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COMMON PESTS AND DISEASES ON ORCHIDS IN THE LOW DESERT

 

A. DIAGNOSIS AND TREATMENT

 

The following discussion is an adaptation of material which the Orchid Society of Arizona, Inc., includes in all New Member Packets. For a more thorough treatment of the subjects, we recommend orchid growers consult ORCHID PESTS AND DISEASES, 1995 edition, published by the American Orchid Society.

 

SCALE - This is probably the most frequently encountered, frustrating-to-control pest found on orchids in the low desert. Our low humidity might be a significant contributing factor to its prevalence. These small pests attach to stems, leaves, pseudobulbs, and rhizomes. They can hide under the dried sheaths, which often makes early detection difficult. Of the armored scales, Boisduval scale, is the most common. Armored females can deposit from 30-150 eggs under the armor which is round and about one millimeter in diameter. The eggs hatch in one to two weeks or longer, depending on the temperature. The males usually occur in clusters that look like white lint. Boisduval is difficult to control, and it is necessary to examine each plant at least once a week to detect a reoccurrence of a problem. 1 tablespoon of Malathion to 1 gallon of water is somewhat effective IF the infestation sites are scrubbed with the solution, the entire plant is dipped in the solution, AND thereafter the plant is sprayed once a month. Rubbing alcohol sprayed directly onto Boisduval males will kill them on contact, but does nothing to the armored Boisduval females. As the eggs hatch underneath the armor, the scale pierces the host and causes chlorotic (yellowing) areas by extracting plant fluids. If just one armored female is undetected, the battle to control scale will have been only temporarily "won". In addition, if ants are present in your growing environment, the acts can "carry" newly hatched scales from plant to plant. (TKO,liquid microencapsulated diazinon - time release - can be used to control ants. Use 1 tablespoon to one gallon of water.) Some growers still use Cygon to control scale, but IT IS A HIGHLY POTENT PRODUCT ASSOCIATED WITH HEALTH RISKS TO THE USER. Also, it can cause distortion in blooms.

 

On cattleya-type orchids, if scale reaches the "eyes" (buds) at the base of a pseudobulb, it can kill the buds from which new growth emerges.

 

Soft scale insects do not have a protective armor like the Boisduval, but they feed on the plant in the same manner.

 

Note: Scale does not appear to be as much of a problem on plants that receive regular misting of the leaves.

 

MEALYBUGS - These are soft-bodied pests which look like cotton. They excrete honeydew in large amounts, and this attracts ants. Adult females are usually oval in shape and from 1/16 to 3/8 inches long. They have well-developed legs, and most mealybugs can move about. Rubbing alcohol in a spray bottle will kill the mealybugs on contact. They favor new growth, but large colonies have been found attached to roots that are near the bottom of the potting medium.

 

SNAILS AND SLUGS - These pests feed at night. They leave a silvery trail of slime. Ortho Snail Bait is effective. Place it around pots, but NOT in the pots. This is a metaldehyde product. Some baits contain apple, which molds quickly. To make doubly sure that a favored bloom will not become lunch for a snail or slug, place cotton wrapped around the stem at the base of the flower.

 

APHIDS - These pests live and feed in colonies on young growth and on buds. They stunt plant growth and cause buds to fail to open. They may be treated with a spray of 1 tablespoon of Malathion to 1 gallon of water, or sprayed with rubbing alcohol.

 

SPIDER MITES AND FALSE SPIDER MITES - These pests are more prevalent in the summer months. They are close relatives of ticks, spiders, and scorpions. Some species spin a fine web similar to those of spiders. FALSE spider mites can only be confirmed by examination of a damaged leaf under a microscope. However, the presence of mites can be determined by rubbing a white cloth over a suspect leaf. If mites or eggs are present, brownish streaks will be seen on the cloth. Also, there is a characteristic silver-like appearance to a leaf infested with spider mites. Spray with "Kelthane Spray". This is actually dicofol, the substitute the Environmental Protection Agency approved when it withdrew Kelthane from the market.

 

WHITE FLIES - No OSA orchid growers have reported damage to orchid blooms in 1998. In previous years, when cotton farmers began to withhold water from their cotton fields, white flies invaded surburbian vegetation. They resembled clouds of near-microscopic snowflakes. The color yellow is a particular "favorite" of white flies. Growers successfully controlled the problem by placing white fly "traps" close to any yellow orchid blooms. The white flies became stuck in the sticky substance that was used in the manufacture of the "traps".

