When Orchids Need Help
Pests And Diseases
Any plant grown in poor conditions can fall victim to insects or
disease, and orchids are no exception. However, when their basic
needs are met orchids are unusually tough and trouble free.
Your greatest asset in handling plant problems will be a sharp eye for
anything that seems abnormal. If you do discover a problem with an orchid, your
first step should be to check the growing conditions to be sure your orchid is getting what it needs to
thrive. Also, ensure that the orchid's leaves are kept clean and that
the growing area itself is clean and free of debris.
Typical danger signs, along with their probable causes and
remedies, are identified below. As the same
poor growing conditions are often the precursor and underlying
cause of troubles involving pests and diseases, it is doubly wise
to avoid them in the first place.
There are many large insects that can chew leaves and flowers. You can use Orthene, Carbaryl or
Diazinon. Scale insects are sap-sucking pests and should be
dealt with immediately any appear as they are
difficult to eliminate if they get established in a
collection. Spraying with Malathion or Diazinon will
control them, but persistent treatment over a long
period is required. Mineral oil sprays sold for use on
green-leafed plants can be used, but follow the
directions carefully. Swabbing with alcohol or methylated
spirits gives immediate results. An effective
home-made treatment is to use a mixture of a mild detergent,
vegetable cooking oil and water in the ratio by
volume of two parts detergent, 10 parts of oil to 1000
parts of water. Put the detergent and oil in a little
water in a blender until it forms a white emulsion,
then add the rest of the water. Adding Malathion or
Diazinon at half full-strength will make the
preparation even more effective. When spraying, be
careful to cover not only the round female scales but
also the dissimilar cotton-like male form.
- Mealy bugs: are sluggish insects that appear white
due to their surrounding cotton-like filaments. Like
scale insects they are also sap-suckers and have a
water-repellent exterior. They tend to hide in
crevices and even in flowers. Any of the treatments
suggested for scale should control them.
- Aphids: often build up in large numbers in buds,
flowers and soft new growths before they are
noticed. They are easy to kill with a wide range of
insecticides including Diazinon, Malathion and
Mavrik. Another option is Orthene which has both
a contact and systemic action, being absorbed into
the sap stream of the plant. Aphids are particularly
unwelcome because they can harbor plant viruses.
- Thrips: are tiny fast-moving insects that rasp the
surface of leaves and flowers and suck the sap. They
leave silvery markings on leaves. Spray with any of
the products suggested for aphids. Check the surface
material of the potting mix as some species like to
- The two-spotted mite (Tetranychus urticae): is a bad
pest of some orchids, including cymbidiums, where
it will leave the underside of the leaves with a silvery
appearance and a feel like sandpaper. The mite is
straw-colored or reddish with the characteristic two
black spots on the back. They proliferate in high
temperatures and low humidity and dislike water.
Two sprays 10 days apart with Kelthane (which does
not kill the eggs) or Pentac are necessary. Do not use
either of these materials more than three times
running in 12 months as the two-spotted mite is
notorious for evolving resistant strains. If necessary,
switch to a spray in a different chemical group, such as Mavrik.
There are other mite species that will do damage,
some small enough to need a magnifying glass to see
them. If the sprays above are not effective on these,
Malathion or Diazinon should be.
- Slugs and snails: need to be kept out of the
growing area of orchids. Baits containing Mesurol or Metaldehyde will kill them.
There is a tiny snail that hides in the pot where its presence is often not
suspected until the orchid is removed. The snail nibbles the ends and
sides of the roots. Ordinary baits are not very
effective. Destroy them physically after shaking the
potting mix from the roots.
- Ants: when ants appear, suspect infestation by mealy bugs, scale,
or aphids. All excrete a sugary honeydew attractive to ants,
which then stand guard over the insect "cows." Ants do
relatively little damage to orchids.
Treatment: Go after the honeydew cause, not the ants.
Ants can be deterred by diatomaceous earth. Fire ants in
particular can be controlled by rotenone. A beneficial natural
predator, Pyemotes mite, will infest ant eggs.
