Uredo acaciae Cooke
This pathogen infects foliage of trees and stems of saplings, as well as flowers and seed pods. Telia, resembling brown powdery masses, develop dry on young host tissue, but become slimy under wet conditions. Spores, that are released under these wet conditions, coat the foliage and mat leaves together forming a brown crust. Rachi, petioles and leaflets may become malformed (twisting or swelling) (McTaggart et al. 2015). In severe cases, wattle rust can reduce growth of young trees by 20 to 40% (Little & Payn 2016).
Early in 2022, a new and previously unknown rust symptom emerged on A. mearnsii. At first, this was thought to be a new rust problem but DNA sequencing showed that the unusual gall symptoms were caused by U. acaciae. This is the first time that gall symptoms have been associated with this rust species, and further studies must be undertaken to determine the basis for this unusual symptom development.
Uromycladium acaciae is a demicyclic rust fungus that is characterised by its telial stage. The telia of U. acacia form as powdery pustules that become slimy under wet conditions. This fungus produces two teliospores per pedicel, with a basal vesicle on the pedicel. This vesicle expands and bursts under wet conditions so it may not be observed. Teliospores are globose, subglobose or broadly ellipsoid, pale brown, with smooth edges and are 18–23 × 15–20 μm in size. Telia often surround spermogonia on the infected plant parts, creating powdery patches. Spermogonia produced by the fungus on the upper surfaces of leaflets or on stems are erumpent and hemispherical. The spermatia are globose to ellipsoidal, hyaline in colour and measure 2.5–5.0 × 2.0–4.0 μm (McTaggart et al. 2015).
Inoculation trials have elucidated many aspects of the life cycle and biology of the fungus. Uromycladium acaciae is an autoecious rust, meaning it can complete its entire life cycle on A. mearnsii. Teliospores are produced dry, but exude in brown, sticky, masses that farmers and foresters refer to as “slime” under wet conditions. Spores can coat the foliage of affected trees, forming a brown crust and matting leaves together. They are therefore thought to mainly disperse in rain or water splash. Under wet conditions, teliospores germinate and produce basidiospores which germinate and infect young host tissue. The optimum conditions for infection are temperatures between 15-20°C with 12 hours or more of dew. Symptoms develop 2 to 5 weeks after infection (Fraser et al. 2017).
Fraser S, McTaggart AR, Wingfield MJ, Roux J. 2017. Effect of temperature, leaf wetness and the developmental stage of host tissue on infection of Acacia mearnsii by Uromycladium acaciae (Pucciniales). Australasian Plant Pathology 46: 407-419.
Little KM, Payn RG. 2016. Screening of fungicides for the management of wattle rust (Uromycladium acaciae) in Acacia mearnsii plantations, South Africa. Southern Forests: a Journal of Forest Science 78: 151-158.
McAlpine D. 1905. A new genus of Uredineae — Uromycladium. Annales Mycologici 3: 303-323.
McTaggart AR, Doungsa-ard C, Wingfield MJ, Roux J. 2015. Uromycladium acaciae, the cause of a sudden, severe disease epidemic on Acacia mearnsii in South Africa. Australasian Plant Pathology 44: 637-645.
Morris MJ, Wingfield MJ, Walker J. 1988. First record of a rust on Acacia mearnsii in Southern Africa. Transactions of the British Mycological Society 90: 324-327.
