Pucciniales

Rust fungi

Rust fungi form a large group of obligate parasites on plants. The have a complicated life cycle in which, in a fixed order, five types of spores are formed. In many species moreover host plant alternation takes place. Yet, many species exhibit an incomplete cycle, in which one or more stages are skipped.

A complete cycle runs as follows.

0. A haploïd spore infects a plant and forms there a sorus (fruiting body); in this group of fungi it is called a spermogonium. In the literature on rusts it is customary to abbreviate this stage name by a 0, and the subsequent stages by roman numbers. Spermogonia often appear on the upper side of a leaf as a group of orange dots of ± 0.1 mm. In the spermogonia special spores are formed, spermatia, that fuse one to one with spermatia derived from other spermogonia, then form a binucleate, haploid mycelium.

I. This mycelium extends and forms, often at the underside of the same leaf a more or less cupulate sorus, that is called an aecium. Here asexually chains of so-called aeciospores spores, that are able to infect other plants. Aeciospores often are yellow, and the walls of the sorus (the peridium) form a white fringe around the opening.

II. Aeciospores that have landed on a suitable plant germinate and form a mycelium that has its own, often more or less dish-shaped sorusL the uredinium. In rust species that have host alternation, the suitable plant is of another species than where the spermogonia and aecia have been formed. Still asexually in the uredinia unicellular spores are formed om stalks: the urediniospores. They generally are light brown in colour. Urediniospores can (re)infect other, but also the same plant.

III. Urediniospores germinate (on the same host plant species as where they were formed) and build a mycelium that form the last type of sorus, the mostly ± pulvinate telium. Here, also asexually, the often stalked, one- or more celled teliospores are formed. Teliospores mostly are formed towards the end of the growing season; they can hibernate, and are thick-walled. Their colour usually is dark brown to black.

IV. Germinating teliospores form a basidium, on wich sexually the four haploid spores are formed that form the beginning of stage 0.

In most cases an infection is local, e.g. restricted tot a single (part of a) leaf. But in some species the mycelium can spread through the complete of the plant. This a called a systemic infection. When the host plant is perennial, a systemic fungus may pass the winter in the below-ground parts of the plant.

In their turn, rusts are parasitised by other fungi, in particular Eudarluca caricis and Tuberculina species, like T. sbrozzii (Lutz, ao). Their spores are the food source for a number of species of the cecidomyiid genus Mycodiplosis.

references

Alexopoulos, Mims & Blackwell (1996a), Gäumann (1959a, 1964a), Hiratsuka & Sato (1982a), Klenke & Scholler (2015a), Lutz, Bauer, Begerow & Oberwinkler (2004a,b), Lutz, Bauer, Begerow ao (2004a), Termorshuizen & Swertz (2011a), Webster (1980a).

03/03/2017