Alan Wood
ARC-PPRI
P. Bag X5017
Stellenbosch
7599

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The Fever Tree (Acacia xanthophloea Benth.) is a popular garden plant in South Africa, being one of the most commonly cultivated of the indigenous African thorn trees. Its characteristic bark colour, attractive form and rapid growth rate (1–1.5 m p.a. under favourable conditions) make it one of the best indigenous trees for frost free gardens in South Africa (Venter and Venter 1996, Joffe 2001).

Recently an outbreak of galls developed on a crop of approximately 1500 young Fever Trees in a shade house in a nursery near Nelspruit, Mpumalanga.. It was found that the galls were caused by infection  with the rust fungus Ravenelia natalensis Syd., P. Syd. & Pole-Evans, aecia being frequent on the galls and uredinia and telia present on minute pustules on leaflets. This is not only the first report of this rust fungus on this host plant, but also the first report of any disease on this tree species in South Africa (Doidge 1950, Crous et al. 2000).

Ravenelia natalensis was previously only known from three collections made by I.B. Pole-Evans and E.M. Doidge at Winkelspruit, Durban, during 1911 and 1912 on Sweet thorn (Acacia karroo Hayne, as A. hirtella E. Mey.) (Doidge 1927). The galls it induces on the host plants are like those caused by Ravenelia macowaniana Pazschke which is frequently observed on Sweet thorn throughout the country. Many of the galls were infested with the mycoparasite Tuberculinia sp., or bored by moth larvae. Twenty four moth species are known to bore in galls of R. macowaniana (Krüger 1998). Although no mortality of plants was reported due to infection by this rust fungus, the galls are unsightly and reduce the retail value of the plants. Subsequently to the outbreak of this disease, the young Fever Trees have been moved out of the shade house and now stand in the open. In addition they are watered with drip irrigation. This has reduced the infection rate substantially.

Ravenelia natalensis is readily distinguished from all other southern African Ravenelia species by its teliospore which is irregular in both number of cells (3–12 cells per spore) and arrangement of these cells which are frequently bunched up one on top of another. Also, its pedicel consists of only 1–3 hyphae.

Figure 2: side view of  teliospores of Ravenelia natalensis, note how cells of teliospore are piled up irregularly which is characteristic of this species.(1 = remnant of pedicel, 2 = basal cell, 3 = hygroscopic cyst typical of Ravenelia, 4 = cell of teliospore).

Estra Strydom, owner of the nursery, and Johan Husselman, ARC-ITSC, are thanked for supplying the specimens and providing information about the disease situation.

References: 

• Crous, P.W., Phillips, A.J.L. & Baxter, A.P. (2000) Phytopathogenic fungi from South Africa.
• Department of Plant Pathology Press, University of Stellenbosch, Stellenbosch
• Doidge, E.M. (1927) A preliminary study of the South African rust fungi. Bothalia 2:1-228
• Doidge, E.M. (1950) The South African fungi and lichens to the end of 1945. Bothalia 5:1-1094
• Joffe, P. (2001) Creative gardening with indigenous plants. Briza, Pretoria
• Kruger, M. (1998) Identification of the adults of lepidoptera inhabiting Ravenelia macowanian
• Pazschke (Uredinales) galls on Acacia karroo Hayne (Fabaceae) in southern Africa. African Entomology 6:55-74.
• Venter, F. & Venter, J-A. (1996) Making the most of indigenous trees. Briza, Pretoria.