References
Common diseases and disorders of Geraniums

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

 

Alternaria leaf spot
Pathogen - Alternaria tenuis
Symptoms - Alternaria leaf spot is characterized by small (less than 1/8 inch wide) spots which are initially water-soaked. These spots turn reddish-brown, may reach 1/4 inch in diameter and are roughly circular with a yellow border. Spots generally do not merge.
Control - Alternaria leaf spot can be controlled through a variety of methods. Total elimination of water on leaves is sufficient to completely control Alternaria leaf spot of many plants. Chlorothalonil (Daconil 2787), mancozeb (Dithane M-45 and Fore), and iprodione (Chipco 26019 50WP) are effective for controlling this disease and are currently labeled for use on geranium in Florida.

Botrytis blight
Pathogen - Botrytis cinerea
Symptoms - Spots usually appear on the leaf underside, on petioles near the pot rim or in contact with the potting medium. Wounded areas are especially susceptible to infection with Botrytis. 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) and iprodione (Chipco 26019) are registered for use on geranium and effectively control Botrytis blight.

Pseudomonas leaf spot
Pathogen - Pseudomonas cichorii
Symptoms - Spots are water-soaked and turn dark green to black. Sometimes under severe conditions, centers of spots drop out and give the plants the appearance they have been attacked by insects. When plants are allowed to dry out the spots turn tan and become papery.
Control - Avoid overhead watering as much as possible to reduce conditions for infection and spread of the pathogen. Preventive applications of a copper bactericide may aid in disease control but none are completely effective. Keep in mind that other crops are hosts of this pathogen (chrysanthemum, schefflera, aroids, lettuce and celery to name a few) and it can spread easily from one crop to another.

Rhizopus blight
Pathogen - Rhizopus stolonifer
Symptoms - Symptoms occur very rapidly and can kill large portions of the plant or a seedling flat in less than 1 week. The black spores of the pathogen form in the dead stems and leaves within a day of first symptom expression. Rhizopus blight is most common when conditions are very hot and light levels are high. Heat stress appears to predispose plants to this disease.
Control - There are no fungicides which are available for this pathogen on geranium. When weather conditions are optimum, plants should be examined daily and discarded as soon as they are found to reduce spread of spores to adjacent plants.

Rust
Pathogen - Puccinia pelargonii-zonalis
Symptoms - Rust disease is easily recognized because of the reddish-brown pustules which occur on leaf undersides. The spots first appear white or light yellow when viewed from the upper surface. Most rust pustules form on the undersides of leaves but some can be found on leaf upper sides as well as petioles and stems. Sometimes the pustules form in concentric rings or zones.
Control - Rust diseases are inhibited by frequent rainfall or irrigation and high temperatures. Fungicides for rust control on geraniums in Florida include chlorothalonil (Daconil 2787), ferbam (Carbamate), triadimefon (Bayleton) and mancozeb (Dithane M-45 and Fore).

Xanthomonas campestris pv. pelargonii
Symptoms - Leaf infections start as small water-soaked spots which are tan and circular to slightly irregular in shape. Occasionally spots are bordered by a yellow halo and under some conditions, they merge and affect large segments of the leaves creating wedge-shaped dead areas. Large areas may be blighted and shoot dieback can occur as well. Systemic infections usually result in plant death since the plants ability to transport water and nutrients is destroyed.
Control - Growers must rely on use of pathogen-free certified plants as well as a vigorous sanitation and scouting program. Ivy geraniums usually do not develop severe symptoms and can have latent or unexpressed infections thus acting as a source of inoculum. Be sure that all of your geraniums remain symptom free by scouting them weekly.

Edema
Symptoms - Small blister-like spots appear on leaf undersides. Sometimes they are clustered around the outer portions of the leaf. These blisters can become corky and sometimes result in leaf drop. Leaves appear yellow from above when edema is severe.
Control - Edema occurs when a water imbalance causes more water to be moved into the leaves than can be utilized. This water collects and causes the blisters described above. Edema is most common during the spring and fall when soil and air temperatures differ substantially. Provide plants with optimal conditions and reduce water applications to limit development of edema.

Bleach phytotoxicity
Symptoms - White sunken areas form all over leaf surfaces. These spots do not enlarge and appear usually within a day or two of using bleach in the greenhouse. This is more common in the winter when the structures are closed tightly to retain heat. Bleach phytotoxicity can occur by direct splashing of the solution onto plants or as an aerosol.
Control - Never use a bleach solution in a tightly closed greenhouse and be careful to apply only to floors or benches well away from plants. If bleach must be used during the winter be sure to vent greenhouses afterward to reduce expose to fumes.

Mancozeb phytotoxicity
Symptoms - Leaf margins turn yellow and then brown within 1 week of a soil drench with mancozeb or maneb. Root death results in severe stunting.
Control - Apply all pesticides at recommended rates only to the labeled site (foliar or drench) and at recommended intervals. Many pesticides are safe when applied to the appropriate site but cause severe damage when applied inaccurately.

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