- Fungus gnats: these common little flies are mostly a nuisance. The more
damaging larvae live within pots, particularly in organic
mixes, where they lay eggs, feed on roots, and break down
mix too quickly. Fungus gnats bring bacterial and fungal root
rots. Plants may wilt, show root rot with maggots, and have
distorted leaves. Overly wet mix and shady conditions
encourage fungus gnats, often introduced via peat.
Treatment: Make sure the potting mix is intact. Keep the
area debris-free. Lay yellow sticky traps horizontally on the
lip of the pot, with another near the base. Horticultural oil
kills adults. To kill larval worms and eggs, use pot drenches
of either insecticidal soap or Bacillus thuringiensis (Bt) strain H-14.
- Scale: these small round insects crawl when young, then attach
for life underneath edges and midribs of tough leaves. Scale
is a sucking insect, sapping juices. There are many species,
but only two basic types. Hard scale is brownish, coated with
a protective waxy armor under which eggs are laid or live
birth given. Soft scale can look like cottony masses of
mealy bugs and secrete sticky honeydew. Damage includes yellow
spotting. Scale also infests roots, sometimes with no
symptoms other than decreased vigor.
Treatment: Clean plants with a soft toothbrush dipped in
insecticidal soap or 70 percent isopropyl alcohol. Use sprays
of insecticidal soap or horticultural oil, or alcohol on swabs.
Avoid over fertilizing. Lacewing is a natural predator.
- Whiteflies: are not flies but are related to mealy bugs and
scale, secreting honeydew. Adults are white waxy fliers; the
more damaging greenish yellow nymphs have sucking
mouthparts and feed heavily. Nymphs attach to the underside of
leaves, with eggs laid in a small circle. Whiteflies are most
active in warm environments. Damage includes wilted leaves
with sooty mold or sticky leaf residue, leaf and plant death.
Treatment: Hang yellow sticky traps vertically a few inches
above plants. Reduce nitrogen fertilizer. Drop temperatures
to decrease activity. Spray with insecticidal soap, horticultural
oil, or pyrethrin. A natural enemy is Encarsia formosa, a
minuscule parasitic wasp not dangerous to humans; Encarsia
work best under warm, humid conditions.
- Spider mites: red spider mite and false spider mite are probably the most serious pests of
orchids because they are the most insidious. The creatures themselves
are so small that they are difficult to see and usually the first warning is the silvery
appearance of a plant's leaves, particularly the underside, which later turn
brown. Red spider mite is often said to be more prevalent in dry conditions but
it can still flourish when the humidity is high. In winter, when the day length
is less than 12 hours, the mites may migrate to the frame of the
and hibernate within webs. So, if possible, wash the frame with dilute bleach
at this time of year to help stop any build-up.
There is a predatory mite that provides a well-established means of
biological control against spider mites. It should be introduced when red spider
mite numbers are rising, that is in spring. It attacks all stages of the mite's life
cycle -egg, nymph and adult. If chemical control is preferred, the insecticide has
to be changed every few years as the mites develop resistance to chemicals.
The eggs are immune to virtually all insecticides and so applications should
be repeated after ten days by which time the eggs will have hatched.
False spider mite can also attack orchids and is susceptible to the same
pesticides as red spider mite. It is particularly serious on pleiones, where
it hides under the pseudo bulbs and thus escapes contact with the
deal with this problem, lift the pleiones, spray with the relevant pesticide and
repot into fresh compost and clean pots.
- Vine weevils: vine weevils, which seem to be on the increase, cause damage in two ways.
The adults eat circular holes in leaves and the grubs, which are white with a
brown head and are about 8mm (3/8 in) long, live below the soil surface and
feed on roots and tubers. The damage caused by adults is unlikely to be fatal
but it is unsightly and long lasting. It is the grubs, however, that pose the biggest
risk. You are unlikely to find grubs in a bark-based compound and they would
certainly not like rock wool, but terrestrial orchids growing in a peat-based
compost could be at risk.
