Pests,
diseases and disorders of interior plants
Indoor
plant displays sometimes do not look their best.
This may be because they are suffering from the
effects of pests and diseases or that there is
something amiss with their environment.
 A
wide variety of pests attack interior plants,
the most common being mealybugs, scale insects
and two-spotted (red) spider mites. If left unchecked,
they not only make the plants unsightly, but they
can severely damage the plants, which may shorten
their life.
The key to keeping plant pests under control is
to understand the reasons why the pests are there
and to know about the full range of control measures
available.
Why
pests damage interior plants
Many insect (and mite) species rely on plants
for their survival. They live, feed and breed
on plants and have evolved to co-exist with them.
Under natural conditions, plants are able to thrive
even if  there
is a large population of insects living on them.
The plants are able to outgrow any damage caused,
and the insects are kept under control by natural
predators such as other insects, birds, reptiles
and small mammals.
Indoor plants do not have the advantages of a
truly natural environment where plants and insects
live in balance. If the plants are unable to outgrow
any damage caused by insects and the insects are
not controlled, then they become pests that need
to be dealt with.
Pest
management
The key to keeping plant pests under control is
to understand the reasons why the pests are there
and to know about the full range of control measures
available.
The management of plant displays to keep pests
at an acceptable level by using a wide variety
of control measures and cultural practices is
called Integrated Pest Management (IPM). The use
of the full range of methods to achieve acceptable
pest levels is called Integrated Pest Control.
The formal definition of Integrated Pest Control,
as written in the UK Plant Protection Products
Regulations (1995), is:
“The rational application of a combination
of biological, biotechnological, chemical, cultural
or plant-breeding measures whereby the use of
chemical plant protection products is limited
to the minimum strictly necessary to maintain
harmful organisms below levels above which economically
unacceptable damage or loss would occur.”
Physical & cultural control
This involves the use of good growing and maintenance
procedures to restrict the development of pests
and the physical removal of the pests from the
plants. This can be achieved in several ways.
First, pruning. The judicious use of a pair of
secateurs can remove the infestation and keep
the plant in a good shape at the same time.
Next, cleaning the plants. Regular cleaning will
remove some pests, especially the mobile stages
of some insects and their eggs and pupae.
 Finally,
physical barriers and plant spacing. Some insect
pests, such as the vine weevil, are unable to
fly. They can only get from plant to plant by
walking. With container plants, it is possible
to prevent the spread of non-flying pests just
by making sure that neighbouring plants don’t
actually touch each other. In landscape beds and
large features, this is harder to achieve. However,
it is still possible to prevent pests spreading
from plant to plant by varying the size and types
of plant in the bed. By mixing plant species so
that susceptible and less-susceptible types are
near each other, it makes it harder for the pests
to find their way to a suitable new host plant.
Biological
control
Biological pest control involves the use of the
natural enemies of plant pests to keep their numbers
under control. The natural enemies may be predators
such as other insects that eat the pest; parasites
that lay their eggs inside the pest and use the
pest as a living host; or diseases that infect
the pest.
Chemical
control
In many ways, the use of chemicals to control
pests is a last resort. There are several reasons
for this, including environmental concerns, legislation
and incompatibility with biological control. However,
if other methods of control fail, chemicals may
be the only effective way of dealing with pests.
Several steps must be followed before using a
chemical pesticide. First, choice of product.
In most countries, the law is very strict about
what chemicals can be used. All registered pesticides
carry a label that specifies exactly what pests
it may be used to control, what plants can be
treated and the conditions under which such treatment
takes place. Once a decision has been made about
the choice of product, then steps have to be put
in place to make sure they can be applied correctly.
A site risk assessment will have to be made to
ensure that any pesticide used can be applied
safely and with minimum impact on non-target areas.
Environmental
issues
The use of pesticides to control plant pests will
have an impact on the environment. Where pesticides
are used inside buildings, the risk is greatly
reduced since exposure to the wider environment
is limited. However, it is necessary for an environmental
risk assessment to be performed before applying
a chemical treatment. Such an assessment will
take into account such factors as drains and ventilation
equipment, the presence of any water such as ponds
and streams and any possible impact on beneficial
insects and other animals.
Soil
fungi
Soil fungi are sometimes found in interior landscapes.
They are seldom damaging to the plants, but they
can look unsightly and rob the plants of valuable
nutrients. The fungi may occur for a few reasons:
over watering, decay of bark mulches or wood-based
material in the compost, decay of wooden plant
containers, decay of damaged plant roots.
Soil fungi are often not apparent until the fruiting
bodies (mushrooms) appear on the surface of the
soil although they may have been present for some
time. Once they appear it is tempting to pull
up the mushrooms, but this will have little effect
since most of the fungus will still be in the
soil.
Diseases
Very few diseases affect interior plants. The
most common are Phomopsis, which affects Ficus
sp., Gliocladium which affects some palms, and
stem and root rots caused by Phytophthora. Many
disease-like symptoms are not caused by infectious
organisms such as fungi or bacteria, but are the
result of physical or nutritional disorders and
damage. Being able to identify the differences
between infectious diseases and physical disorders
requires a lot of skill and practice and the use
of specialized equipment.
Disorders
 Indoor
plants are in an environment very different from
those in which they evolved. Buildings are a relatively
hostile environment for plants. They are devoid
of direct sunlight and cleansing rain and full
of pollutants, but short on humidity. The plants
will be in containers or small beds very different
from open ground in the wild. Pests and diseases
on the plants often thrive in the very conditions
that are putting their hosts under stress. Given
this range of problems it is hardly surprising
that even the healthiest plants in the best environments
may decline and need replacement.
Plants under stress show many symptoms. However,
it is not always easy to diagnose the cause of
a problem from visible symptoms alone. Discoloration,
defoliation or abnormal growth may be the result
of nutritional imbalances, fungal diseases, pollution
or physical damage. It is only by a thorough investigation
that the real cause of a problem can be identified
and a solution discovered.
Vandalism
It is a sad fact that indoor plants are sometimes
vandalised. The extent of vandalism varies from
outright destruction to damage caused by neglect
or thoughtlessness. Physical damage to plants,
such as broken branches or torn leaves not only
makes the plant unattractive, but may also be
dangerous. Irritating sap may be released, or
the plant may become unstable and fall over.
More subtle forms of vandalism occur when the
occupants of offices pour their cold coffee into
the plant container rather than down a drain.
This encourages pests and soil diseases and may
upset the nutritional balance of the soil. Likewise,
office cleaners should avoid tipping the contents
of their buckets in plant beds, since the components
of some cleaning solutions are very bad for the
plants and may kill them.
Detecting the damaging effects of such thoughtless
actions is possible and diagnostic equipment is
available to determine whether detergents and
other products have been added to the soil. Putting
the damage right, however, is very difficult and
expensive and may involve replanting or the replacement
of soil.
Author:
Kenneth
Freeman
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