PEST AND DISEASE

A few African Violet pests and disease problems.

Some pesticides damage plants so be certain the product you select is recommended for use on African violets. In many cases, the label provides a list of plants known to be sensitive to the pesticide, as well as those plants for which it is recommended. When possible, test-treat a few plants or a few leaves and look for signs of plant injury after 2 to 7 days before treating remaining plants.

Three of the most common ailments violets may get are (1) Blossom Thrips, (2) Mealy bugs, (3) Powdery Mildew. Below is listed the full set of aliments.

Root or Crown Rot: This is from a fungus Pythium ultimum, caused by over-watering. The crown and roots of these infected plants turn dark and soft, and the leaves usually wilt. Infected plants can be lifted easily from the soil. This disease is not a problem when plants are grown in a pasteurized soil mixture in a container that allows for proper drainage. Destroy badly diseased plants, and thoroughly clean and disinfect their containers before reusing them. Highly prized plants can sometimes be saved if the crown is not badly decayed. Remove the rotten portion of the crown well above the line of decay and reroot the plant in sterile medium.

Petiole Rot: This normally begins as an orange-brown or rust-colored lesion where the petiole touches the rim of pot or where it contacts the soil. This is aggravated by the accumulation of fertilizer salts on the rim of the porous pot or on the soil surface. An aluminum foil collar or ring on the rim of the pot and flushing the soil occasionally with heavy watering will prevent this problem.

Ring Spot or Chlorosis: The unsightly yellow or white ring and line patterns that occasionally appear on the leaves are thought to be caused by a sudden chilling of the leaves by cold water or by the sun shinning on wet leaves. This condition can be avoided by carefully watering the plants from below with water slightly warmer than air temperature.

Powdery mildew: This appears as a light gray, powdery substance on the stems and petioles. This shortens the life of blooms and makes the violet unthrifty. Mildew grows conditions where the violet is in a cool, moist area with still, stagnant air. The best cure is air circulation - a small fan moving the air in the room will keep it from developing.

Control through use of system fungicide:

1. Karathane is a good control, as is dusting sulfur when a very small amount is blown lightly over the plants.
   
   
2. Physan: Spray once or twice with a system fungicide (1/4 teaspoon. per pint of warm water), or mix 5 drops Physan 20 in 1 cup warm water and spray the entire plant both the affected and the unaffected areas. Phyton 27 is a new fungicide. It leaves no residue.

Root-knot Nematodes: African violets infested with root-knot nematodes are stunted and weakened. Galls form on the roots, and the crown and leaves become thickened and distorted. Blisterlike galls also develop on the leaves. Destroy all infested plants, sterilize their containers, and propagate new plants only from nematode-free plants.

Botrytis blight: The leaves of plants grown under greenhouse conditions may become infected by the fungus Botrytis cinerea. Botrytis blight first appears as small water-soaked lesions on the underside of the leaf. These lesions enlarge until much of the leaf turns brown to black. Affected plant parts and flowers become covered with gray mold. High humidity, poor air circulation, and low light intensity contribute to the development of this disease. Spores of the pathogen are spread by direct contact of leaves. Spacing the plants so that the leaves of adjacent plants will not touch reduces spreading to healthy plants.

Since infection by Botrytis often follows mite injury, controlling these pests aids in controlling disease. The application of a foliar fungicide may be needed to control severe outbreaks.

Cyclamen mites: Insects and mites occasionally damage the foliage and flowers of African violets. Cyclamen mites are extremely small insects, not visible without magnification. Cyclamen mites feed in the plant crown, on the growing tip of the plant, causing young leaves and new leaf growth to be stunted, twisted, contorted, curled, discolored, brittle, and look hairy. Corky layers develop over injury causing puckering, twisting and distortion of leaves in the center of the plant.

When infestations are heavy, leaf hairs become matted and flower buds fail to open.

