Lilac issues 2020

People have been reporting issues with lilacs dying to Master Gardeners this season (Fig. 1). The following is information to help you diagnose and advise people on what may be causing the problem and what they can do (if anything) to remedy it.

WHAT SPECIES OF LILACS ARE AFFECTED AND HOW?

1. Syringa vulgaris, our common lilac, is reportedly the most affected species. S. reticulata the tree lilac, is also susceptible. Whether specific cultivars are affected more than others is unknown mainly because most people don't know the cultivar.

2. Affected plants are reportedly mature and "never been a problem" before this year.

3. Symptoms: Leaves turn yellow, then brown, and drop. Branch dieback has been reported as random, on one shrub in the middle of a hedge, all one side of a hedge, on individual branches.

HOW DOES WEATHER AFFECT THE EMERGENCE OF A PATHOGEN OR CONDITION?

• According to the UMN Plant Disease Clinic (PDC), in May 2020, we had about a week of very cold temperatures without snow cover, possibly compromising root systems.

• MN Weathertalk, a weekly blog by climatologist Dr. Mark Seeley, reports that we experienced a stretch of very tropical temperatures in June and July that set records. July 2020 was the 14th warmest and 14th wettest since 1895. Day temperatures were in the 90s, nights in the 70s and dew points >70 degrees F. Conditions like these can stress plants in various ways, slow down development, and volatilize herbicides.

• Drought and flooding varied throughout the state as well.

• Storms can also cause damage e.g. saturated soil, standing water, erosion

• Insects can cause damage and vector diseases.

WHAT ARE KNOWN LILAC ISSUES THAT MIGHT PRODUCE THESE SYMPTOMS?

Fungal diseases:

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Fig. 1 Dieback on common lilac. Photo: Linda Brammer.

• The Plant Disease Clinic (www.pdc.umn.edu) has analyzed lilac samples this season and found the fungal disease Lilac Pseudocercospora leaf spot to be present (Fig. 2). Symptoms are the same as reported (yellowing then browning leaves, dieback) https://hortnews.extension.iastate.edu/lilac-pseudocercospora- leaf-spot. The PDC emphasizes sanitation as it seems that the spores of Pseudocercospora can persist for several years on plant debris (leaves, stems, dead flowers, bark). Cleaning up fallen leaves can be tedious, but will assist in reducing the likelihood of reinfection next season. Renewal pruning to decrease density and consistent watering will help support the stressed plant.

• Verticillium wilt affects a number of plants in our landscapes,

lilac being one. Individual branches turn brown and die suddenly

due to the two fungi Verticillium dahliae and Verticillium albo-

atrum blocking the vascular system of the branch, cutting off

water and nutrient movement. There is no cure for a plant with

this fungal infection, but owners can up the watering and

fertilizing to extend the life of the plant. The best prevention is to purchase resistant cultivars and practice good sanitation as all parts of the infected plant can spread the disease.

Insects

The insect, the Lilac borer (also called ash borer and a type of clearwing borer moth) tunnels into lilac branches. Signs include sawdust, sap and frass (excrement). Some plants tolerate this damage. Others can experience dieback of branches and sometimes death of the entire plant. However, Extension Entomologist, Jeff Hahn, confirmed he has NOT seen lilac borer samples coming into his lab this year, He also noted that people are unlikely to notice them.

Herbicide damage: With people spending more time at home, they may be more aware of changes in their landscapes. If applied during times of high heat and humidity, broadleaf weed treatments on lawns can volatilize, turning to a gas, and drift onto non-targeted plants. Cupping and browning, both symptoms of herbicide damage (Fig. 3), would be seen in patterns; that is, along one side of a hedge and not the other, at the same level across a base of shrubs planted in or near lawn areas, and most importantly, downwind from prevailing winds for the property. Lilac hedges along farm fields or other large areas treated with herbicides may also reflect damage in this way.

Fig. 2 Pseudocercospora on lilac leaf. Photo: Grace Anderson, UMN Plant Disease Clinic.

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Figure 3 Cupping is a sign of herbicide damage on lilac. Leaves may become brittle and fall from shrub and stems may dieback.

DIAGNOSING A LILAC PROBLEM

1. Try to determine if damage to the lilac is:

• Abiotic – Herbicide damage, storm damage, drought, flooding, mechanical (lawn

mowers, trimmers, animal browsing)

• Biotic – Insects; disease-causing pathogens (fungi, bacteria, virus)

• Note that damage could be a combination of abiotic and biotic causes.

2. If abiotic, the client should implement renewal pruning and proper plant care (watering, mulching, fertilizing) to help the plant recover. Advise keeping an eye on the plant in the 2021 season for signs of permanent damage and the possibility the plant may need to be replaced. Possible replacements for lilac can be found in the Plant Elements of Design plant database.

3. Have the client look for signs of insect infestation (sawdust, sap, frass) and damage like exit holes on the branches. Pruning off damaged branches could prevent further damage. Advise clients to monitor next spring as lilac starts blooming for the adults and set traps using pheromones (see Lilac borer). Sprays can be applied. Advise they contact a certified arborist for treatment.

4. If your diagnosis leads to the conclusion the damage is likely caused by a pathogen, advise clients to submit a sample to the UMN Plant Disease Clinic. They should visit the webpage for the Plant Disease Clinic and read how to submit a sample to the PDC for analysis.

CONSIDERATIONS FOR THE CLIENT IF PLANT REMOVAL IS A POSSIBILITY

• What is the value of the plant (property value and / or sentimental value)? There are many great plants including resistant lilacs that could be good replacements. You can find some of them in the Plant Elements of Design plant database.

• Is the lilac at the end of its life? Lilacs are relatively long-lived plants (25 years to 50+ years) depending on how they have been cared for and the growing conditions they have endured)

• What is the potential for spread to other plants of the same kind and / or another susceptible species?

• Can the issue be managed by other methods of management (pruning, sanitizing, improved plant care).

• Do they need a certified arborist to assess the plant(s)?

• Has this problem occurred repeatedly for multiple years? Have different methods of

management failed?

QUESTIONS OR COMMENTS?

Julie Weisenhorn

Extension Educator – Horticulture <personal data hidden>

Grace Anderson

Research scientist and Plant Disease Clinic diagnostician Extension Master Gardener – Hennepin County <personal data hidden>

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