Michelle Grabowski, University of Minnesota Extension
As spring slowly unfolds in Minnesota this year, many gardeners are anxiously watching the buds on their favorite landscape shrubs open up for the season. Some shrubs have one to several clusters of thin weak twigs arising from one point on a larger branch. These unusual growths are known as witches' brooms and they can form at the tip of the branch or lower down the stem. Many different problems can result in a witches' broom. Mites, fungi, aphids, salt or other damage to buds can all result in a proliferation of small branches, where only one should have emerged.
Another less known and little understood pathogen causes witches' brooms in Minnesota landscapes. Phytoplasmas are single celled organisms in a group known as fastidious bacteria. These tiny pathogens were unknown to scientists prior to 1967. They are very difficult and often impossible to grow in a laboratory and can only be seen with a powerful electron microscope.
As a result, little is known about phytoplasmas. Many of them do not even have proper scientific names (a genus and species) because they have not been properly described. Often they are categorized into groups of phytoplasmas that are believed to be related to one another. To truly identify a phytoplasma in a landscape shrub, DNA analysis needs to be done.
Phytoplasmas live in plant sap and interfere with photosynthesis, plant growth, development and seed production. Infected shrubs often have yellow, curled or distorted leaves.
Branches are often stunted or unusually thin. Witches' brooms are very common in phytoplasma infected shrubs. The entire plant may be stunted and in very severe cases killed.
Phytoplasmas are carried from plant to plant by insects that feed on the sap. They can survive from year to year within the insect or the host plant. Once a shrub is infected, it cannot be cured. Some shrubs tolerate the infection; others decline for several years and then die. The diseases below are phytoplasma diseases that can occur in Minnesota.
Willow Witches' Broom
Witches' brooms in willows can be seen in willows growing alongside roads as well as in landscapes throughout the state. Black willow (Salix nigra) and pussy willow (S. discolor) are both susceptible to a phytoplasma in the aster yellows group. Willows infected with this phytoplasma have witches' brooms on one or more branches. These brooms often have stunted yellow leaves and may die during winter months. Exact identification of a phytoplasma requires a costly DNA analysis. Although the willows with witches' brooms in Minnesota have not undergone this analysis, phytoplasma is the most likely cause of the problem.
Willows are considered tolerant of the disease and often continue to grow vigorously despite infection. In fact, it is believed that the willow is able to isolate the pathogen in the infected branch. The brooms themselves have a very high concentration of phytoplasmas, where as other branches in the same shrub are often completely free of the pathogen. Gardeners with infected willows should prune out any branches with witches' brooms.
Lilac Witches' Broom
Lilac witches' broom is caused by Candidatus Phytoplasma fraxini, a specific member of the aster yellows phytoplasma group that only infects lilac and ash trees. Over 20 species of Syringa are susceptible to lilac witches' broom. Common lilac (S. vulgaris) is tolerant and often shows no symptoms other than slower growth and shorter twigs. Japanese tree lilac (S. reticulata) and many hybrid lilacs are very susceptible to lilac witches' broom. These lilacs have yellow, distorted small leaves that often scorch brown on the edges by midsummer. Many tiny thin shoots form in clumps at the base of the plant. The shrub declines and is often killed a few years after the first witches brooms appear.
Dogwood Witches' Broom and Stunt
Witches' brooms caused by phytoplasmas have been identified on silky dogwood (Cornus amomum), gray dogwood (C. racemosa) and red osier dogwood (C. sericea). It is believed that the phytoplasma responsible for witches' brooms in dogwoods is a member of the aster yellows group. It is unknown how commonly this disease occurs in Minnesota.