Control:

1. Cyclamen mites can be controlled by treating infested plants at weekly intervals for three weeks with dienochlor (Pentac 50WP) at a rate of 1 teaspoon per gallon of water, or
   
   
2. with 2 or 3 applications of dicofol (Kelthane) as directed on the label. Adding a few drops of liquid detergent to the spray will increase coverage and provide better control.

Isolate infested plants and be careful while handling them so you do not accidentally transfer mites from one plant to another.

Red Spiders: These pests injure the plant by puncturing and sucking sap. In large numbers, they develop a web over leaves, flowers, etc. Plants appear stunted. Insects are small, reddish in color and visible to the naked eye. If you shake a leaf on a white piece of paper, they can be observed crawling.

Mealybugs: These are small, soft-bodied insects about ¼ inch in length that occur in white, cotton clusters on the surfaces of leaves, on leaf petioles, and near the bases of leaf stems and don't move. They injure plants by piercing and sucking plant juices, which causes stunting and distortion of the leaves. Mealybugs also excrete a shiny, sticky substance called honeydew that is highly attractive to ants and also supports unsightly growths of a dark sooty mold. Heavy mealybug infestations may cause leaves and plants to wither and die.

Control:

1. When infestation are heavy or when many plants are involved, the best approach is to make 2 to 3 weekly applications of 57% Malathion at a rate of 2 teaspoons per gallon of water or 1/4 teaspoon per quart of warm water.
   
   
2. Safer Insecticidal Soap (4 1/4 teaspoon per pint of warm water).
   
   
3. Disulfoton (DiSyston), Marathon (Imidacloprid liquid or granular) a systemic granular insecticide, will also provide satisfactory control if scratched into the soil and watered in. Note that Soil Mealy Bugs may climb upwards when pots are drenched. It is advisable to spray overhead also or to drench from the top down. Granular Marathon can be applied to the root zone. It must make contact with the roots to become systemic.
   
   
4. Mealybugs can often be eliminated by spraying plants with a jet of lukewarm water or by removing them when a cotton swab dipped in alcohol, although I find this almost an useless method.
   
   
5. Hanging a Vapona pest strip in the vicinity of plants may help prevent reinfestation.

Aphids: Stunted plants with curled or distorted leaves may be an indication of feeding by aphids. Both adult and nymphs are similar in appearance with soft, pear-shaped bodies, long legs, and antennae. Also called plant lice, they are usually green or black. Like mealy bugs, aphids excrete honeydew which gives leave surfaces a shiny appearance and supports the formation of sooty mold.

Control:

1. These may require treatment with Malathion Carefully read the label and follow directions for applying pesticides or
   
   
2. Aphids can be washed from infested African violets with a spray of lukewarm water, or
   
   
3. treated with an insecticide such as 57% Malathion at a rate of 2 teaspoons per gallon of water plus a few drops of liquid detergent. Repeat this treatment after 7 to 10 days. When only a few plants are involved, consider using a premixed, commercial house plant spray that lists both aphids and African violets on the label.

Thrips: These are tiny, slender insects characterized by long, hair-fringed wings. They are fast moving insects and damage African violets by feeding on the leaves and blossom petals. You will NEVER get rid of all of your Thrips if you do not remove ALL(100%) of your blooms.

Typical leaf injury appears as irregular or streaked silvered areas dotted with small, black drops of excrement. Flower feeding causes distorted blooms, discoloration or streaking of petals, and shorter flower life.

Control: Thrips can be controlled following one of the following:

1. Spray twice a week for several weeks directly into the blooms and on the foliage with either Safer Insecticidal Soap (4 1/4 teaspoons per pint of warm water ) or with Neem Oil (2 1/2 teaspoons per pint of warm water).
   
   
2. Spray twice a week for several weeks directly into With 1 or 2 applications of premixed, commercial house plant spray containing Malathion or Orthene. Be sure that the product is specifically labeled for both thrips and African violets.
   
   When only a few plants are involved, consider using a premixed, commercial house plant spray that lists both aphids and African violets on the label.

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