What is Lilac Witches’ Broom?

Lilac witches’ broom is a serious systemic disease of lilacs for which there is no cure. Infected lilacs typically gradually decline until they die. The characteristic symptom is the development of witches’ broom (tight clusters of short, thin branches that all stem from the same point). These witches’ brooms tend to form near the base of the lilac but they can also occur at the ends of branches. The brooms typically have a very upright growth pattern. Other symptoms of the disease may include:

• The leaves on the overall lilac may appear smaller than normal, they may yellow or appear distorted.
• The leaves on the brooms tend to remain green on the shrub long after the rest of the plant has turned colour in the fall and dropped.
• Brooms may die back in the winter.
• New growth may have shortened internodes.
• Slow apical and radial grow (dwarfing).
• Diminished apical dominance.
• There may be twig dieback.
• The lilac may bloom out of season, typically earlier.
•  Affected lilacs may be abnormally sensitive to freezing.
• There may be overall loss of vitality and premature death.

What Causes Lilac Witches’ Broom?

It is caused by the Phytoplasma Candidatus Phytoplasma fraxini (This is the same species that causes Ash Yellows). Phytoplasma are a unique group of bacteria like micro-organisms. Unique in that they are single celled and lack a cell wall. The disease is vectored (spread) by phloem feeding insects mostly leafhoppers, planthoppers, and psyllids. These insects are able to harbour the phytoplasma in their bodies for as much as two years after becoming infected. Once inside the plant the Phytoplasma colonize the phloem (where the sugars produced by the plant are sent down to the branches, trunk and roots) causing blockages. These blockages reduce the plants food supply and kill the growing points which results in the dense cluster of growth on side shoots.

Are there Other Plants In the Landscape Susceptible to Witches’ Broom?

There are a variety of conifers and deciduous tree species that may form witches’ brooms. There are also a variety of causal agents, like fungi, mites, genetic mutations, parasites, environmental factors and Phytoplasmas. Of the Phytoplasmas, witches’-brooms and bunch disorders can develop on pecan, hickory, lilac, walnut, willow, dogwood, ash, honeylocust, peach and elm (Missouri Botanical Garden, n.d.). How ever the specific Phytoplasma species Candidatus Phytoplasma fraxini is the one responsible for lilac witches’ broom and Ash Yellows. In Canada it has also been found in peaches (S. Zunnoon‐Khan et. al. 2010).

Are Some Varieties More Susceptible Than Others?

Common lilac and hybrids of common lilac and Syringa relexa or Syringa villosa are more tolerant of the disease (Olis J. and Hudelson B, rev. 2012). While Preston lilacs (Syringa x prestoniae), Japanese tree lilac (S. reticulata), Syringa josikaea, Syringa reticulata and Syringa sweginowii or hybrids of these species are all susceptible. Late flowering varieties tend to be more at risk. Collections of lilacs also appear to be more at risk than individually grown lilacs. Single blooming varieties are said to be more susceptible than doubles and plants with magenta, purple and blue flowers are more susceptible than those with lilac colors (Mathieson, M. 2009).

Lilac Witches ‘ Broom has been found in approximately 35 taxa. Following is an partial list of some varieties the disease has been found on. The bolded ones are the more susceptible varieties.

• Syringa x diversifolia, 
• Syringa x henryi
• Syringa x josiflexa
• Syringa x nanceiana
• Syringa oblata
• Syringa x prestoniae
• Syringa pubescens 
• Syringa tomentella
• Syringa vulgaris (common lilac)
• Syringa josikaea (Hungarian lilac)
• Syringa Komarowii (nodding lilac)
• Syringa × laciniata (cut-leaf lilac)
• Syringa meyeri (Korean lilac)
• Syringa ×persica L. (Persian lilac)
• Syringa sweginzowii (Chengtu lilac)
• Syringa villosa (late lilac)
• Syringa yunnanensis (Yunnan lilac) ( Sinclair, WA et. al. 1996 )
• Syringa reticulata (Japanese tree lilac)

How Do You Treat Lilac Witches ‘ Broom?

At present there is no cure for this disease. However a few cultural practices may help to prolong the lilacs life or help to prevent the infection.

• Prune out the witches’ broom, disinfecting your pruning tool between cuts.
• Keep the shrub properly watered during periods of drought.
• Lightly fertilize with low nitrogen fertilizer.
• Choose varieties that are less susceptible to the disease.
• Manage phloem feeding insect populations.
• Proper site selection.

Photo credits: all photos taken by the author.

References

Mathieson, M. 2009. Possible diseases of Lilacs. Rainy River Record. Retrieved on April 11, 2021. Possible diseases of Lilacs | Rainy River Record

Missouri Botanical Garden n.d., Witches’ broom, Retrieved on April 11, 2021 from: Witches’ broom (missouribotanicalgarden.org)

Olis J. and Hudelson B., UW-Madison Plant Pathology. Revised:  8/6/2012. Ash Yellows. Retrieved on Apr. 12, 2021 from: Ash Yellows – Wisconsin Horticulture

Sinclair, WA; Griffiths, HA; Davis, RE (1996). “Ash yellows and lilac witches’-broom: phytoplasmal diseases of concern in forestry and horticulture”. Plant Disease. 80 (5): 468–475. Retrieved on April 11, 2021 from: PlantDisease80n05_468.pdf (usda.gov)

S. Zunnoon‐Khan, Y. Arocha‐Rosete, J. Scott, W. Crosby, A. Bertaccini, R. Michelutti, 2010. First report of ‘Candidatus Phytoplasma fraxini’ (group 16SrVII phytoplasma) associated with a peach disease in Canada. British Society for Plant Pathology. Retrieved on April 11, 2021 from: First report of ‘Candidatus Phytoplasma fraxini’ (group 16SrVII phytoplasma) associated with a peach disease in Canada – Zunnoon‐Khan – 2010 – Plant Pathology – Wiley Online Library

Other resources:

Ash Yellows (wisc.edu)

Candidatus Phytoplasma fraxini – Wikipedia

Lilac Diseases (psu.edu)

What’s wrong with my plant? : Garden : University of Minnesota Extension (umn.edu)

Lilac Witches’ Broom – Tips For Treating Lilacs With Witches’ Broom (gardeningknowhow.com)

Japanese Lilac, few blooms #332341 – Ask Extension

PlantDisease70n04_342.pdf (apsnet.org)

ASH YELLOWS PDF model (arborcareservices.net)

Knowledge Bank | Phytoplasma fraxini (plantwise.org)

Responses of 11 Fraxinus Cultivars to Ash Yellows Phytoplasma Strains of Differing Aggressiveness (apsnet.org)

Phytoplasma fraxini (ash yellows) (cabi.org)

yglilacs.pdf (msuextension.org)

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