|Disease and Pest Problems|
One of the major problems associated with African violet care is excessive watering, which can lead to root or crown rot. Root and crown rot caused by the fungus Pythium ultimum may occur on overwatered plants. All ages of plants may be affected. 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 rotted portion of the crown well above the line of decay and reroot the plant in sterile medium.
Petiole rot begins as an orange-brown or rust-colored lesion where the petiole touches the rim of the 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 on the rim of the pot and flushing the soil occasionally with heavy watering will prevent this problem.
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 shining on wet leaves. This condition is known as ring spot or chlorosis, and can be avoided by carefully watering the plants from below with water slightly warmer than air temperature.
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.
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. Flowers may be similarly affected. 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 the disease. The application of a foliar fungicide may be needed to control severe outbreaks.
Powdery mildew is a light grey, powdery substance on the stems and petioles. This shortens the life of blooms and makes the violet unthrifty. Mildew grows in cool, moist, stagnant air. The best cure is air circulation--a small fan moving the air in the room will keep it from developing. Karathane is a good control, as is dusting sulfur when a very small amount is blown lightly over the plants.
Insects and mites occasionally damage the foliage and flowers of African violets. Cyclamen mites feed in the plant crown, causing young leaves and new growth to be stunted, twisted, discolored, and brittle. When infestations are heavy, leaf hairs become matted and flower buds fail to open. 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 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.
Mealybugs are small, soft-bodied insects that occur in white, cotton clusters on the surfaces of leaves, on leaf petioles, and near the bases of leaf stems. They injure plants by 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. Mealybugs can often be eliminated by spraying plants with a jet of lukewarm water or by removing them with a cotton swab dipped in alcohol. When infestations 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. Disulfoton (DiSyston), a systemic granular insecticide, will also provide satisfactory control if scratched into the soil and watered in. Hanging a Vapona pest strip in the vicinity of plants may help prevent reinfestation. Ground or root mealybugs are tiny, whitish insects that feed on African violet roots, causing poor plant growth and wilting of leaves between waterings. To control these pests, avoid overwatering and drench soils with an insecticide such as malathion or acephate (Orthene).
Stunted plants with curled or distorted leaves may be an indication of feeding by aphids. Both adults and nymphs are similar in appearance with soft, pear-shaped bodies, long legs, and antennae. Like mealybugs, aphids excrete honeydew which gives leaf surfaces a shiny appearance and supports the formation of sooty mold. Aphids can be washed from infested African violets with a spray of lukewarm water, or 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 are small, slender insects characterized by long, hair-fringed wings. They damage African violets by feeding on the leaves and flowers. 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. Thrips can be controlled 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.
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 and look for signs of plant injury after 2 to 7 days before treating remaining plants.