Rust Fungus

Deformity on stem of Vachellia Karroo, caused by Sweet Thorn Rust (Ravenelia macowaniana).

Rust fungi belong to the order Pucciniales and are represented by 540 species (in 40 genera) in Southern Africa and around 7 000 species globally.[1] Despite their prevalence in the environment, rust fungi are generally overlooked in the natural environment. The relationship of rust fungus to the host plant is a pathogenic parasitic relationship. The fungus is an obligate biotroph, meaning that they cannot live outside of their host organisms. Rust fungi exhibit complicated life cycles and some develop five distinct types of spores at the different stages of their life cycles.

Vachellia karroo, normal plant structure. No galls present.

 One of the most widespread species in Southern Africa is Ravenelia macowaniana or Sweet Thorn Rust. This species has all the spore types present in its life cycle. It lives in the Vachellia karroo (Acacia karroo) plant making this species of rust fungus particularly widespread and common. The life cycle of the fungus is complicated.

Ravenelia macowaniana gall. Note the small holes on the gall, these were made by moth larva feeding on the gall.

Summarised from Alan R. Wood, the life cycle can be described as follows:

In the beginning of the growing season, the galls develop on new growth and on these galls “blister-like structures” (pycnia) manifest. These produce minute spores (pycniospores) in a sweet suspension that attracts insects which feed on the substance and spread the spores. This creates a situation of “cross fertilization” and results in “small cup-like aecia [that] are produced all over the galls” and in these asexual spores (aeciospores) are produced which go on to infect the leaflets of the plant. These leaflet infections then initially produce another asexual spore that appears like blisters (uredinia) on the surface of the plant. These uredinia go on to produce urediniospores. Towards the end of the growing season structures called the telia produce the large teliospores which “after overwintering, germinate in the next rain season to begin the cycle again. When germinating these teliospores produce four basidiospores, each of which can infect the new, developing growth producing the [pycnia] galls.”[2]

Ravenelia macowaniana gall. Aecial cups are visible here, indicative of the early parts of the fungus life cycle.

In addition to the fungi and tree relationship, there is a third relationship and that is between the galls and Lepidoptera species (gall moths). There are at least 24 species and they belong to the Tineidae, Oecophoridae, GraciIlariidae, Gelechiidae, Cosmopterigidae, Tortricidae, Pyralidae and Noctuidae families. [3] M. Krüger (1998) groups the moths into three guilds: “putative obligate species, i.e. those feeding on gall tissue … A second significant guild comprises polyphagous species that develop as borers, mostly in fruits and pods of their host plant … The third guild includes species that are probably predators of other larvae” (55–56).  The life history of these moths is, as Krüger himself puts it, “obscure”.  

The obligate species of moths associated with the Ravenelia macowaniana are an example coevolution. That makes the relationship between the tree, the fungus and the moths incredibly old. All three have been evolving together into their current coupling. The tree seems to be relatively defenseless against the rust. I have noticed that trees growing in less than perfect locations exhibit more of the disease. It would be interesting to find out more about the trees defenses against the rust.

Thinking about the intricacies in the relationship between this tree and the fungus that lives inside it and the relationship of the moths to the galls produced by the fungus in collaboration (even though it is through disease) with the tree, just demonstrates that we are surrounded by biodiversity of such a magnificent scale. Just looking at something as small as fungal structures on the surface of tree can reflect relationships and interactions that are not even well understood. In trying to find out what the cause of the structures are; moths and a strange world of fungus is called up. This is the mysterious world of nature that has makes my brain burn with wonder.

 

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[1] For an overview of the taxonomy of the Pucciniales see: http://www.plantmanagementnetwork.org/proceedings/FCRS/2011/Presentations/Aime.pdf

[2] Adapted from Wood, A. R. (2012). Rust fungi on South African plants. Veld & Flora, 3 (98), 123– 125. 

[3] For a discussion of the moths see: Krüger, M. (1998). Identification of the adults of Lepidoptera inhabiting Ravenelia macowaniana Pazschke (Uredinales) galls on Acacia karroo Hayne (Fabaceae) in southern Africa. African Entomology 6, 55–74.