The grubs can be killed by the insecticide Sybol, but biological control is also
possible in the form of a parasitic nematode (a type of eel worm), which attacks
both adults and larvae. Adults can enter a greenhouse through open vents and
one way of controlling them is to use an insectocutor, an electrical insect killer,
although it will kill any flying insect that is attracted to light. Vine weevils are very
cryptic and emerge only at night. It is said that if one goes into a greenhouse
after dark, any vine weevils there can be traced by the sound of crunching jaws.
- Woodlice: woodlice feed mainly on decaying vegetable matter and many people believe
that they do no harm to living plants, but they definitely eat the growing tips
of orchid roots. They lurk under pots that are standing on a solid surface and you
may occasionally find one in a pot when reporting. They probably do most
damage to mounted orchids where they can hide under the plant or on the back of
a mount, so it is always worth checking the backs of mounted plants from time
to time. Woodlice are susceptible to almost any insecticide.
- Garlic snails: garlic snails are tiny snails with a flat
shell, about 5mm (1/4 in) in diameter, which are often found in orchid houses, being
passed around between growers on pots and plants. They tend to live in
the pots during the day and emerge in the evening. They do not seem to be
affected by slug pellets, but reasonable control can be achieved by walking
round the orchid house in the evening and crushing any that are seen. When
crushed they smell of garlic, hence the name. At least growers intemperate
climates are spared the depredations of the giant African snail, which grows
to 15cm (6 in) long and can demolish an orchid in a single day.
Fungi and bacteria
Water is almost always involved in the
establishment of these diseases. Some fungi release spores
into the air and they may fall on healthy plants and
infect them. The spores of the fungus Botrytis will
germinate and infect when the relative humidity is
near 100 percent, particularly if there is water (e.g.
the merest film of dew) on the surface. This disease
is the main one responsible for the spotting of
flowers and it takes just six hours of favorable
conditions for germination and infection to occur.
Some diseases are spread by water infected at the
source or splashing around from infected surfaces.
These include bacterial rots and the soft black or
brown rots caused by Pythium or Phytophthora. Do not allow stagnant water to remain in the crown or elsewhere on the orchid.
There is a bewildering array of chemicals available to control diseases. Most
are preventative; if your orchid is coated with them the disease finds the
environment too toxic to get established. Some
fungicides are systemic, being able to enter, move
around and protect the plant from within. Many
modern fungicides are effective against some diseases
but not against others and may even make them
worse. The problem for the orchid grower is diagnosis
and often hit-or-miss tactics have to be employed.
The following shortlist is a sample of the chemicals
available and the names under which they have
- Captan (Orthocide): a preventative fungicide
effective against a wide range of fungal diseases. Can be
used to drench potting media.
- Mancozeb (Dithane, Manzate, Fore): another
preventative fungicide effective against many diseases.
- Rovral (Iprodione, Chipco): a preventative systemic
fungicide with some eradicant action on established
infections. Very effective against Botrytis.
Unfortunately over-use will generate resistant strains of
diseases and this product should not be applied more
than three times in 12 months.
- Benlate (Benomyl): another systemic fungicide.
Resistant strains quickly build up but being in a
different chemical group it is useful to alternate with
Rovral. Neither of these products should be
regularly applied but they are reliable to have in reserve
for emergencies. Neither is effective against
Phytophthora or Pythium.
- Aliette (fosetyl aluminum): a systemic absorbed by
leaves and roots to give long-term protection against
Phytophthora and Pythium. Has some curative effect.
- Terrazole (Truban, Etridiazole): a soil drench
effective against the same diseases as Aliette.
- Quaternary ammonium compounds: trade names
come and go for these, which are marketed for
anything from eliminating moss and algae to
treatment of swimming pools. They have been sold
for plant protection under the names of Consan,
Physan and RD20 among others. It is claimed they
are effective against both bacterial and fungal
diseases. Follow the directions, as they should be
used greatly diluted. These are the only products
among those mentioned above that will kill bacteria
and they will often clean up small patches of wet rot
on leaves if swabbed on or applied to diseased areas
after the rotten tissue has been cut out. Useful, too,
for controlling algae on walls and roofs.
If a fungal or bacterial problem cannot be diagnosed,
try everything until an effective solution is found.
