Pests and Diseases

Bacterial     Fungal    Viruses    Insects    Other pests

Optimum growing conditions make for healthy orchids that are resistant to disease. But even healthy orchids suffer to some degree from the various common pathogens. Gardening is impossible without at least cameo appearances by insect and non-insect pests and diseases caused by bacteria, fungi, and viruses Without vigilance, minor outbreaks can lead to heartbreak when a favourite or expensive plant becomes so infested or infected that it must be discarded.

In general Regular observation of your plants will help you spot and treat problems before they pass the point of no return. A wide selection of pesticides, bactericides, and fungicides is available on the market. Because brand names differ around the world and because chemicals are continually added or removed from the list of approved, registered pesticides, we suggest that you consult your local orchid society, plant nursery, or agricultural agent for recommendations of specific products to control pests and diseases and also ways to improve your growing conditions to prevent future problems.

Safety in the application and storage of all chemicals cannot be overemphasized. Before opening any pesticide container, read and heed the label. One item on the label often disregarded or forgotten is the need for a follow up spraying a few to several days later to eradicate any juveniles that might have hatched.
 

Diseases

Bacterial and fungal diseases can be difficult to diagnose. Water left standing on a leaf or in the crown of a Phalaenopsis encourages the growth of bacteria and becomes a watery, brown to black lesion which spreads through the plant rapidly, even overnight. Fungal diseases include black rot, caused by Phytophtora and Pythium, and affect many different orchid genera. Leaf spot fungi such as Cercospora cause dark, sunken spots on orchid leaves. For all of these diseases it is important to remove all affected areas then treat with the appropriate product. Increase air circulation and make sure that stems and leaves are dry by dusk. Tilt potted plants so that water does not lodge in leaf crowns

Bacterial

Bacterial Brown Spot This disease is most prevalent in phalaenopsis, and is as deadly as black rot is with cattleyas. It is especially active under wet and coot conditions. Symptoms: Infection by Pseudomonas cattleyas, the bacterium involved, starts as a brown, watery blister on a leaf and quickly spreads, engulfing whole leaves. If it reaches the growing center (the crown), then the plant is essentially gone. What to do: Act immediately. Remove infected leaves with a sterile cutting edge. Spray immediately with Physan at the rate Of '12 tablespoon per gallon of water. Repeat in a few days.

 

Brown Rot This rot, caused by the bacterium Erwinia cypripedii, plagues paphiopedilums in particular. Symptoms: Infection starts as a small brown spot on a leaf, but spreads quickly, forming a chestnut-brown patch. Infected plants give off an unusual, almost spicy odour. If the infection reaches the base of a paphiopedilum, it can kill the entire plant. What to do: Remove infected leaves. Treat immediately as above for Pseudomonas, bacterial brown rot. Repeat in a few days.   Back to the top

Fungal

Black Rot These fungi - Pythium ultimum and Phytophthora cactorum can kill an orchid quickly, so keep an eye out for the following symptoms of infection. Symptoms: Black rot is an apt name for these pathogens. Infected areas, which can be any part of the plant, turn black and watery. The leading edge of the infection can appear fluid - much like a fast-rising black tide. Cattleyas are most often attacked. If the infection starts in or reaches the rhizome and the plant is not treated, in a matter of days it will be dead. What to do: If the infection is isolated in the pseudobulb or leaf, remove it. Spray or drench with an appropriate fungicide available in your state at label strength and recommended frequency. A change in cultural practices coupled with removal of infected tissue is the best solution. Watch the plant until you are sure the infection has stopped progressing.

Root Rot As the name implies, root rot, caused by the fungus Rhizoctonia typically attacks the roots. Orchids in decayed potting media, or those that are poorly aerated, are most susceptible.

Symptoms: Root rot is not usually as swift a killer as black rot. Infected roots eventually die, and after some time the rhizome is affected. Infected tissue appears more brown than black. The most noticeable symptom is a decline in the vitality of the infected plant, including shrivelled pseudobulbs and leaves, and smaller new growths.

What to do: Unpot the infected plant, remove all diseased tissue, repot in fresh potting medium and drench with a recommended fungicide, following the instructions on the label. Repeat the fungicide in a week.

Leaf-spotting Fungi Many fungi infect orchid leaves, but unlike the pathogens above, they are not usually lethal. Most growers, and their orchids, just live with them.

