Yellow rust symptoms and management in wheat

A combination of varietal resistance and fungicides is usually effective at preventing unacceptable economic losses to this foliar disease. However, the pathogen that causes wheat yellow rust is highly adaptable, so close crop monitoring is essential.

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 varieties.

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

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.

The pathogen that causes yellow rust - Puccinia striiformis - 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.

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.

• Susceptible variety
• Eastern regions of the UK
• Green bridge from previous crop
• Later-sown wheat
• Cool, damp weather conditions
• Mild winter
• Humid microclimate

Varietal resistance can be very effective. The main AHDB Recommended List (RL) wheat yellow rust ratings (1–9 scale) reflect the resistance of adult plants during the main part of the growing season (from about stem extension onwards). Some RL wheat varieties are susceptible to yellow rust at the young-plant (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. From the 2023–24 edition, the RL includes information on the resistance (r) or susceptibility (s) status of wheat varieties.

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.

Based on Recommended Lists (RL) trial results, the wheat yellow rust watch list can reveal varieties that performed out of line with their RL disease resistance rating at some sites. Use the list to help understand disease risk and focus in-season management.

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.

• Grow a variety with a high resistance rating, but monitor disease levels throughout the season
• Eradicate volunteers to remove ‘green bridge’
• Generally, the later winter wheat is sown in the autumn, the more disease is present on the crop in the following year. This may be due to crops being at an earlier (more susceptible) growth stage when spore pressures are high
• the later winter wheat is sown in the autumn, the more disease is present on the crop in the following year
• Manage nitrogen applications to avoid excessive concentrations in plants (for example, apply relatively late doses of nitrogen)
• 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

With a focus on yellow rust and septoria tritici in wheat, this video shows how disease severity is assessed in RL trials using a standardised scale (0 to 100 per cent)

Published control curves are based on septoria data (black solid lines). However, the faster-cycling rusts, with their shorter latent period, result in tighter curves (yellow dotted lines).