With soft brown or black rots try swabbing or
flooding the infected area with Physan and drenching the
roots with Aliette or Terrazole. If possible cut out
the rot first. As a last resort try Rovral or Benlate,
which treat some diseases with these symptoms. For
leaf spotting try Rovral or Benlate once and then
maintain regular spraying with a protective
fungicide such as Captan or Mancozeb. Above all, try to
correct the conditions that allowed the
establishment of the disease, given that this may be easier
said than done. Rotten roots may be hosting diseases
but the prime cause is likely to be media remaining
wet too long because it has broken down or has been
watered too frequently. Tip the plant out of the pot
and re-pot in a fresh pot with new medium after
removing the decayed roots.
Symptoms on the foliage include yellowish or dark
streaks and blotches, or mottling, often with sunken
leaf surfaces and occasionally in a diamond or
mosaic pattern. Flowers may be streaked with brown,
white or more intense colored markings. Diagnosis is
often difficult as many of these symptoms can have
origins other than a virus. If the same markings
appear on every leaf or flower every year and other
similar plants are "clean", a virus must be suspected.
There is no cure and infected plants should be
destroyed or isolated so they cannot infect others. A
virus can be spread by insects but is more likely to be
spread by the grower when handling plants,
especially when re-potting or cutting off flowers.
Wash hands between re-potting valuable plants and
sterilize cutting tools by heating to a cherry red.
Never use the same pot for another plant without
sterilizing it first.
Keep pots weed-free. Ferns that come up may look nice but they are competitors for the orchid and
should be removed. A variety of Oxalis corniculata has become a worldwide pest of orchids. It fires shiny
black seeds a considerable distance and the seedlings come up everywhere. They are easy to pullout
when small but, when a large plant gets established next to a pseudo bulb, removing the plant from the
pot and shaking off all the potting medium can be necessary to remove it. This can be avoided by
painting the weed with a solution of weed killer. Glyphosate can be applied with a small artist's brush.
Be very careful not to allow the weed killer to contact any part of the orchid.
To be your own orchid problem-solver, practice being a keen observer. Examine the
troubled plant in good work light, the sort a dentist or doctor uses, and, if you need them,
use your best reading glasses. To comprehend what you see, it will be necessary to know the
growing conditions needed by the type of orchid you are inspecting.
- Often cited as the most common problem with orchids, overwatering
results in pseudo bulbs (and leaves, if succulent) that are shriveled and
growing slowly or not at all. Inspect the roots and you will find evidence of
rot. The treatment is to reduce watering or to repot if the medium has decayed.
Keep the orchid shaded in a humid
area until new roots become established.
Sometimes this condition is the result of planting in a pot that is too large and in
which the medium has begun to decompose. Again, the solution is to repot, this time into
a smaller pot, using fresh medium.
- Possibly the second most common problem with orchids, underwatering has symptoms exactly the same
as overwatering-with one important exception: the roots will be firm and white when you
inspect them. The treatment in this situation is to water the orchid several times in
succession until the medium is soaked. The pseudo bulbs should plump up in a day or two. In the future, water
more frequently. However, if you wait to water an orchid until it has dried out
completely, the roots
may not be able to quickly take up moisture. Frequent watering, then, can lead to root rot.
- This condition can occur from too-frequent applications or from haphazard measuring. Symptoms include leaf
edges and tips that are burned and roots that are withered, especially at the tips. Treat by
leaching out the fertilizer by pouring several gallons of plain water-deionized if you can
find it-through the growing medium.
- Scaly or powdery white mineral deposits
- Found on the rims and exterior walls of a pot and on the surface of the
medium, these deposits indicate that your water contains high concentrations of
minerals. Leaf tips may show signs of being burned by excess salts, and new growth may
be stunted. The solution: pour several gallons of plain or deionized water through the
medium to leach salts. Or repot the plant. When you water, do so thoroughly over the
entire surface of the growing medium, not in one spot alone. If your water is extremely
hard, mix it with deionized water or rainwater to reduce the concentration of minerals.
