Common diseases and disorders of African Violet

A.R. Chase is a Plant Pathologist and President of Chase Research Gardens, Inc.
C.R.G. is a private research and consulting corporation specializing in ornamental plants.
Both hourly and contract services are available.
You can reach C.R.G. at P.O. Box 168, Mt. Aukum, CA 95656 Phone and Fax: 916-620-1624 - E-mail: MTAUKUM@AOL.COM

Tomato Spotted Wilt
Pathogen - Tomato Spotted Wilt Virus
Symptoms - TSWV can infect many flowering and foliage potted plants as well as vegetables such as tomato and lettuce. Symptoms on African violet are dramatic with concentric rings of sunken tissue that can be black. These spots look like waves and are generally easier to see on leaf undersides. This virus is spread by thrips such as Western flower thrips as well as by using infected leaves for new plants.
Control - Thrips control must be the first step in controlling this virus disease. Since the host range of TSWV is so large all plants should be examined before entering the greenhouse. Do not use any plants or parts which come from stock with these symptoms.

Botrytis blight
Pathogen - Botrytis cinerea
Symptoms - Spots usually appear on the leaf underside, especially on petioles near the pot rim or in contact with the potting medium. A small, water-soaked spot can rapidly enlarge and cover the entire leaf. Sporulation on necrotic leaves or flowers appears as a powdery grayish-green mass.
Control - Watch for Botrytis when the following conditions occur - low light, high humidity, poor air circulation and warm days with cool nights. Fungicides such as vinclozolin (Ornalin 50WP) are registered for use on African violet and effectively control Botrytis blight. Although iprodione (Chipco 26019 50WP) is effective against botrytis blight on many plants it is not registered for African violet and may cause stunting and chlorosis.

Phytophthora stem and root rot
Pathogen - Phytophthora parasitica
Symptoms - Phytophthora stem and root rot appears very similar to bacterial blight caused by E. chrysanthemi. Mixed infections with the two pathogens sometimes occur. Spots can form on leaves or petioles and are watery and collapse rapidly. The plant usually dies. Culture of the pathogen is necessary prior to developing a control program for either disease.
Control - Avoid overwatering since water-logged roots are easily attacked by P. parasitica. Use pathogen-free pots, potting media and plant material. A wide variety of fungicides are registered and effective for control of Phytophthora stem and root rot of African violet. Etridiazol is available in a variety of formulations (Terrazole or Truban). In addition, a combination product of ethazol and thiophanate methyl (Banrot 40WP) is also registered on African violet. Two other compounds, metalaxyl (Subdue 2E) and propamocarb (Banol 66.5EC) are available for Phytophthora stem and root rot of African violet. Since the pathogen is present in the root system and/or potting medium, these fungicides must be applied to the potting medium to achieve disease control.

Powdery mildew
Pathogen - Oidium sp.
Symptoms - Spots appear on flowers, petioles and leaves. A powdery white coating can form up to 1/2 inch round areas or can coalesce to cover the entire leaf.
Control - The disease apparently does not cause serious losses in Florida since many growers do not apply fungicides during an outbreak. Dodemorph (Milban 39EC) is effective and registered for powdery mildew control on African violet.

Foliar nematode
Pathogen - Aphelenchoides ritzemabosi
Symptoms - Small, tan, interveinal sunken areas appear on lower leaf surfaces. These spots eventually are visible on the upper leaf surface as well. Lower leaf surfaces become shiny, brown and slightly cupped. Severe reduction of leaf size as well as distortion also are common.
Control - Both preventive and eradicative treatments with either aldicarb (Temik 10G) or oxamyl (Vydate 2L) are effective, registered, and safe for African violets infected with foliar nematodes.

Root knot nematode
Pathogen - Meloidogyne spp.
Symptoms - Galls occur on roots and the root system may be drastically reduced; plant stunting and wilting occur when severe infestations are present.
Control - Use sterile soil and grow plants off the ground if possible. Dasanit, Mocap, Temik and Vydate will aid in control. Check labels for this plant and application methods.

Cold water damage
Symptoms - Round spots which are light yellow or green appear on the upper surface of leaves. Spots can appear on margins or blades and are sometimes irregular or donut-shaped and white.
Control - Water that is colder than the leaf surface will cause spotting and is most common in Florida during winter. Deep well water is usually near 70F and rarely causes problems, except when it is stored outdoors in tanks. Be sure temperatures of overhead water applications are near leaf surface temperatures.

Ethylene damage
Symptoms - Plants develop watery black spots on leaves and petioles and even in flower centers. The spots look the same as those caused by some fungi and bacteria.
Control - These symptoms appear during shipping when plants are exposed to ethylene. Since fruits and vegetables generate this gas, avoid shipping African violets with produce. Storing plants in high humidity and temperature conditions can also promote ethylene damage.



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