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.
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
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.
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.
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.
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.
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.
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 70°F and rarely causes problems, except
when it is stored outdoors in tanks. Be sure temperatures of overhead
water applications are near leaf surface temperatures.
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