Yellow rust in wheat

Pathogen

Puccinia striiformis

Hosts

The disease affects wheat, barley and triticale. There are distinct crop-specific forms of the fungus:

  • P. striiformis f.sp. tritici mainly attacks wheat, but can also attack barley
  • P. striiformis f.sp. hordei can only attack barley

Within the forms of P. striiformis, there are races that only affect particular varieties.

Symptoms

The characteristic symptom of yellow rust is of parallel rows of yellowish orange coloured pustules on the leaves of adult plants. Epidemics of yellow rust often start as individual plants, usually in the autumn. Symptoms develop slowly over winter and are often missed until the early spring when small patches or foci of infected plants can be seen in fields. These patches usually spread in the direction of prevailing wind.

Early on, the pustules are very difficult to distinguish from brown rust. However, yellow rust lesions tend to spread as a band up and down young leaves, often with a yellow band on the leaf moving ahead of the sporulating lesion. Pustules are also often scattered at random on young leaves before becoming more obvious stripes as the leaf gets older. Under hot, dry conditions – or after fungicide use – the epidemic has been checked meaning pustules may be difficult to detect.

Infected leaves can rapidly become chlorotic and then necrotic in May/June, if weather conditions are conducive. In severe attacks, yellow rust infection of the ears can occur, resulting in the formation of masses of spores between the grain and the glumes. At the end of the season, secondary black spores (teliospores or telia) are sometimes produced amongst the stripes of pustules.

Life cycle

Yellow rust requires living green plant material to survive. In the winter, the fungus survives as dormant mycelium or active sporulating pustules on volunteers and autumn-sown crops. Although, low temperatures kill pustules, mycelium within plant tissue can survive temperatures of -5°C. Cooler winter temperatures may also kill infected lower leaves and, therefore, the fungus.

The epidemic takes off as temperatures warm in April/May. Temperatures of 10–15°C and a relative humidity of 100% are optimal for spore germination, penetration and production of new spores. These are spread by wind or leaf-to-leaf contact. Cool, damp weather in the spring, with overnight dew or rain, provides optimum conditions for disease development.

The complete cycle from infection to the production of new spores can take as little as 10 days during ideal conditions, so leaf tips may show symptoms before leaves fully emerge. The disease cycle may repeat many times in one season.

Temperatures over 20°C slow the fungus, although there are strains tolerant to high temperatures.  A prolonged spell of warm, dry weather often checks an epidemic. This is due to the direct effect on the fungus and increasing host resistance at higher temperatures.

At the end of the season, secondary black spores (teliospores or telia) may be produced as part of the sexual stage. In wheat, the basidiospores produced from these teliospores have an alternative host (Berberis spp.). This means sexual recombination can take place, forming new races. However, this sexual pathway is not known in the UK.  In barley, there is no known alternate host, meaning sexual recombination cannot take place.

Importance

The main economic loss from yellow rust is in wheat. The disease does occur in barley but it is rare due to effective varietal resistance. The disease is most important in the East, although infection can occur all across the UK. Areas that have cool, damp summers and mild winters are prone to yellow rust infections, for example, coastal regions or regions around rivers or estuaries.

Yield losses of 40–50% often occur in untreated susceptible wheat varieties. However, varietal resistance and well-timed fungicide sprays are usually very effective, so annual losses are often small. Yellow rust can also affect grain quality.

Single major genes control varietal resistance to yellow rust in wheat and can be overcome by new races in a short time period. The United Kingdom Cereal Pathogen Virulence Survey (UKCPVS) monitors changes in pathogen populations and assesses the risk to currently recommended varieties of new incursions.

Local sources of infection are very important but long distance spread can also occur, including across countries.

High-risk factors

  • Susceptible variety
  • Eastern regions of the UK
  • Early sowing
  • Green bridge from previous crop
  • Cool, damp weather conditions
  • Mild winter
  • Humid microclimate

Management

Varietal resistance is a very effective. The AHDB Recommended List (RL) wheat yellow rust ratings reflect the resistance of adult plants during the main part of the growing season (from about stem extension onwards). Most RL wheat varieties are susceptible to yellow rust at the seedling stage (up to stem elongation) but resistant at the adult stage. Therefore, infection in the autumn and early spring does not usually mean there has been varietal breakdown to a new race of disease.

Resistance ratings do not provide any guarantee of durability of resistance, as the pathogen population can change and new rust races can develop to overcome these resistances. Therefore, all varieties need to be checked regularly for symptoms.

Growing large areas of susceptible varieties can exacerbate a yellow rust epidemic. This risk can be mitigated by growing resistant varieties alongside more susceptible varieties to limit the spread of the disease. Previously, grouping varieties according to their underlying resistance genes enabled growers to use varieties with a diverse mix of resistance genes, meaning that the spread of yellow rust could be limited should one resistance gene be overcome. However, since the incursion of the Warrior race in 2011 this grouping is no longer possible due to the diversity of the yellow rust population.

Destruction of volunteers will help to prevent carry-over of the disease, as they act as a green bridge from one season to another. Early sown crops are at greater risk of yellow rust as fungal spores spread from the previous year’s infected crops and from volunteers in late summer. Therefore, delaying drilling of susceptible varieties can reduce risk. Managing N to avoid excessive concentrations in plants, for example from late doses of nitrogen, can also help.

Fungicides

Fungicides can provide very effective yellow rust control. The pathogen is also not particularly prone to becoming resistant to fungicides. Systemic seed treatments may help delay epidemics developing, where risk is high.

Winter wheat

T0 – two to four weeks earlier than T1

  • Control at this timing can be economic where risk is high and the disease is active
  • Limit spread of visible infection in susceptible varieties with an azole or strobilurin
  • Consider adding a multisite to reduce the risk of fungicide resistance

T1 – as soon as leaf 3 is fully emerged (GS31–33)

  • Limit spread of visible infection in susceptible varieties with an azole or strobilurin
  • Sprays applied for septoria tritici control will normally also control rusts

T2 – fully emerged flag leaf (GS39)

  • If yellow rust is active at this timing, then apply an azole or strobilurin
  • Sprays applied for septoria tritici control will normally also control rusts

T3 – ear spray

  • Control visible infection on the ear with an azole or strobilurin. This will protect the ear and top two leaves from yellow rust
  • Hot, dry spells will limit yellow rust development, but could favour the development of brown rust
  • Products applied at this timing will usually be targeted towards ear disease and brown rust, but will normally also control yellow rust

Winter barley

Fungicide treatment is rarely needed to control yellow rust in barley. If the disease is seen, then strobilurins, azoles and SDHIs are effective

Summary

  • Grow a variety with a high resistance rating, but monitor disease levels throughout the season
  • Eradicate volunteers to remove ‘green bridge’
  • Delay drilling of susceptible varieties
  • Manage nitrogen applications to avoid excessive concentrations in plants
  • In wheat, apply an azole or strobilurin when yellow rust is seen. SDHIs used for septoria control will also give control of yellow rust
  • Fungicides are rarely required to control yellow rust in barley
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