Fall 2024
A common question I have been asked many times this season pertains to the lilacs: What is happening to them and what are ways to mitigate the situation?
The good news is that it is not nearly as bad as you think. Your lilacs are able to survive these diseases, as these diseases are not typically fatal.
An important thing to note is that one cannot necessarily point to a single disease, as there are a few common culprits. In this blog post, I will discuss the likely three.
Bacterial Leaf Blight
This disease is associated with wet conditions, which describes our current season to a T. Bacterial Leaf Blight is caused by the bacteria Pseudomonas syringae. Common symptoms and responses of lilacs are leaf spots that cause wilting and eventual leaf drop. Being that this specific leaf blight is caused by a bacterial pathogen, common fungicides even when used preventatively will not be of much help. I will further discuss prevention and possible control later in this post, so stay tuned.
Fungal Leaf Blights
The likely two fungal blights that you may currently be seeing are either Septoria Leaf Blight or Pseduocercosporia Leaf Blight. Symptoms include small, brown to black spots on leaves that create distortion and cause leaf drop. Because these particular blights are fungal, fungicides may be used preventatively early in the season. They however, are not recommended to be used late in the season, as the disease has already progressed by this point.
Why are my Lilacs Reblooming?
This also is a very common question. Simply put, it is the lilac's "stress response". They are fooled into reblooming during this blight period. This is still under speculation by many gardeners.
Mitigation
Whether it be bacterial or fungal, the methods of control are nearly the same. These control methods include some chemical, but mostly cultural. Cultural control methods help a plant remain healthy by altering its environment. For all diseases mentioned in this post, cultural control methods include:
Cleaning up leaf litter throughout the season, with one final cleanup before winter.
Proper watering using a trickle hose and/or soaker hose to ensure no overhead watering (this promotes a environment which diseases thrive).
Promoting air circulation by pruning out crossing branches and diseased or dying branches (making sure to remove no more than 1/3 of total branches).
Mulching to prevent pathogens from reaching the lower leaves and to retain moisture.
As for chemical control by use of fungicides, there are a few available to homeowners. Copper sulfate is one way to try to prevent and control blights caused by fungal pathogens.
One Final Thing to Note
Diagnosis is hard to determine by looking at the leaves only. If you are curious about your lilacs having a bacterial or fungal disease, leaf samples can be submitted to the Plant Disease Clinic at the University of Minnesota. They can either be mailed in or personally delivered.
Enjoy your fall gardening!
Aubrey B
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