 

BACTERIAL ROT - This problem presents as soft, dark brown/black areas on leaves, and is frequently circular. On pseudobulbs, the blackened area can extend to the rhizome. Cut off infected parts and seal with copper bordeaux powder made into a paste with water or sprinkle the fresh cut with ground cinnamon. Also, a paste made of Captan powder and premixed Physan (1 1/2 teaspoons to 1 gallon water) can be "painted" on a small, suspicious spot. Fungus and bacteria thrive in high humidity and still air. Therefore it is important to maintain good air movement around plants.

 

A word of caution: Never spray a dry plant. Many pesticide labels clearly state that plants should be sprayed the day after the plants have been watered.

 

THE ORCHID SOCIETY OF ARIZONA, INC. DOES NOT RECOMMEND USING ANY CHEMICAL ON ORCHIDS WHICH HAS NOT SPECIFICALLY BEEN CLEARED FOR USE ON ORCHIDS. ALSO, NEVER USE A STRONGER DILUTION THAN IS LISTED ON THE PRODUCT LABEL.

 

Note: Pests tend to build up resistance to chemicals, and therefore, it is necessary to alternate products used for effective control.

 

 

B. PEST/DISEASES CONTROL OVERVIEW - BASICS

 

The growing challenge for orchid growers is the control of pests and diseases without using previously reliable and approved potions. What is today termed non-toxic may well be considered poisonous, tomorrow. In 1991, Orthoganic Insecticididal Soap was recalled because it was contaminated with oxyfluorfen, an herbicide. For a frightening, in-depth discussion of the devastation caused by Benlate DF (dry flowable), see the article: "Phantom of the Greenhouse", pp. 1190-1195 in the December, 1991 AOS BULLETIN (now known as ORCHIDS Magazine). This article also mentions that Benlate WP (wettable powder) is no longer approved for use on orchids and that Tersan 1991 DF, a Benlate-related product was recalled in March, 1991.

 

These revelations have forced us to reevaluate our cultural practices - or risk losing a portion or all of our orchid collections. Think in terms of:

 

1)Bug Glasnost.It is unrealistic to expect to eradicate every bug. There is truth in the phrase, "they breed like flies". Insects multiply rapidly. A "terminated" adult population often leaves behind eggs or larvae. Control may mean learning to live with a few teenage bugs.

 

2) More Vigilant Cultural Practices. Thoroughly examine each plant at least once a week. This includes rapidly removing plants or parts of them that show infection, using sterilized tools, using a magnifying glass to detect small but not microscopic-size pests, gently passing cotton balls over leaves to detect the presence of false spider mites, and investigating pitted leaves with a microscope.

A roll of paper towels near your growing area is a must. After watering, especially in the case of phalaenopsis plants, each leaf should be dried and all water removed from the crown of the plant. (Bacteria thrives on wet leaf surfaces and easily develops into Pseudomonas, a bacterial soft rot.)

3) Home Remedies. You don't need to own a copy of COMMON SENSE PEST CONTROL (an excellent "encyclopedia" with pest control solutions for home, garden, and pets), but it would be helpful to have a veteran orchid grower handy.

 

OSA's orchid mentor, Deacon Bell, who has been growing orchids for 50 years, successfully uses a NON-TOXIC SPRAY FOR RED SPIDERS, concocted from the following ingredients:

2 tablespoons cold water (liquid)Surf detergent

1 tablespoon methyl alcohol or rubbing alcohol

1 tablespoon cooking oil

Mix thoroughly the above ingredients in a gallon of water and spray 1-3 times at 10 day intervals. If a precipitate forms, strain the solution through fine cheese cloth before putting it into a spray bottle.

FREE consultations for nontoxic pest control are available from ARBICO, P.O. Box 4247, Tucson, Az., 85738. ARBICO also has a web site on the internet: http://www.usit.net/BICONET

BIBLIOGRAPHY

Olkowski, William, Daar, Sheila, and Olkowski, Helga. COMMON-SENSE PEST CONTROL. Newtown, Ct., The Taunton Press, 1991.

 

"Tolerance and 'Soft' Techniques...Ways to Control Greenhouse Pests", SUNSET MAGAZINE, November 1990, pages 154-155.

 

 

 

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