Lilacs are often a staple of our groves and yards here in southwestern Minnesota. In the past year or two, the growing conditions have been difficult for these prized plants and they have been suffering through some different problems.
There are several different varieties of lilac but the most common one that we have planted in our area is the common lilac, which its Latin name is Syringa vulgaris. If you happen to have a tree lilac, then you may have seen some trouble with these plants as well.
These are often mature plants and you may have never seen a problem with them in the past. The symptoms that most gardeners and rural residents may have seen are yellowing leaves as well as branches dying for no apparent reason. So where do we lay the blame on this latest plant mystery?
The climate is the first culprit on the list. We had three years of very wet growing conditions which is enough to comprise any healthy tree or shrub as it is. However, as we were all at home trying to figure out what was happening with the pandemic, May 2020 experienced some of the coldest temperatures that we have had in some time and it lasted for about a week. This is followed up by one of the hottest Julys we have had in some time in Minnesota. Remember 90-100 degree days?
Fungal diseases have also been diagnosed in lilac plants this year throughout Minnesota at the Plant Disease Clinic at the U of M. These cause the same symptoms as the climate symptoms. While it may be easy to clean up one or two lilacs in your yard, if you have a whole line of them for your grove, going and cleaning up any fallen leaves can be a big job. Unfortunately, these fungal diseases persist in the dropped leaves and dead branches which can cause the fungal disease to last more than three years. Another fungal disease is Verticulum wilt which looks similar to the first fungal disease. The only way to combat this fungal disease is to purchase lilacs that are resistant to it.
Any resident can try renewal pruning which is cutting 1/3 of the lilac shrub down to the ground but not pulling them out of the ground. Pruning the largest branches out but leaving the smaller branches will help for air movement through the plant, which helps to decrease fungal problems and overgrowth. They will come back from this base and it usually only takes a few years before they are back to full force. This will help keep the lilacs from becoming so dense that it invites fungal diseases to take off.
Insects can also be a problem but at this time we have not seen any of the lilac borers in Minnesota. In the past, a good sign that there are insects is that there will be something that looks like sawdust on the branches or your lilac branches may be oozing some sap.
Herbicide (weed killer) can also cause problems with our lilacs and it might be seen more in the rural areas where lilacs are used for a grove. If the leaves of your lilac are/were cupping, then there is a good chance that there was herbicide damage.
For more information about growing lilacs, please go to https://extension.umn.edu/trees-and-shrubs/lilacs#pruning-1923160 or you can also learn more about lilac diseases this year at https://extension.umn.edu/yard-and-garden-news/lilac-issues-season.
If you are interested in becoming a Master Gardener, you can go online to learn more about the program at https://extension.umn.edu/news/apply-extension-master-gardener-program-through-oct-1
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