Plant Pathogenic Rust Fungi

Host Morphology Alteration and Floral Mimicry

Some rust fungi are able to change the morphology of the plants they parasitise. One example of this is Uromyces pisi, which can parasitise the plant Euphorbia cyparissias (Cypress Spurge). In this case the rust fungus is able to cause the plant to produce pseudoflowers. These pseudoflowers are rosettes of leaves which are covered with fungal gametes in sugary nectar of fungal origin. The rust’s gametes are on separate and distinct mycelia (they are heterothallic). It is thought that this could be to entice insects towards the fungus in order to cross fertilize their mating types, and that the fungi depends on the insects to carry out sexual reproduction. This theory was tested by Pfunder and Roy in 1999 by a simple insect exclusion experiment (e). The infected plants were enclosed in a fine mesh gauze to prevent the entry of insects. With insects excluded from the infected plant, they concluded that insect pseudoflower visitations were required for the reproduction of U. pisi.

Left: Uninfected flowering Euphorbia cyparissias plant and Right E. cyparissias infected by the rust fungus Uromyces pisi. Source: 1. Smoczyk (2003) 2. Pfunder and Roy (1999) (F) (E)

It is also suggested that pseudoflowers can affect the reproductive success of nearby flowering plants due to their highly successful rates of attraction of insects (R). This was found particularly with regard to the rust fungus Puccinia monoica , which inhibits flowering in Arabis species, whilst radically transforming their morphology to include elevated clusters of infected leaf rosettes.

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