MAJOR GREENHOUSE PESTS AND DISEASES

Correct identification of the pest or disease cannot be understated. How can a pest be controlled if it is unknown? Unless the pest is known, proper treatment cannot be administered. For instance, a doctor would not prescribe cold medicine to treat athlete's feet. Following are a number of the more common pests and diseases found in the greenhouse.

Pests

Aphids—Aphids are pear-shaped, soft-bodied, usually wingless insects. They are often green or yellowish in color. Aphids have the ability to reproduce very rapidly. Astonishingly, they give birth to live young that are pregnant! Aphids use their mouthparts to pierce the plant and suck out juices. Aphids attack a wide variety of greenhouse plants.

Fungus gnats—Fungus gnats are

long-legged, winged, gray-black insects less than an eighth of an inch long. The larvae of fungus gnats feed on root hairs and tunnel into plant stems. They prefer growing media that is constantly moist.

Leaf miners—Leaf miners are small stocky flies. The adult deposits eggs inside a leaf. The eggs hatch and the larva feeds on the interior of the leaf, making tunnels as it moves along. Chrysanthemums are subject to leaf miner damage.

Mealybugs—Mealybugs are slow-moving, oval-shaped, whitish insects. They have a waxy finish and produce small cottony masses. Mealybugs pierce plant leaves and suck the plant juices. As with aphids, they give birth to living nymph.

Mites—Mites are not insects. They have eight legs and are related to spiders. Mites pierce plant leaf tissues and suck juice. Symptoms include a yellow speckled appearance to the leaf, and in severe cases, yellowing of the leaves and defoliation. TWo-spotted or red spider mites are among the most serious greenhouse pests. Two-spotted mites have two dark spots on their back and appear as tiny specks. Cyclamen mites infest a broad range of plants and are not visible to the naked eye. Bulb mites damage lily bulbs and the developing shoots. Spider mites create a complex network of webbing as they move about a plant.

Scale—Many types of scale insects infest greenhouse plants. 'Typically, they have flat, oval, often brown bodies. They may or may not be covered with an armored shell. Scale insects pierce plant leaves and stems and suck juices.

Western flower thrips—Western flower thrips are small insects with two pairs of fringed wings. They are dark brown in color.

Whiteffies—Whiteflies are small insects that are not surprisingly, white. They generally camp out on the undersides of leaves where they pierce the tissues and suck juices. Their flat, scale-like larvae feed on the undersides of the leaves. The whitefly is a major pest of poinsettias and fuchsias.

Worms or caterpillars—Caterpillars are the larva of various moth species. They damage greenhouse crops by eating the plants.

Diseases

Botrytis blight—Botrytis blight is a fungal disease that can attack nearly all greenhouse crops, and is a common problem with cut flower storage. It causes a brown rotting and develops fuzzy, gray mold as it produces spores. Botryt,is is most common when temperatures are between 60 and 70°F, air circulation is poor, and humidity is high.

Erwinia—Erwinia is a bacterial disease that causes rotting of plant tissues. The bacteria enter the plant through wounds. It is a common problem on Dieffenbachia, Philodendron, chrysanthemum, and cyclamen.

Powdery mildew—Powdery mildew is a fungal disease. Characteristic symptoms include a white dusty coating to leaves and flowers. Infected plants can become stunted. High humidity contributes to powdery mildew problems.

Pythium—Pythium is a fungal disease that attacks greenhouse plants under cool, wet conditions. It is one fungus that can cause damping- off. Damping-off is a term used to describe the early death of seedlings that have germinated. More mature plants can develop root and stem rots as a result of Pythium, particularly when growing medium has poor aeration.

Phytophthora—Phytophthora is a fungal disease similar to Pythium. It causes crown and stem rots in cool, wet conditions.

Rhizoctonia—Rhizoctonia is a fungal disease prevalent under wet and warm conditions. It is a common disease problem in the southeastern region of the country. Rhizoctonia causes damping-off, as well as, root and stem rots.

Thielaviopsis—Thielaviopsis is a fungus that causes root and stem rots. Problems caused by Thielaviopsis are favored by cool, moist conditions.

Virus—Tobacco mosaic and aster yellows are two common viral diseases associated with greenhouse crops. Symptoms of infection include discoloration of plant tissues, stunting of growth, and deformed growth. The spread from one plant to another is primarily by feeding greenhouse insects. Tobacco mosaic virus can be spread to the plants from the hands of workers who smoke.

PEST CONTROL STRATEGIES

For successful management of pests, the IPM program must be a year- round program. Also, IPM control measures for a specific crop, poinsettias for example, should begin before the plants enter the greenhouse. The strength of IPM is the combination of control measures used. Four broad areas of control include sanitation, cultural/physical control, biological control, and chemical control.

Greenhouse Sanitation

Many pest problems can be greatly reduced, if not eliminated, with greenhouse sanitation. Greenhouse sanitation is simply the efforts made to keep a greenhouse clean. Many modern greenhouse ranges have been built with concrete floors partly because they are easier to keep clean than gravel floors.

One aspect of greenhouse sanitation involves the removal of weeds from the interior of the greenhouse and the immediate area outside the greenhouse. The importance of weed control cannot be understated. Weeds harbor pests. Control measures applied to a crop fail to control pests that find safety on the weeds. After awhile, the pests migrate to the crop to cause damage. Only a few herbicides are labeled for use in the greenhouse.

Another important sanitation practice is the removal of plant debris and other debris from the floors and benches. Debris often houses disease organisms and pests. Severely infested or infected plants should also be removed and disposed of properly.

flower guide