- Top-heavy plant in small pot
- This is an obvious visual clue that it's time to divide and repot an orchid plant. The less
obvious symptoms include gradual, even yellowing of the leaves, the oldest affected
first. There maybe an overall dullness about the appearance of the foliage. New growth
likely will be stunted and the pseudo bulbs will extend out over the edges
of the pot, or be packed to the point of beginning to grow on top of each other.
- Sunburn or too much light
- Sunburn is indicated by scorched blotches on leaves and exposed surfaces of
pseudo bulbs, or a general, overall yellowing of the plant. In extreme cases, the flower buds
may be deformed. Provide less light, more shade, lower daytime
temperature, or increase
humidity and improve air movement to prevent heat buildup.
- Too little light
- Inadequate light is indicated by foliage that is unnaturally dark
green but otherwise healthy and a plant that remains flowerless. Increase light gradually
over a period of a month. If the plant is growing under fluorescent lights, increase the
number of lamps, replace them if they have been in constant use for a year or more, raise
the plants so that they are closer to the light, or increase the number of hours the lights are
turned on each day. However, don't light the plants for longer than 14 to 16 hours a day.
- Air pollution
- Certain types of air pollution can be a problem if not detected
and corrected. Ethylene or sulfur dioxide in the air from smog, pilot lights, stoves, or
heaters can result in flower damage ranging from drying and discoloring of the tips of the
sepals to rapid wilting of the flower. Buds may falloff. Sheaths may yellow and dry before
buds appear. Don't leave flowering orchids in a closed room with ripe
apples or hyacinth
flowers, both of which give off ethylene. Improve ventilation and make sure gas
appliances are adjusted properly.
- Bud drop
- Bud drop can be caused by temperature fluctuations, reduced humidity,
or a change in environment-as well as air pollution. A large swing in temperature in a
brief period, for example 20°F or more, is one of the most common causes of bud drop. Also,
moving an orchid in bud from ideal light, moisture, and temperatures, such as in a
greenhouse, sunroom, or light garden, to a relatively dark, dry, home situation may also
result in buds shriveling and dropping. It's better to wait until flowers have opened
before moving plants.
- Pleated leaves
- This condition occurs among orchids having relatively thin leaves,
miltonia for example, and orchid plants that in general are weak, stunted, and
shriveled. Pleated leaves are a signal of inadequate watering.
If the growing medium dries out too much between waterings, roots never have a chance
to become established sufficiently to boost vigorous growth. All they are doing is
surviving. Water more often. Be consistent. Mark your calendar or date book if you
have to, then keep appointments with your orchids the same as you do your friends and
Another possible cause for pleated leaves and overall lackluster orchid plants is too little
of the good things orchids need, namely, a moist atmosphere and fairly strong light. It
may be that you need to add a cool-vapor humidifier to your growing area during that
time of the year when the heating system is being used, along with some fluorescent or
other supplementary lights.
- Lack of rest
- This can keep orchids from thriving in the same way it can cause humans to
malfunction. It is more likely to be a problem if plants are growing
under lights, in which case, use a timer to assure they receive uniform
amounts of light and dark over each 24-hour period. Leaving the lights
on nonstop is just as detrimental as leaving the orchids in total darkness
for a similar period.
- Potted too high
- Particularly with phalaenopsis, doritis, and doritaenopsis, an otherwise healthy
plant may develop shriveled leaves. Check to be sure that it is not set too high in the
growing medium. If you see that the lower-most leaf is an inch or more from the
surface of the medium, replanting is in order so that the bottom leaf emerges from
the stem at the surface of the medium rather than above it. Remove the plant
and do a complete repotting. Do not attempt simply to push it down into the
existing old medium.
- These can be a problem with a collection of potted orchids, because there
are several endemic to the orchid world that aren't going to go away any time soon.
Oxalis acetosella is one; another is a small acanthus with big roots, such as a
Chamaeranthemum. The cure for weeds is persistence in pulling them as soon as
they are noticed, along with every root.
- Within the world of orchids, there are some species having such an unusual
appearance that you could be fooled into thinking something is amiss. For example, consider the
Restrepia x anthophthalma, whose tiny flowers appear on top of the leaves. Now
is the time to use that magnifying glass for something more rewarding than
looking for trouble.