Common diseases and disorders of Begonias

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


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).

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.

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 85F, 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.

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.

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.

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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