Witches broom can occur
on trees and shrubs for several months to several years depending on its
severity or cause. Although witches broom can be an eye sore it has no serious
threat to the host tree. Currently there is no treatment however it is possible
to prune out the broom growth several inches below the point from where the brooms
began to form. Pruning out infected branches is currently the only option if the appearance
of the brooms is intolerable. If feasible, pruning out branches
that are infected or cutting back shoots to the point of origination on larger
branches and trunks although shoots may regrow requiring on going removal every
few years. When fungi, virus or mycoplasma-like organisms are responsible for
witches’ brooms, the disease may have spread throughout the tree, so pruning
may not provide control.
The disease cycle begins
when spores of Moniliophthora perniciosa which are dispersed by either
wind or water and enter pores of a tree’s leaves. Due to the high humidity and
moisture in brazil the spores are able to germinate, and the tree undergoes
severe physiological and hormonal changes causing the plant’s energy to divert from
normal growth to the production of the brooms. This has a negative impact on the
cacao pods, which contain the beans used in the production of chocolate. After about
two to three months of infection, the brooms die, and the fungus lifecycle is
complete by giving rise to clusters of pink mushrooms, which then again release
In order to maximize the
supply of cacao beans which produces chocolate, large monocultures of cacao
trees were planted in both South and Central America from a handful of seeds.
This accidentally placed the trees in a position that wasn’t ideal, since
genetically similar populations are more at risk of succumbing to these devastating
pathogens. Ten years after the first cases in Bahia, Brazil, nearly 75 percent
of the native cacao trees have been eradicated due to the rapid spread of witches’
However, some witches brooms have had a malicious effect on the economy.
One witch’s broom disease caused by a fungus (Moniliophthora perniciosa) attacks the cocoa tree preventing the production of
chocolate. The fungus’s spores are spread by wind making it difficult and expensive
to control, which in return has drastic effect on production. The disease found
its way to Brazil’s cocoa-producing area where the country went from being
second in the world in cocoa production to a country that imports more then it exports.
Due to human error the stage for the spread of Moniliophthora
set and the aggressive fungus responsible for witches’ broom
disease took over cacao trees in Brazil.
Generally, the type of
tree is used to distinguish the cause of witches’ broom. For instance, mites
are typically responsible for witches’ broom in willow trees whereas pine
brooms are commonly caused by rust fungus (Gymnosporangium and Pucciniastrum)
or brooms found on hackberry tress are normally caused by both mites and fungus.
Some other common trees that develop witches’-brooms include oak caused by
powdery mildew (Sphaerotheca lanestris), incense cedar caused by a
rust, hackberry caused by powdery mildew and an eriophyid mite, and rose
rosette caused by a virus.
Phytoplasmas are related
to bacteria, they lack a rigid cell wall, and have an amoeba like shape. Phytoplasmas
appear to colonize in the sap conducting tissue called the phloem and damage
the tissue by interrupting the sap flow. The stress results in deformed bundles
of twigs and branches which appear to look broom like. The growth around the
witches’ broom may become less vigorous, indicating that the witches broom may prevent
nutrients from reaching other parts of the plant. When witches’ brooms are
caused by mutation, gardeners sometimes propagate them for breeding of dwarf
plants. The signs and symptoms of witches’ broom can vary. There can be a
single broom on a tree or in some cases the brooms can be quite large and
scattered throughout the tree, while others may appear small and less
noticeable. When the broom is caused by a genetic mutation there is usually one
broom per tree. However, when there are environmental stressors that injure the
growing point of the branches broom can be triggered and begin to form in abundance
making it difficult to distinguish the exact cause.
Both biological and
environmental stresses can lead to witches’ broom occurring in trees or shrubs.
Organisms such as phytoplasmas, infestation of mites, fungi (Apiosporina, Exobasidium, and Taphrina),
dwarf mistletoe, aphids, or genetic mutations of vegetative cells are all factors
that can kill the terminal bud of shoots and cause abnormal growth. Once they
begin attacking the host tree it triggers the formation of brooms.
Witches broom can be a symptom
of a fungal or viral infection that typically occur on a number of conifer trees
such as hackberry, maple, and willow, and deciduous trees such as pine and
spruce. The terms deciduous and coniferous tell us of classifying trees in two
aspects which are according to their leaves and the manner of seed production.
Deciduous trees are also known as angiosperms or flowering plants which drop
their leaves at the end of the growing season and go dormant during the cold
weather. Once the weather becomes warmer in the spring they begin producing new
leaves. Whereas coniferous trees aren’t described by their leaves falling off
during fall or winter, but rather by their seed-bearing natures. These trees produce
seeds in structures known as cones. They are, therefore, known as gymnosperms
(having naked seeds)
January 12, 2018