CAES: Orchid (Orchidiceae)

Orchid (Orchidiceae)
Orchid (Orchidiceae)

Plant Health Problems

Diseases caused by Fungi:

Anthracnose, Gloeosporium spp. and Colletotrichum spp.
Spots may occur on leaves or tubers and vary from yellowish to light brown, are more or less circular, soft and sunken. Spore pustules are pinkish or reddish-orange and often arranged in concentric rings. Leaves which have been stressed or injured by cold, sun, or poor root systems are most susceptible.

In most cases, these diseases are considered more aesthetic than life-threatening. Efforts for reducing the spread of disease include avoiding overhead sprinkling, and reducing humidity by improving ventilation. Removal of infected leaves is also very helpful since it reduces the amount of the fungus available to infect new tissues. Control can also be achieved with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are thiophanate methyl and mancozeb. Consult the label for dosage rates, safety precautions, species sensitivities, and indoor use.

Root rot, Rhizoctonia spp.
Infected plants yellow and wilt and may show a slow, progressive decline. This disease can destroy susceptible orchids at any plant age. A brown mycelium can sometimes be found on tubers and basal portion of leaves.

This cosmopolitan soilborne fungus can be managed by careful attention to sanitation. It is important to use clean pots and sterile potting media. Since recovery is unlikely when the entire root system is infected, roguing and removing symptomatic plants is often prudent.

Petal blight, Botrytis cinerea.
Small, circular spots appear anywhere on petals and sepals. These rapidly enlarge and rot the entire flower. A gray fuzzy growth is often visible on the infected tissue if high humidity and low temperatures persist.

This disease can be minimized by removing diseased or spent flowers as soon as they appear. Watering early in the day and avoiding wetting of flowers or leaves can be effective. Maintaining fairly high temperatures and improving air circulation are also helpful.

Diseases caused by Bacteria:

Bacterial soft rot or brown rot, Erwinia spp.
Circular or oval greasy brown spots appear on leaves which rapidly become a deep chestnut. Lesions are sunken with well-defined margins. If the temperature is 65 F or above and the humidity is high, the disease progresses rapidly to the growing point, and plants droop.

Sanitation is very important to minimize the spread of this highly contagious bacterial disease to healthy plants. It is often necessary to disinfest all equipment with 10% bleach or 70% alcohol solutions. Maintaining good air circulation and avoiding wetting the leaves during watering are also helpful. Heavily infected tissues can be excised. Control can also be achieved with the use of bactericide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are copper sulfate pentahydrate and benzalkonium chloride. Consult the label for dosage rates, safety precautions, species sensitivities, and indoor use.

Brown spot, Acidovorax cattleya.
This bacterial spot is very serious on Phalaenopsis, attacking seedlings in community pots or mature plants. It starts as a green water-soaked spot which becomes brown and eventually black. Seedlings may be decimated by the disease, and mature plants can be killed when the bacteria reach the growing point. On Cattleya the discoloration is more limited, appearing usually on older leaves, and lesions are black and sunken. Overhead irrigation serves to spread this highly contagious disease.

It is helpful to isolate infected plants as soon as symptoms are recognized. Since watering practices help to spread the disease, it is prudent to avoid overhead irrigation and splashing water, and to water early in the day to encourage rapid drying of leaf surfaces. Control can also be achieved with the use of bactericide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are copper sulfate pentahydrate and benzalkonium chloride. Consult the label for dosage rates, safety precautions, species sensitivities, and indoor use.

Diseases caused by Viruses:

Mosaic, Ringspot, Necrotic pitting, viruses.
There are many virus diseases of orchids and the symptoms vary considerably with the type of virus, genus, and environmental conditions. Symptoms can include colorbreaking in flowers, mottling of leaves with yellow and green blotches or streaks, and necrotic pitting of leaves. Other viruses cause ring-like concentric patterns in leaves, either yellow or brown, and necrosis may follow.

Once plants are infected, they cannot be cured so it is often prudent to destroy them in order to reduce the risk of spread to healthy plants. Since the predominant means for the spread of virus diseases is mechanical (spread during dividing of plants) careful attention to sanitation and handling is very important.

