Alternaria Panax on Umbrella Tree.

Abstract

The umbrella tree (Schefflera actinophylla (Endl.) Harms. (synonym: Brassaia actinophylla Endl.), which is indigenous to humid coastal Queensland north from about the Tropic of Capricorn, is a popular garden tree as well as being grown as a potted indoor and outdoor plant in southern Queensland. In June 1981 potted umbrella tree plants growing in a Brisbane nursery were severely affected by a leaf spot and leaf blight disease. Leaf symptoms ranged from very small light brown necrotic spots to large irregular dark brownlblackish necrotic areas (Fig. 1). Leaf spots were often surrounded by chlorotic zones. Severely affected plants were defoliated. A species of Alternaria occurred on lesions, and was consistently isolated. The fungus was subsequently identified as Alternaria panax Whetzel. This identification was confirmed by Dr S.M. Francis of the Commonwealth Mycological lnstitute (IMI 269850; BRlP 13804). Pathogenicity tests were performed by spraying a spore suspension (1 x 105/ml) from cultures derived from a single spore isolate of the fungus onto healthy plants of S. actinophylla and S. arboricola (Hayata) Merrill (dwarf umbrella tree). Control plants of each species were sprayed with sterile water. All plants were enclosed in plastic bags for 3 days at 20°C. After 6 days the temperature was raised to 26OC. Symptoms identical with those seen in natural infections developed on the S. actinophylla plants within 4 days. S. arboricola symptoms were quite different. Small light brown necrotic spots (round to irregular) surrounded by a diffuse chlorotic zone were present on mature leaves 12 days after inoculation. On the undersurface of the leaves the spots were much more prominent, appeared corky, and were slightly raised. On soft immature leaves, small irregular dark red spots were produced and again they were much more prominent when viewed from the undersurface. Immature leaves were distorted and spots also appeared on petioles. However, there was no defoliation. A. panax was successfully reisolated from all inoculated plants. With S. arboricola percent recovery was always lower than with S. actinophylla and with time the fungus became harder to isolate. Alternaria panax is an important pathogen of many plants in the family Araliceae. Recorded hosts include Schefflera actinophylla, S. arboricola, Dizygotheca elegantissima (Hort. Veitch) R. Vig. & Guillaum, Tupidanthus calyptratus Hook. f. & T. Thomas, and plants in the genera Aralia, Panax, Polyscias, Pseudopanax and Meryta (1, 2, 3). Many of these plants are grown as ornamentals in Queensland. A. panax has been recorded on Araliaceae in New Zealand, Spain, U.S.A. and Venezuela (2). The fungus has recently been recorded on Polyscias fruticosa (L.) Harms. in Queensland. Phytophthora parasitica Dastur (P. nicotianae B. de Haan var. parasitica (Dastur) Waterh.) has been reported to cause a leaf spot on S. actinophylla (4) similar to that caused by an Alternaria species. In Queensland Phytophthora palmivora (Butler) Butler has been found on one occasion causing a leaf spot of S. actinophylla nursery plants. Symptoms were very similar to Alternaria leaf spot on this host. To my knowledge this is the first recording of Alternaria panax of S. actinophylla in Queensland.