Master Gardener Sue Morris: Lilac problems reported in many parts of Minnesota
Many master gardeners throughout the state have been reporting all sorts of issues on lilacs this year with many lilacs dying. I haven’t had any questions concerning this problem in our area nor have my lilacs suffered any this year.
I only have two varieties — Japanese tree lilac and a Miss Kim. I’ve had them for more than 20 years with no problems.
Julie Weisenhorn, Extension educator, and Grace Anderson, research scientist with the University of Minnesota, have been looking into these issues and I would like to share with you their findings and possible causes.
They have found that the common lilac is reportedly the most affected species. Affected plants are mature and never had a problem before this year.
Symptoms include leaves turning yellow, then brown and drop. Branch dieback has been random (i.e., one shrub in the middle of a hedge, all one side of a hedge, on individual branches.)
Weather affects the emergence of a pathogen or a condition.
In May we had about a week of very cold temperatures without snow cover, possibly comprising root systems.
We also experienced a stretch of very tropical temperatures in June and July that set records. July 2020 was the 14th warmest and wettest since 1895. (Haven’t had much rain around this area to date — so that might be a clue why I haven’t heard of any problems locally.)
Day temperatures were in the 90s, nights in the 70s and dew points over 70 degrees.
Conditions like these can stress plants in various ways, slow down development and volatilize herbicides.
The Plant Disease Clinic has analyzed lilac samples this season and found the fungal disease lilac leaf spot to be present.
If you have this, what should you do?
They say sanitation needs to be done because these spores can persist for several years on plant debris (leaves, stems, dead flowers, bark).
Cleaning can be tedious but will assist in reducing the likelihood of reinfection next year.
Renewal pruning to decrease density and consistent water will help the stressed plant.
Verticillium wilt affects a number of plants, lilac being one.
Individual branches turn brown and die suddenly because it blocks the vascular system of the branch, cutting off water and nutrient movement. There is no cure for a plant with this fungal infection but you can up the watering and fertilize to extend the life of the plant.
Good sanitation helps prevent the spread of the disease.
There is a lilac borer that tunnels into lilac branches.
Signs include sawdust, sap and frass. Some plants tolerate this damage. Others experience dieback of branches and sometimes death of the entire plant. There has been no such reported damage this year.
Herbicide damage will show up as cupping or browning of leaves, usually on one side.
The good news is that lilacs are relatively long lived — from 25 to 50-plus years. I’m thinking since no one in this area has had questions, we have escaped this problem.