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Rust fungus is a common fungus found on many different types of plants. It has a complex life cycles, requiring two separate plants species to complete.

What is a Rust Fungus?

Much of the food that you eat on a daily basis requires good growing conditions for the plants that they come from.

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There are a variety of things that can go wrong, from bad weather, to pests, to fungi. Wheat used to be particularly vulnerable to a type of fungus called rust fungus. Modern wheat varieties have been developed to be much more resistant to these fungi, but only 100 years ago farmers would be quite upset if they began to see the red or yellow spots along the stem of their wheat leaves. Every so often even modern wheat varieties can be hit with a new variety of this rust fungus, which can be very hard on the crop.

The rust fungus is seen as red bumps on leaves, in this case blackberry leaves
Rust fungus on leaves

Rust fungus is any fungus caused by the Pucciniales order of fungi.

There are thousands of different species of this rust fungi. They typically present a red color on the plants that they attack, thus the name ‘rust’. Specific species tend to focus on specific plant types.

Life Cycle of the Rust Fungus

Rust fungi are interesting because often they require two different plant species in order to complete their life cycle. Let’s look at the specific rust fungus that tends to attack wheat, Puccini graminis. This species is also known as the black stem rust.

In order for it to complete its life cycle it also needs to have access to a shrub called barberry. It can travel on the winds in order to transfer between the wheat and the barberry plants.The fungi usually starts at the uredinum stage, coming from infected plants. When this gets transferred to another plant (on the wind) it forms the telium or teliospores stage. At this point the fungus can be transferred to the barberry plant, and is able to survive even harsh weather conditions.

Rust fungus grow on two separate plants, the teliospores stage starts on the wheat plants and is transferred to the barberry plant.

The aeciospores start on the barberry plant and are transferred to the wheat plant.

Life cycle

On the barberry plant the fungus undergoes meiosis and forms the basidiospores haploid stage which will then form the pycnia stage. The fungus gets transferred to the wheat plants as the next stage, aeciospores.

This turns into the uredospores which can in turn infect more wheat plants, restarting the cycle.Under the right conditions it is possible for this fungus to survive even without barberry plants present. If temperatures become cold and harsh enough, then the rust fungus needs the barberry stages in order to become hardened and survive these harsh conditions.

But under the temperate climates in parts of North America it doesn’t need this hardening stage, and can survive simply passing from one wheat plant to another. In fact in these conditions the fungus won’t even form the teliospores stage.Under typical conditions the telispores will form as temperatures begin to drop, forming a thick protective wall. As temperatures begin warming up in the spring, they will undergo karyogamy (forming the diploid nucleus) and meiosis. This eventually forms the thin walled basidiospore stage, at which point it begins taking over the host barberry, colonizing on the leaves.The pycnia spores will form a sticky residue, attracting insects to ensure it becomes cross pollinated, in order to ensure it can continue to grow. Once the spores have grown through the barberry leaves it forms the aeciospores, which can be transferred to the wheat.

On the wheat the aeciospores germinate and grow, eventually breaking through to stem to form the uredospores. This is the repeating stage, where they are able to multiply on the host wheat plant. Once it begins to get cold again, the aredospores will form the telispores to be transferred to the barberry plant, and withstand another cold winter.

Lesson Summary

Rust fungus is a fungus in the Pucciniales order, forming a red or yellow colored fungus on plants. It often needs two different plant species in order to complete its life cycle. The wheat-barberry variety has a 5 step life cycle:

  • Uredinum on the wheat, transfers to other wheat plants
  • At winter changes into the teliospores and transfers to the Barberry plant
  • In spring changes into the basiodiospores stage, and populates the Barberry plant
  • Grows into the pycnia, allowing it to be fertilized
  • Forms the aeciospores, which can be transferred to the wheat plants
  • Repeats, returning to the uredinum stage, which can infect other plants as well

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