WHAT ARE RUSTS?

Plant rusts, commonly known as rust fungi or simply rusts, are a group of parasitic fungi that belong to the order Pucciniales in the phylum Basidiomycota. Rust fungi are obligate biotrophs that can only grow and reproduce on living host plants. These fungi are named after the characteristic reddish-brown or rusty colour of their spores. Rusts are notorious plant pathogens and are responsible for causing various diseases in a wide range of plants, including crops, ornamental plants, and wild vegetation.

Some rust fungi have a complex life cycle that involves multiple hosts or spore stages. They often require two unrelated plant species to complete their life cycle, known as the primary host and the alternate host. Rust fungi are termed macrocyclic when their life cycle includes five spores stages (spermatia [often called pycniospores] = stage 0, aeciospores = stage I, urediniospores = stage II, teliospores = stage III, and basidiospores = stage IV).

Some macrocyclic rusts require different hosts (heteroecism), whereas others can complete all spore stages on a single host plant species (autoecism). Those rusts lacking one or more spore stages are called microcyclic, demicyclic or hemicyclic, depending on the functional and missing stages.

Urediniospores: These are the most common spores produced by rusts and are responsible for the rapid spread of the disease. They are typically produced in uredinia on the leaves of the infected plant and can be easily dispersed by wind, rain, or other means.
Teliospores: These spores are produced later in the life cycle and are usually darker in colour. They are formed in structures called telia and are responsible for overwintering, allowing the fungus to survive adverse conditions.
Basidiospores: After overwintering, the teliospores germinate to produce basidiospores, which are the spores that infect the alternate host.
Aeciospores: After infection by basidiospores, structures known as spermogonia (termed pycnia in some cases) are formed, eventually giving rise to cup-shaped aecia containing the aeciospores. Aeciospores can infect the primary host, thus completing the life cycle of the rust fungus.

Rust infections can lead to various symptoms in plants, including the appearance of small, round, or elongated pustules on the leaves, stems, or other parts of the plant. These pustules contain masses of spores and are often surrounded by a characteristic yellowish or orange-coloured halo. Severe infections can result in leaf drop, reduced growth, and even death of the plant. To control rust diseases, various management strategies are employed, including the use of resistant plant varieties, fungicides, crop rotation, and good cultural practices. It’s important to note that the specific types of rust fungi and the plants they infect can vary widely, resulting in different rust diseases that impact agriculture and ecosystems globally.

SOURCES

OpenAI, ChatGPT to Z.A. Pretorius, 23 July 2023.

Lorraine C. et al, 2019. Advances in understanding obligate biotrophy in rust fungi. New Phytologist 222: 1190-1206.