Symptoms: The fungi involved, Cercospora species for the most part, cause varying degrees of brown to black spotting on the leaves (and sometimes the pseudobulbs).

What to do: Several fungicides are available that can be sprayed onto orchids regularly. Making sure the plant has adequate ventilation, and is being grown under optimum conditions, helps reduce leaf-spotting. Reduce misting the leaves with water. If you can afford furnish with filtering system for the water.

Petal Blight Botrytis cinerea is a fungus that spots orchid flowers. To infect, it requires water on the flower surface, so it is a problem when humidity levels are so high as to lead to water condensation on the flowers. Water persisting on flowers after a rain or watering will encourage Botrytis Symptoms: Black dots appear on the flower and, in severe infections, can coalesce. What to do: Although a fungicide can be applied, once a flower is spotted, the damage is done. Increasing the air movement and decreasing the humidity will help keep the flowers dry, thereby minimizing the chances of infection. When watering, keep water off of the flowers. Remove infected flowers from the growing area to reduce sources of reinfection.

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Viruses

Several viruses are known for orchids in cultivation. Three viruses are prevalent among orchids: cymbidium mosaic virus (CyMV), tobacco mosaic virus (TMV) and bean yellow mosaic.

For detailed info on orchid viruses visit

Symptoms are non-specific and can easily be confused with symptoms of bacterial and fungal diseases or even cultural problems. But plants with yellow to black streaks on the leaves and pseudobulbs or colour break in the flowers are suspect and should be examined by a virus testing laboratory. Infected plants must be discarded. There is no known cure at present, but genetic engineering in progress holds promise for creating virus free collections.
The only way to control viruses is by prevention, isolation and disposal of infected plants and sterilizing tools between plants when dividing and repotting. Pots should also be sterilized before reuse. Eradicate aphids as soon as they are discovered.

Symptoms: Classic symptoms of CyMV include sunken spots and lines of necrotic (dead) tissue on leaves. Similar necrosis can also appear in the flowers. TMV causes similar symptoms, but can induce colour break (streaks of darker coloration) in flowers. Contaminated plants do not necessarily have obvious symptoms, though most plants exhibit a lack of vigour. What to do: You may not know whether any orchid you buy is contaminated with virus, but the seller should. Inquire whether the grower takes control measures for virus, and tests regularly. The only way to control viruses is by prevention, isolation and disposal of infected plants and sterilizing tools between plants when dividing and repotting. Pots should also be sterilized before reuse. Eradicate aphids as soon as they are discovered. Consider using one of the virus testing services available (see advertisements in Orchids) for your plant.

Most importantly, always use sterile cutting edges to avoid transmitting the virus from one plant to another. Single-edge razor blades are suitable for this purpose. If wiped clean and heated in an oven set at 300º F (132 º C) or higher for one hour, they may be reused.

Viruses have been known to be transmitted by dirty pots. Clean all used pots thoroughly with a brush and then soak in a solution of household bleach (1 cup in a gallon of water) for 30 minutes to one hour. Allow to dry before reusing. Soak disinfected clay pots in clean water (from known source), to wash of the bleach. Clay pots absorb and retain the solution. Plastic pots can be used after disinfection  without rinsing.

Viruses could also be transmitted by dirty hands. Some growers use disposable plastic gloves when repotting more than one plant. They change gloves with each batch plants they repot. Remember, for many good reasons, including virus control, do not recycle used orchid-potting medium.

An additional precaution against transmitting viruses is to place a stack of unfolded newspapers on the potting surface, and remove one sheet each time you pot or repot a specimen.

All these practices are important because once a plant is infected with virus, it will stay that way - there is no practical cure.

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Insects

Orchids in cultivation have their share of pests. Here are the most prevalent.

Scale Quite a few species of scale infest orchids. Scale insects secrete a hard, waxy covering that repels most pesticides unless a wetting agent is added. Their presence is often first noticed by the sticky "honeydew" that they secrete. Large infestations can be very difficult to eradicate even with highly toxic pesticides. Symptoms: Scale usually appears as a circular raised mound attached to any part of the plant. Left to its devices, it can proliferate and take over a plant, killing tissue. Cattleyas are the most likely to be infested with scale.

What to do: Beginners have the advantage of owning just a few plants. A cotton swab dipped in alcohol and applied to the scale can control minor infestations. Pesticides such a Malathion, offer stronger control. Be careful to follow directions and minimize your, as well as others, exposure.

