Unusual yellow rust spreads: Winter wheat on trial blog

Paul Gosling, who manages the Recommended Lists for cereals and oilseeds (RL), provides an update on our winter wheat on trial site in Norfolk, including observations of unusual levels of yellow rust.

April was warmer than average in the UK and May had the warmest start to the month since records began. In addition to the warmth, it has been very dry.

Despite the lack of rain, crops are moving rapidly through growth stages in many areas, including the 50 winter wheat varieties (35 recommended, 15 candidates) at our RL trial site in Norfolk (Terrington).

Earlier this year, I blogged about the RL fungicide programme, including why we use a ‘belt-and-braces’ approach and how it may be closer to commercial practice than you think.

The Norfolk site has a high yield potential, which is why the host farm’s commercial fungicide programme has a similar intensity to the RL programme.

We have followed the core RL programme at T1 (Univoq plus Arizona and a low dose of Imtrex for rust control).

The trial operators also have an optional spray at T1.5 at their disposal. This season, it was needed at the Norfolk site to help keep disease as low as possible.

The T1.5 azole option can help damp down rusts and septoria (we used RevyPro at the Norfolk site).

Lying close to The Wash, the Terrington site is notorious for yellow rust.

But there is something afoot with yellow rust this year, which was the main target of our T1.5 spray. Thankfully, the dry conditions are keeping septoria tritici pressures low, for now.

The alarm bells started ringing in early April. Not in the yellow rust hotspot of Terrington, but much further north, where yellow rust is not usually a particular concern.

At our RL trial site in Sunderland, all the varieties rated as resistant at the young plant (seedling) stage on the RL showed signs of yellow rust. Soon after, sites in Scotland showed similar patterns.

It wasn’t until last week that we started to see yellow rust in the fungicide-untreated trial at Terrington.

Mark Bollebakker, who manages the RL trials, provided an update (vlog) last week. At the time of his inspection, the variety plots were around growth stage 37.

Compared to the sites in the RL North region, yellow rust was less advanced (Norfolk is in the RL East region).

Once again, the plots are starting to show a similar pattern to the trials in the North, in terms of the varieties affected in the fungicide-untreated plots.

Some varieties were showing symptoms on the leaf tips or along the middle of the leaves, but the classic stripiness was also starting to appear.

In these videos (YouTube shorts), Mark Bollebakker, who manages the RL trials at AHDB, provides an update on yellow rust.

We have sent diseased leaf samples from the North region sites to the UK Cereal Pathogen Virulence Survey (UKCPVS) at NIAB to characterise the yellow rust isolates, but it will take a few weeks to bulk up the samples for varietal screens.

As the yellow rust pathogen population is highly diverse, it is not unusual to detect new strains. We know that new yellow rust strains can blow in from overseas (potentially from outside Europe).

The latest population shift could be a result of such a movement, but it could also be a mutation of a current UK strain – time will tell.

UKCPVS has confirmed that an important yellow rust resistance gene has failed (Yr15).

Pathogen isolates from disease leaf samples not only infected a test variety known to carry the Yr15 resistance gene but went on to sporulate profusely.

NIAB-funded work also confirmed the presence of the Yr15 gene, using molecular markers, in several of the varieties impacted at the young-plant stage.

From a crop management perspective, it is academic. But the observations suggest that the latest change is significant.

At present, we do not know how much it will impact adult stage resistance and varieties may still outgrow initial infection. It all depends on the genetics in each variety.

The advice is to monitor all winter wheat crops (any disease rating) – even in regions where yellow rust is not often problematic – and manage them accordingly.