Strategies to minimise the threat from yellow rust

Wheat growers should continue to ‘expect the unexpected’ with yellow rust

After high levels of the disease were found on varieties with good disease resistance ratings in 2019 – reminding us all how rapidly it can change within a season.

Alarm bells started ringing earlier this year when yellow rust suddenly appeared in wheat varieties that shouldn’t be susceptible to the disease – prompting fears that there had been further changes to the rust races found in the UK.

Knee-jerk reactions to certain varieties are understandable in such situations, even if the suspicions prove to be unfounded. Yellow rust has the potential to hit yields very hard and new races are aggressive and more difficult to eradicate.

A complex and very diverse pathogen, it is a master at adapting to its surroundings. Originally named after the varieties that the disease was found on, different races are now known by a colour and number combination, reflecting both their genetic heritage and their ability to cause disease.

As we wait for clarification on the 2019 developments and the varieties affected, it is worth noting that we aren’t alone. Yellow rust was also a major problem across Europe this year, with unusual patterns of varieties affected.

Early testing in some countries suggested the presence of new races, putting everyone on high alert.

The EU-wide RustWatch project, which NIAB is a partner in, keeps a watching brief. While what’s happening in Europe doesn’t always reflect the UK situation, it does provide an early warning of what may be coming our way.

Knowing whether variety resistance ratings are correct or not would certainly help. Scores given to varieties in the RL system show their performance in previous years. This means the ratings are only as good as last season and don’t reflect how the variety will react if new races are confirmed.

There are also two types of genetic resistance; seedling and adult plant resistance. Only three varieties on the Recommended List have complete resistance to yellow rust – others may be susceptible at the seedling stage, but then shrug off the disease as they go through stem extension in the spring.

Making good use of variety resistance ratings is especially important if spraying capacity is stretched. In the right conditions, yellow rust can take just seven days to appear from the initial infection event.

For crops going into the ground this autumn, there are several things that growers can do. Drilling a mix of different varieties is a sensible precaution, just in case one of them breaks down to rust.

A later drilling date and destroying the green bridge will also help. These reducing the opportunity for spores to transfer from the previous crop or volunteers, as the pathogen requires living green plant material to survive.

As far as spray programmes are concerned, the majority are designed around septoria control and in most cases, fungicides which contribute to yellow rust control will be used in preference where yellow rust is anticipated to be an issue. In a particularly susceptible variety, there may need to be tweaks – perhaps adding in an azole or a strobilurn at T0, +/- T1, for example.

Although they are not effective against septoria anymore, the strobilurns still offer useful yellow rust protection. They are limited to two applications per year.

With the imminent withdrawal of key triazole fungicides, future rust control will fall to new generations of fungicides . In 2020, there will be two new fungicides on the market – Revysol and Inatreq – and growers will need to learn about their effects on rust.

New chemistry is not always as effective and generally more expensive than the products they replace, placing more reliance on competitive resistance traits to keep costs low.