Glossary

Below are some of the terms most commonly used when talking about rust. Click on the symbol to read a definition of each one.

Adult Plant Resistance

 

Some varieties are susceptible to early disease and then develop resistance as the plant matures. Depending on the variety, the resistance can be first detected at different growth stages or under different environmental conditions. Generally, the resistance increases with plant age and as the temperature rises. The resistance is controlled by minor genes that work in a number of different ways but the effect is to slow down the rate of the epidemic.

Epidemic

 

This term describes the increase and spread of a disease through a crop, district or region.

Exotic

 

This term is used to describe a pathogen isolate (in this case rust) that has been introduced into Australia from somewhere overseas.

Germplasm

 

Germplasm in this context refers to the plant material that is used by plant breeders – it includes the plants used as parents for crossing, breeding lines under development, and established varieties.

Green bridge

 

The term ‘green bridge’ describes the weeds and crop volunteers that establish during the non-cropping phase that help pests and diseases cross from one cropping season into the next. This mass of vegetation grows in paddocks, roadsides and non-crop land after summer rain or under irrigation. The green bridge is particularly important for diseases like rust or pests that cannot survive through the summer without a green host.

Incursion

 

The appearance of a new pathogen or pest in Australia, which has come from somewhere else.

Inoculum

 

A pathogen, a disease causing organism, that can cause infection when transferred through inoculation.

Inoculation

 

The process of applying a pest or pathogen onto healthy plants to generate disease.

Long season wheat

 

Wheat varieties are generally classified as either spring wheats, or long season (winter) wheats. The latter require a period of cold temperature at crucial growth stages to induce flowering – because of this, they are often sown earlier than spring wheats, and remain in the ground longer (hence “long season”). Wheats that are sown earlier in the season are potentially a concern in providing a green ramp – rust inoculum can move from the green bridge onto the green ramp, and from there into spring wheat crops.

Mutation

 

A change in the DNA of a plant, pest or pathogen that changes its genetic composition. Mutations occur in all organisms and are a random process. In rust, the most well-known case of mutation is where a pathotype mutates and acquires the ability to overcome a resistance gene in a variety, rendering it less resistant. In the case of rust, it is generally a change in the expression of virulence that allows it to overcome resistance in a plant. It could also be a change that increases fitness or environmental adaptation in the rust pathogen.

Pathogen

 

A pathogen is a disease-causing organism (life form).

Pathology

 

The area of science that deals with controlling diseases.

Pathotype

 

A particular variant of a pathogen. In this case, a variant of a rust fungus that has a particular combination of virulence for resistance genes in its host.

Pustule

 

The part of the rust pathogen that emerges through the leaf or stem of the plant. It is visible as a mass of fungal spores that are released and spread the disease to other plants.

Spores/urediniospores

 

Rust produces spores in pustules that vary in shape, size and colour depending on the type of rust – leaf, stem or stripe. The urediniospores (aka urediospores) move readily on the wind and are responsible for rust transmission between plants, within fields and across large regions.

Teliospores

 

In high disease pressure and as crops begin to turn, darker coloured spores become evident as teliospores (aka teleutospores) are produced. The teliospores are resting spores, which do not have any function in wheat rust and oat rust pathogens in Australia. In the barley leaf rust pathogen, these spores can germinate to produce basidiospores, which infect the Star of Bethlehem, on which this pathogen undergoes sexual recombination to produce aeciospores. Aeciospores are then able to reinfect barley to complete the life cycle.

UG99

 

In Uganda, Africa, in 1999, a new pathotype of stem rust appeared that overcame one of the most widely used resistance genes in world wheat production. This pathotype was given the nickname Ug99. Since then, Ug99 has moved out of Uganda into neighbouring countries of east Africa and as far east as Iran and down to South Africa. Like all rust pathogens, it has mutated and is developing into new pathotypes (races) capable of overcoming other important resistance genes.

Click here to see a video about UG99

Virulence

 

Virulence is the ability of a pathotype of rust to overcome the resistance effects of a gene in the host. For example, the resistance gene Lr24 was effective in Australia in providing protection in wheat to leaf rust from 1983 until 2000. In 2000, a new pathotype with virulence for this gene was detected, and many cultivars with this resistance gene were rendered susceptible.