Mealybugs These insects are a particular problem with paphiopedilums and other leafy orchids. Symptoms: The white, sticky infestations typically start as a patch under the leaves or in some other less than obvious location. Under warm conditions, they spread quickly. An infestation not controlled early can persist for years, weakening and even killing some plants. What to do: The trick with this and all orchid pests is constant surveillance, so that you catch any infestation before the pest can multiply. An isolated mealybug can be killed with alcohol. An entrenched infestation requires repeated spraying with pesticides found at your garden centre, such as Malathion. Most  insecticides do not affect the eggs, so spray with two weeks interval to ensure  eradication of newly hatched insects. More frequent application of insecticide (as well as fungicide) is unnecessary and only causes pollution and potential risk for your health.

Caution: Pesticides are potentially dangerous. Check the label for whether the chemical is recommended for orchids or ornamentals. Do not use it if it is not. Follow directions carefully. Wear a mask and gloves, and spray outside away from people and pets. Wash thoroughly after spraying, and store the pesticide in a cool, dry place out of the reach of children.

Spider mites (mites) multiply quickly in dry conditions, leaving pits on the lower surfaces of leaves and giving the leaves a silvery appearance. These pests are hard to see, but the damage they do is not.

Symptoms: Mites suck dry the leaves they attack, so that infested leaves look silvery and dry.

What to do: Mites often benefit from dry and warm air conditions. Increase the humidity and air circulation and apply contact insecticide. At least two sprayings several days apart are necessary to kill all adults and juveniles. Raising humidity in the growing area will help prevent outbreaks.
Chrysanthemum extract (available in nearly ever garden center) – Pyrethrum or Pyretin is a good choice it is non toxic for mammals it has very short lasting effect, but is powerful. Repeated spraying with it or an insecticidal soap also offers some control. Commercial miticides offer more.

Aphids are small green to yellow insects which mass on flower buds or new leaves - are  easily controlled, even with soapy water.They have piercing mouthparts that penetrate plant surfaces, which allows the transmission of some orchid viruses (or fungi), so it is important that they be eradicated as soon as they are spotted. What to do: Chrysanthemum extract (available in nearly ever garden center) – Pyrethrum or Pyretin is a good choice it is non toxic for mammals it has very short lasting effect, but is powerful. Repeated spraying with it or an insecticidal soap also offers some control.

White fly - very easy to recognise.

What to do: Use sticky traps or contact insecticedes. Repeate spraying with two-three days interval five to six times.



 

Slugs and Snails feed on leaves, roots, and flowers in the evening. They are more a problem outdoors and in greenhouses than indoors but still these slimy molluscs have a cruel preference for orchid buds and flowers. A number of baits are available, though of limited effectiveness they have proven to be the most effective control. What to do: Clean up the growing area to remove hiding places. Baits in pellet or meal form are scattered around the plants. Liquid forms can be diluted and poured on the medium. Some growers spread diatomaceous earth for organic control. Warning: Baits are toxic to pets. Hobbyists over the years have placed shallow bowls of beer in the growing area with some success. Make sure you change the beer every other day  (you need just a little) because it starts to stink terribly. Beer and pellets attract the slugs away from the flowers, still present they prefer beer much better.   Back to the top

 

Other pests

Some times unsuspected creatures cause the mechanical damage on the leaves and flowers. In usually crowded collector's greenhouses the collector himself unintentionally breaks leaves and flowers while watering trying to squeeze trough tide spaces or to reach shelves and corners. Avoid wearing heavy cloths and long sleeves while in the greenhouse. Favourite pets (like cats and dogs) and kids like to play with the flowers sometimes causing damage that looks like nothing else. Overfeeding cause burning that resembles black rot or basal rot. Water on the flowers looks like mite damage. When observing the plants try to observe your habits and routines. When the plants (hanging baskets) are on the way try to move them instead trying to avoid them gently. Be careful when watering to move slow. It happens frequently to knock down a pot and then to step on it. This usually happens to the most favourite specimens.
Some times unsuspected creatures cause the mechanical damage on the leaves and flowers. In usually crowded collector's greenhouses the collector himself unintentionally.

There is no manual that can cover all pests and diseases you can encounter but the key for success fighting pests is to observe.