Bacterial leaf blight of fishtail palm
Acidovorax avenae subsp. avenae
Fishtail palms, Caryota spp., comprise a genus of 12 species of single- and cluster-stemmed palms from India and Sri Lanka to S. E. Asia, N. Australia and the Solomon Islands. In Hawaii, Caryota mitis (the Burmese fishtail palm) is widely grown in landscapes and sometimes is exported for interiorscapes or houseplants.
The Burmese fishtail palms are prone to damage caused by a number of parasites or problems, including spider mites and scale insects, nutrient deficiencies and fungal leaf spots. Under high rainfall or overhead irrigation, these palms may also be infected by plant pathogenic bacteria that cause blight of leaves. The distinct disease symptoms include conspicuous, darkly colored stripes along and around leaf veins.
Here we discuss the bacterial leaf blight of C. mitis caused by the plant pathogenic bacterium Acidovorax avenae subsp. avenae and what can be done to recognize, prevent and minimize the damage caused by the disease.
Disease name(s): Bacterial leaf blight, leaf striping
Pathogen: Acidovorax avenae subsp. avenae (Manns) Willems et al.
Pathogen synonyms: Pseudomonas avenae Mann 1905, Pseudomonas alboprecipitans Rosen 1922
Host (scientific name): Caryota mitis
Host (common names): Burmese fishtail palm, Clustered fishtail palm.
Photo: the distinctive symptoms of bacterial leaf blight of Burmese fishtail palm (Caryota mitis) in Hawaii at a palm nursery in North Kohala on the island of Hawaii, caused by Acidovorax avenae subsp. avenae (Photo - S. Nelson, UH-CTAHR).
Host origin: native from Burma to the Malay Peninsula, Java and the Philippines. These palms are grown in greenhouses, nurseries and outdoors in soils at private properties and in landscapes within warm regions of the United States such as Florida and Hawaii. Fishtail palms are also used as interiorscape plants and houseplants in some locations. Outdoors they can grow in large clusters of plants, up to 12-40 feet in height.
Disease history: this is a relatively newly described plant disease, having been first studied by plant pathologists in 1978. The disease has been in Hawaii since at least 1995, according to the University of Hawaii Agricultural Diagnostic Service Center in Hilo. We do not know when or how this disease was introduced or first reported in Hawaii. Relatively little published research has appeared on the pathogen and the disease in Hawaii or elsewhere. The disease is relatively common in wet areas or in nurseries under overhead irrigation in Hawaii.
Pathogen host range: reported hosts of Acidovorax avenae subsp. avenae include corn (Zea mays) and rice (Oryza sativa). In Iran, twelve isolates of A. avenae subsp. avenae isolated from rice were reported pathogenic on corn, sorghum, barley, oat, sugarcane, barnyard millet, Italian millet, wheat, barnyard grass and Johnson grass, indicating a wide host range (Rostami and Taghavi). It is unknown if the pathogen affects these hosts in Hawaii.
Pathogen strains: there are a number of strains or subspecies of Acidovorax avenae that attack other plant hosts but not C. mitis.
Type of pathogen: a bacterium.
Bacterial facts: gram-negative, rod shaped, motile with single polar flagellum.
Symptoms:
- initial symptoms are small, water-soaked, translucent to light yellow to light brown banded areas running along and around leaf veins
- mature lesions develop a brown to black color and may have a chlorotic (yellow) halo; lesions range from a minimum of 1-2 mm wide and in length up to the entire length of affected leaves.
Infection: initial infections often occur through hydathodes at leaf tips or margins.
Disease cycle and epidemiology: free water on C. mitis leaves in Hawaii is the most important environmental factor for infection and disease development (e.g., high rainfall, frequent dew, overhead irrigation). Splashing water disperses the pathogenic bacteria from leaf to leaf or between plants and enhances bacterial survival. The bacteria can enter wounds or natural openings in leaves of fishtail palm, in particular the bacteria probably enter most frequently through the hydathodes at leaf margins. Infections and disease symptoms appear and move along and near leaf veins, resulting in the black stripe symptoms. Symptoms of this disease can appear at about 5-10 days after inoculation in greenhouses with air temperature ranging from 18 to 32 C. The pathogen reproduces and survives primarily within infected Caryota leaf tissues.
Disease occurrence and distribution in Hawaii: a relatively common disease; bacterial leaf blight of fishtail palm is most likely to occur in palm nurseries where C. mitis plants are clustered together and irrigated using overhead sprinklers or in high-rainfall areas.
Diagnosis:
The symptoms are diagnostic for the disease. For bacterial isolation and identification:
1. Wash the symptomatic leaf tissues in fresh water and excise some young lesions.
2. Crush excised lesions in sterile water (or phosphate buffer).
3. Streak the suspension onto agar (beef-yeast extract agar or yeast extract-dextrose-calcium carbonate agar) and incubate; alternately, place excised lesions directly onto the selected agar.
4. Observe colony growth and morphology characteristics on growth media, or use commercial testing kits.
Similar diseases of C. mitis: none, the mature symptoms are unique to this disease.
Integrated management practices for bacterial blight of C. mitis:
- eliminate overhead irrigation that impacts leaves
- irrigate in the morning instead of the evening
- grow plants under cover from frequent rainfall
- use preventive sprays of copper-containing or antibiotic-containing pesticides
- remove symptomatic leaves and destroy them; remove and destroy entire plants if they are affected severely
- provide good air circulation around plants to allow leaf drying after they become wet
- do not purchase, sell, or distribute diseased plants
- increase spacing among plants in nurseries or greenhouses
- intercrop C. mitis with non-susceptible plants only (not with reported hosts of the bacterium)
- do not transplant symptomatic plants into landscapes
Symptom photographs of bacterial blight of C. mitis:
Above: initial symptoms of bacterial leaf blight of fishtail palm (Caryota mitis) are small, water-soaked, translucent to light yellow banded areas running along leaf veins (Photo - S. Nelson, UH-CTAHR).
Above: mature lesions develop a brown to black color and may have a chlorotic (yellow) halo; lesions initially range from a minimum of 1-2 mm wide and in length up to the entire length of affected leaves (Photo - S. Nelson, UH-CTAHR).
Above: Adjacent lesions may expand and coalesce under wet conditions. Infections usually start at the jagged leaf edges. Symptoms may include light brown, water-soaked tissues (Photo - S. Nelson, UH-CTAHR).
Above: older lesions may coalesce and develop tan to light-colored central regions that become colonized by fungi that are not necessarily pathogenic to C. mitis, especially under wet conditions for leaves. Under dry conditions, the bacterial lesions may stop expanding altogether (Photo - S. Nelson, UH-CTAHR).
