Common diseases and disorders of Hibiscus

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


Bromine toxicity
Pathogen - Excess bromine from foliar applications of Agribrom
Symptoms - Distortion and burning of new leaves especially. Failure to root out and develop good shoot growth are also common. The plants on the lower row in the figure were rooted with Agribrom at 55 ppm while those on the upper row were rooted with water alone.
Control - Rates of Agribrom delivered in an overhead misting system should stay be approximately 25 ppm bromine to insure benefits of disease and algae control without causing bromine toxicity. Using as little as 55 ppm bromine caused these symptoms.

Cercospora leaf spot
Pathogen - Cercospora spp.
Symptoms - Leaf spots appear very similar to those caused by the bacterial pathogens. Laboratory culturing is required to obtain an accurate diagnosis and design the appropriate control program.
Control - Thiophanate methyl compounds (Cleary's 3336, Domain and Topsin M 4.5F) are effective for control of Cercospora leaf spot on many ornamentals. Be sure to direct the sprays to the lower leaf surfaces in order to make contact with the spores and reduce disease spread. Avoid using stock plants with a history of Cercospora leaf spot.

Choanephora blight
Pathogen - Choanephora cucurbitarum or C. infundibulifera
Symptoms - Symptoms occur very rapidly on spent flowers. The black spores of the pathogen form in the dead flowers and leaves within a day of first symptom expression. Choanephora blight is most common when conditions are very hot and light levels are high.
Control - There are no fungicides which are available for this pathogen on hibiscus. When weather conditions are optimum, plants should be examined daily and old flowers removed and discarded to reduce spread of spores to adjacent plants.

Root rot
Pathogen - Fusarium, Phytophthora, Pythium or Rhizoctonia spp.
Symptoms - This condition is usually not a problem once plants are established in the landscape. In containers, plants are small, unthrifty and can give the appearance of lacking fertilizer. Roots should be examined to determine their health. Rotted roots are mushy, wet and disintegrate when handled. They are also usually brown to black. Since these symptoms can be caused by nematodes as well as pathogenic fungi a laboratory diagnosis is required for best course of treatment.
Control - Diagnosis of the causal organism is the most important step toward control of root diseases. The combination product of etridiazol and thiophanate methyl (Banrot 40WP and 8G) and iprodione (Chipco 26019 50WP) are effective in controlling Fusarium and Rhizoctonia. Metalaxyl (Subdue formulations), Banrot, fosetyl aluminum (Aliette 80WP) and etridiazol (Terrazole and Truban formulations) are all effective in controlling Pythium and Phytophthora.

Pathogen - Puccinia heterospora
Symptoms - This disease occurs only on Hibiscus syriacus (rose of sharon, althea). Rust disease is easily recognized because of the yellowish-tan 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.
Control - Rust diseases are inhibited by frequent rainfall or irrigation and high temperatures. Fungicides for rust control include chlorothalonil (Daconil 2787), ferbam (Carbamate), triadimefon (Bayleton) and mancozeb (Dithane M-45 and Fore).

Pathogen - Bright sunlight after long periods of cloudy days or sudden exposure to direct sunlight following pruning.
Symptoms -Large areas of leaves become brown and dead. It is common to find only a few leaves in the highest light affected with surrounding leaves apparently undamaged. Sunburn usually appears within a week of the damaging incident.
Control - Be careful to protect plants from sudden exposure to bright light following pruning. Avoid drastic changes in the plant canopy and those surrounding it by regular pruning. Since the damage will not continue it can usually be tolerated or carefully removed to improve plant appearance.

Xanthomonas leaf spot
Pathogen - Xanthomonas campestris pv. malvacearum
Symptoms - Foliar infections on hibiscus start as tiny pinpoint water-soaked areas which can rapidly enlarge. They tend to remain confined to the areas between leaf veins and sometimes have a bright yellow border. In severe infections leaf drop is common. All hibiscus species tested have been found susceptible to the pathogen, although some cultivars are more resistant than others. Two other bacterial pathogens can cause similar symptoms on hibiscus; Pseudomonas cichorii and Pseudomonas syringae pv. hibisci.
Control - Minimize foliar wetting through irrigation or rainfall to reduce spread of the bacterium and its ability to infect. Preventative applications with copper containing compounds are sometimes helpful but cannot control the disease under severe conditions. This disease is more of a problem during the warm wet periods of the year. Never use infected plants for cuttings even when the cuttings appear healthy since latent infections will simply continue to develop as the cuttings root.


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