We’ve all seen it. The disease that is affecting our older lilac shrubs. Stems with dark brown shriveled leaves start showing up mid-summer and are not very pretty at all.
This is the 4th or 5th summer we’ve seen this condition on our older lilac shrubs in central Missouri. The old-fashioned lilac (French Lilac) has always been durable, blooming reliably every year for what seems like generations. So, what is this?
A fungal disease (or two) has become predominant in our lilacs recently affecting their mid-summer and fall display. Septoria and Pseudocercospora are both called Lilac Blight and either one can be present.
There is no magic trick to fixing this problem except time and persistance. Keeping the shrub healthy and the area free of disease are the best controls and may take a few years to get it to clear up.
Start by removing any diseased and dead stems. Cracks or sunken areas on the stem or mushrooms starting to grow are a good sign that the branch is infected or dying. Remove at the first sign of symptoms regardless of the time of year.
Prune out the oldest branches, removing a total of 1/3 of the stems. The best time of year to do this is in May, but can still be done as late as August. This opens up the shrub allowing for good aeration and encourages new growth.
Fertilize lightly each year in early spring with a granular fertilizer low in nitrogen.
Clean up all leaves, branches, and stems that may sit at the base of the shrub at least once a year, preferably burning them to keep spores from spreading. Good sanitation practices keep the disease from being present and is less likely for infection to happen again.
Water thoroughly during times of drought. Lilac has always been that tough plant that can survive anything, but in order to control these diseases, they should not endure excessively dry conditions. Water heavily at least once a month if little or no rain.
Do all of these things until the shrub has been free of disease for at least 3 years.
The disease starts on the leaves as early as late spring as light brown angular spots. It continues to spread all summer until highly visible in late summer.
Fungal sprays are for prevention only and do not cure the disease. Spray starting in mid-spring for a couple years to break the infection cycle along with the sanitation practices described above.
Bacterial Leaf Blight can also show up on lilac with the same symptoms as their fungal cousins. Sanitation, removal of older branches, fertilizing, and watering during drought remain the best course of action.
