Witches’ brooms on plants are an abnormal cluster of twigs or shoots arising from a single point on a stem or branch and can be caused by physical injury or the invasion of an organism. Mistletoes (see “Mistletoe“, one of my earlier posts), which most of us are familiar with, induce witches’ brooms.
Wandering around Baum Lake (Shasta County CA) my attention was drawn to witches’ brooms on several juniper trees (Juniperus species – for example, see “Western Juniper“). Certain rust fungi belonging to the genus Gymnosporangium can cause witches’ brooms on junipers. (Depending on the Gymnosporangiam species swellings instead of witches’ brooms may occur.) Fungi have no chlorophyll so depend on living (parasites) or dead (saprophytes) organic matter for their nutritional needs. Fungi reproduce by spores.
Gymnospore species have different life cycles, many of which are complex and involve alternate infection of two different hosts. The Gymnosporangium species that infect junipers also have a stage that must occur on species of the rose family (hawthorns, quince, serviceberry, chokecherry, apples, pears, etc.). The rust fungi that cause witches’ brooms usually do slight damage to the juniper, most often only deforming the tree. However a massive infection can cause reduced growth and vigor and could possibly kill the tree. On the secondary rose family host the rust causes lesions on the fruit, leaves and twigs. Again, in the wild these infections usually are not serious, but if the rust infects apple or pear orchards they do affect the commercial value of the harvest.
As a very simplified explanation of the juniper witches’ broom life cycle, the rust fungus overwinters on the juniper in the telial (resting) stage as teliospores (thick-walled resting spores). After spring rains the teliospores germinate to become reproductive basidiospores, which are then carried by the wind or insects to nearby rose family hosts. After being infected by the basidiospores, cup-shaped structures (pycnia) develop in the rose family host followed by the production of vegetative aeciospores. The infection on the rose family host looks like orange or yellow spots on the leaves, fuits or stems. By early fall the aeciospores mature and are blown by the wind to junipers where they again form witches’ brooms and begin the cycle anew. The two hosts must be present for the Gymnosporangium to complete its life cycle.
I am not positive because identification of the different gymnospores is complex, but I believe the juniper witches’ brooms in the photographs may be caused by Gymnosporangium nidis-avis. The one pictured juniper must have a massive infection because the branches were bare of healthy vegetation and only multiple witches’ brooms remained. Another photograph illustrates a normal juniper branch tip as well as an abnormal twig cluster taken from a witches’ broom.
Rust fungi are much more complex organisms that one might expect.