Diseases caused by Nematodes:

Foliar nematodes, Aphelenchoides spp.
The most diagnostic symptom appears on unopened flower buds. Infected buds turn yellow, may be deformed, and drop. Brown or blackish spots delimited by veins can also appear on leaves and result in premature leaf drop.

Since nematodes are spread by splashing water, careful watering is essential. Improved ventilation permits water to evaporate from plant parts quickly, and nematodes cannot spread easily. Removal of symptomatic plant parts is also helpful.

Insect Problems

Aphids, Macrosiphum luteum and Neomyzus circumflexus.
The orchid aphid, Macrosiphum luteum, and the crescent-marked lily aphid, Neomyzus circumflexus, occur on orchids. Among the compounds registered for control of this pest in Connecticut are insecticidal soap and imidacloprid. Apply the former as a spray if needed. Use lower rates on sensitive plants. Imidacloprid is used as a systemic to be taken up by the roots. Consult the label for dosage rates and safety precautions.

Orchid weevil, Diorymerellus laevimargo.
This 1/8" long, shiny-black weevil feeds on roots, tender leaves, flower bud sheaths and bulbs. It will also feed on flower petals prior to opening, allowing secondary pathogens to enter and destroy the blooms. The white, legless grub is even smaller and hollows out new roots. This insect is only a problem where large numbers of orchids are grown.

Orchid bulb borer, Metamasius graphipterus.
The black and yellow, 1/2" long beetles feed on leaves. The larvae feed inside the bulbs, resulting in secondary infection of bulb rots. Control is seldom needed.

Orchid fly, Eurytoma orchidearum.
This black, 1/8" long wasp lays eggs at the base of the pseudobulb, or sometimes on new foliage and rhizomes. Eggs hatch within ten to fourteen days and the larvae feed within the pseudobulbs. Flowering is reduced as a result. Destroying infected plants should control the problem.

Orchid mealybug, Pseudococcus microcirculans.
This bug is found on plants grown in the southern United States and sold here in Connecticut. It can be controlled with imidacloprid applied as a drench to the roots. Examine plants closely before purchasing.

Cattleya midge, Parrallelodiplosis cattleyae.
The 1/8" yellow maggots found in the tips of young roots cause galls to form. For control, cut off and destroy the galls.

Cattleya weevil, Cholus cattleyae.
Just under 1/2" in length, with white marks on their back, these weevils feed on the surface of the pseudobulbs and puncture foliage. Larvae feed on the leaves and develop on stems and pseudobulbs. Again, as a result of this feeding, the bulbs succumb to rot and fail to bloom. Control is rarely needed. Among the compounds registered for control of this pest in Connecticut are permethrin for indoor use against adult weevils and methoxychlor if the plants can be treated outdoors. Consult the label for dosage rates and safety precautions. Use with care on sensitive plants.

Dendrobium borer, Xyleborus morigerus.
Tiny brown beetles bore into the pseudobulb and lay eggs. The larval feeding makes long tunnels. Cut out and destroy infested bulbs.

Orchid plant bug, Tenthecoris bicolor.
This orange to red true bug with black designs on its back feeds on the undersides of leaves resulting in irregular white spots. Control is seldom needed.

Scales, Saissetia oleae, S. coffeae, Lepidosaphes ulmi and Fiorinia theae.
Black, hemispherical, oystershell and tea scales are among those that attack orchids. When needed, apply malathion, which is registered for use against this pest in Connecticut, when crawlers are present. For the honeydew producing Saissetis species, imidacloprid, drenched around the roots, is effective. Consult the labels for dosage rates and safety precautions.

Thrips, Heliothrips hemorrhoidalis.
The greenhouse thrips and others injure leaves and buds with their rasping-sucking mouthparts. Symptoms include a browning or blotching of the leaves and bud destruction. If needed, malation or spinosad, which are among the pesticides registered for use against this pest in Connecticut, can be sprayed. Consult the label for dosage rates and safety precautions.

Mite blotch.
There is an unidentified mite that causes brown sunken areas in the leaves of Coelogyne spp. Control is seldom necessary.

Slugs, Limax maximus.
These snails without a shell eat buds, blossoms and tender plant tissue. In a small collection they can be handpicked or use a beer bait trap.

 



Content Last Modified on 4/10/2007 2:38:53 PM