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Hosts

Disease Names:

Leaf spot and blight of octopus tree (Schefflera actinophylla, formerly Brassaiaactinophylla.

Leaf spot and blight of ginseng (Panax quinquefolius).

In Hawaii, Alternaria panax has been isolated from diseased Schefflera actinophylla, Schefflera elegantissima (= Dizygotheca), and Schefflera calyptratus (= Tupidanthus). Pathogenicity on all hosts was confirmed. In addition, inoculation of local panax (Polyscias guilfoylei) showed that it was susceptible also, but the disease was not severe. In Florida, Alternaria panax has been isolated from diseased Aralia spp., Fatsia japonica, kalanchoe, Polyscias spp. and Schefflera arboricola. It has been reported on Eleutherococcus (= Acanthopanax) leaf spots in Missouri. In the U.S. ginseng plants are attacked by this pathogen.

Distribution

This pathogen occurs on all major islands in Hawaii. It also occurs in the eastern United States and in Florida. Alternaria panax is a major pathogen of ginseng in Asia.

Symptoms

Before the introduction of this fungus to Hawaii in the 1970s, the was virtually no severe foliar disease on Schefflera in the islands. After its introduction, epidemics caused by this pathogen became a major problem on potted Schefflera for nearly a decade. Sporadic outbreaks still occur today at local nurseries. 

Leaf spots caused by Alternaria panax begin as very small water-soaked spots. These increase in size to form sunken, brown to rusty-brown spots on mature Schefflera leaves. Circular brown spots are commonly formed when the environment becomes dry following leaf infection. On softer immature leaves or young plants, spots are more diffuse, gray and grow to cover large areas of the leaf with dark rots called blights. Yellow areas are sometimes associated with leaf lesions. Blighted leaves commonly abscise (fall away from the plant). 

Large numbers of spotted and blighted leaves make ornamental Schefflera plants unmarketable. Young seedling are stunted or killed by this fungus as cotyledons and small leaves are rotted.

Biology

Taxonomy

Kingdom: Fungi 

Phylum: Ascomycota (teleomorph or sexual stage) 

Order: Dothideales

Family: Pleosporaceae

Genus: Lewia

Traditional: Fungi Imperfecti: Deuteromycotina (anamorph or asexual stage) 

Hyphomycetes 

Genus: Alternaria

The teleomorph or sexual stage of Alternaria panax is not known. 

Other Alternaria species are reported to have Lewia as the sexual stage. In older literature, Alternaria panax was called Alternaria actinophylla.

Biology

Alternaria panax produces microscopic, dark, asexual spores called conidia. These conidia are composed of many cells and are produced on specialized fungal threads called conidiophores. The production of conidiophores and conidia is affected by relative humidity, light, and temperature. High humidity and cooler temperatures (16° C or 68° F) favor conidiophore production. Similar conditions also favor spore formation on these conidiophores. 

The shape or morphology of the spore is strongly influenced by light and temperature. Long, slender spores are formed at lower temperatures while wider spores are formed at higher temperatures. In general, larger numbers of spores are produced at lower temperatures 16°-20°C than at higher temperatures (28°-31°C).

Epidemiology

This pathogen produces large numbers of conidia on the surface of diseased leaves. Dark, black-gray growth over the rotted leaves indicate spore formation. Spores are spread by wind and splashing water. Contact with clothing, gloves, and equipment will also spread the pathogen. 

Moisture is needed for spore germination. The fungus germinates by producing germ tubes which are thread-like growths. These penetrate the epidermis (skin) of the leaf and the fungus grows within the leaf. Alternaria panax releases enzymes and other by-products which kill living cells. Nutrients released from dying cells are absorbed by the fungus. The metabolic activities of the fungus causes tissue yellowing and death of the cells. Groups of dead cells are observed as brown spots on the leaf. 

Conidiophores later emerge from the diseased leaf and are present on the epidermis or surface of the leaf. Hundreds to thousands of conidiophores commonly emerge from stomates, which are natural openings on the leaf. In a few hours spores are produced on the conidiophores. Distribution of these spores by rain or splashing water begins the disease cycle again. In moist tropical environments, blight formation and subsequent spore formation can occur in 5-7 days. Epidemics become common in areas of the State with prolonged wet periods.

Management

In high rainfall areas, solid covered greenhouses are strongly recommended. In addition, cultural practices that reduce humidity also reduce disease levels. These include increasing the spacing between plants, use of drip irrigation, and encouraging good air circulation by adjusting row (bench) configurations and trimming large shrubs or trees surrounding greenhouses. 

Collecting, removing, and destroying diseased leaves reduces the number of spores, minimizing the development of new spots. Breaking the disease cycle by sanitation also maximizes the benefits of fungicides that protect leaves from new infections. Fungicides such as Dithane M45 and iprodione can be used. Follow label directions with care when using any pesticide.

References

Alfieri, S. A. Jr., K. R. Langdon, J. W. Kimbrough, N. E. El-Gholl and C. Wehlburg. 1994. Diseases and Disorders of Plants in Florida. Bulletin No. 14. Florida Department of Agriculture and Consumer Services. Gainesville, Florida. 

Uchida, J. Y., M. Aragaki, and M. A. Yoshimura. 1984. Alternaria leaf spots of Brassaia actinophylla, Dizygotheca elegantissima and Tupidanthus calyptratus. Plant Disease 68:447-449.

BA-PANA

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