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
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Botrytis
blight
Pathogen - Botrytis cinerea
Symptoms - Botrytis leaf blight usually appear on lower leaves
of cuttings in contact with the potting medium. The water-soaked lesion
may enlarge rapidly to encompass a large portion of the leaf blade
or even the entire cutting. The area dies and turns dark brown to
black with age. When night temperatures are cool, day temperatures
warm, and moisture high, the pathogen readily sporulates on leaves,
covering them with grayish-green dusty masses of conidia.
Control - Controlling Botrytis blight of foliage plants is
particularly important during the winter months in Florida. Methods
which improve foliage drying and reduce moisture condensation on foliage
during the nights reduce the need for fungicide application. During
the past few years, use of iprodione (Chipco 26019 50WP) or vinclozolin
(Ornalin 50WP) has greatly reduced severity of this disease in foliage
production. However, the only effective fungicide which is labeled
for use on this plant is thiophanate methyl (Domain or Cleary 3336). |
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Fusarium
stem rot
Pathogen - Fusarium solani
Symptoms - Fusarium stem rot typically appears as a soft, mushy
rot at the base of a cutting or rooted plant. The rotten area frequently
has a purplish or reddish margin. Infection of leaves under very wet
conditions results in tan, papery leaf spots with concentric rings
of light and dark tissue. Fusarium solani forms tiny, bright red,
globular structures (fruiting bodies) at stem bases on severely infected
plants.
Control - If stem rot or cutting rot is a problem, treatment
of the cuttings with a dip or a post-sticking drench of thiophanate
methyl should diminish losses. Remove infected plants from stock areas
as soon as they are detected. Since Fusarium stem rot appears similar
to Erwinia stem rot, accurate disease diagnosis is very important
prior to choice and application of pesticides. Thiophanate methyl
compounds (Domain or Cleary's 3336) have been effective in controlling
Fusarium diseases on foliage plants. |
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Myrothecium
leaf spot
Pathogen - Myrothecium roridum
Symptoms - Spots generally appear at edges, tips and at broken
leaf veins of plants. Dead areas are dark-brown and initially appear
water-soaked. Examination of the lower leaf surface generally reveals
sporodochia which are irregularly shaped, black and have a white fringe
of mycelium. Sporodochia form in concentric rings within the dead
areas.
Control - Using fungicides when temperatures are between 70
and 85°F, minimizing wounding, and fertilizing at recommended levels
contribute to minimizing severity of Myrothecium leaf spot of foliage
plants. Chlorothalonil (Daconil 2787) has been effective for Myrothecium
control on many foliage plants and mancozeb compounds (Dithane M-45
or Manzate 200 80WP) also provide excellent control of this disease.
Although not registered, iprodione (Chipco 26019 50WP) and Daconil
2787 4.17F are both safe and effective on begonias. |
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Powdery
mildew
Pathogen -Oidium sp.
Symptoms - Spots of frosty white growth appear on leaves. The
powdery coating can form up to 1/2 inch circular areas as single spots
or can join to cover the entire leaf. Most spots are found on leaf
undersides. The disease is most common during the drier periods of
the year such as the winter months in Florida.
Control - The disease apparently does not cause serious losses
in Florida since many growers do not apply fungicides during an outbreak.
Thiophanate methyl compounds (Domain or Cleary's 3336) have been effective
in controlling powdery mildew diseases on some plants. Labels must
be checked for appropriate and legal usage. |
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Pythium
root rot
Pathogen - Pythium splendens
Symptoms - Cuttings usually show poor rooting and have yellow
leaves. Examination of the stem and roots reveals a mushy black rot
extending from the cut end into the upper portions of the stem and
leaves. Root and stem rots usually occur in patches on a propagation
bench where it spreads into uninfected cuttings.
Control - Control should be based on use of disease-free propagation
material, sterilized potting media and raised benches. Preplant treatments
with fungicides such as etridiazol (Truban and Terrazole formulations)
or postplant treatment with metalaxyl (Subdue 2E) or fosetyl aluminum
(Aliette 80WP) can be very effective in control of Pythium root rot.
Reducing water applications to the minimum level for good rooting
also reduces root and stem rot diseases. |
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Xanthomonas
leaf spot
Pathogen - Xanthomonas campestris pv. begoniae
Symptoms - Begonias are commonly infected with this bacterium
although some producers do not recognize the symptoms as a disease
problem. Dead margins and speckling are the most common symptoms.
All types of begonias have been found susceptible to this pathogen.
Control - The best way to control this disease is to establish
plants that are free of the disease for cuttings. All symptomatic
plants should be collected and destroyed. Minimizing overhead irrigation
will also reduce disease development and spread. Using slightly higher
than recommended rates of fertilizer also have been shown to reduce
disease severity. |
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