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.

Insect Pests

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.

  • Mealybugs: 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 lurk there.
  • 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 is 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 mealybugs 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 works 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 greenhouse 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 pseudobulbs and thus escapes contact with the pesticide. To 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 eelworm), 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 the 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 is 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 around 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 short list is a sample of the chemicals available and the names under which they have been marketed.

  • 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 the 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 a 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 pull out when small but, when a large plant gets established next to a pseudobulb, 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.

Cultural Problems

To be your own orchid problem-solver, practice being a keen observer. Examine the troubled plant in good work light, the sorta 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 pseudobulbs (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 a fresh medium.
Possibly the second most common problem with orchids, under-watering has symptoms exactly the same as over-watering-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 pseudobulbs 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 re-pot 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 re-pot an orchid plant. The less obvious symptoms include gradual, even yellowing of the leaves, the oldest affected first. There may be an overall dullness about the appearance of the foliage. New growth likely will be stunted and the pseudobulbs 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 pseudobulbs, 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, sun-room, 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 watering, 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 business associates.
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 isn’t going to go away anytime 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.