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