Stay vigilant for yellow rust in resistant wheat

• Signs of disease at an early stage

• Growers advised to keep eye out

• Send samples away to be tested

Growers should remain vigilant after signs of yellow rust in winter wheat varieties classed as resistant to the disease.

Unusual early observations of yellow rust on several winter wheat varieties in some Recommended Lists (RL) trials mean farmers will need to monitor crops more closely this season rather than relying on disease ratings.

Trial site inspections in the north of England last month found signs of the foliar disease on almost all wheat varieties classified as resistant at the young plant stage in the latest Recommended List – including some widely grown varieties.

Highly diverse

The Agriculture and Horticulture Development Board said there were also the “first hints of something unusual” starting to happen in other regions too. And it called on growers to have any suspicious samples properly analysed.

The yellow rust pathogen population is highly diverse – so new strains of the disease are not unusual. But these early observations suggest there may be a significant new strain or strains that has not been seen before in the UK pathogen population.

Unpredictable

Some varieties historically classified as susceptible at the young plant stage are currently relatively clean at the impacted trial sites. This suggests a potential pathogen population shift that may have displaced some other yellow rust strains.

Varieties impacted by yellow rust at the young plant stage are more likely to require treatment with fungicides that have good rust activity during the T0–T2 fungicide period -– even if they have a high adult plant disease resistance rating.

AHDB recommended list trials manager Mark Bollebakker said: “The trials in question were at about growth stage 30 at the time of inspection, so adult plant resistance had not kicked in.

“When it does, from stem extension onwards, these varieties may outgrow initial infection. However, it is difficult to predict what will happen. We have sent samples for testing to give us a better understanding of what we are seeing.”

Further updates

Farmers and agronomists should inspect all varieties before final spray decisions and not rely on the RL ratings until we know more, said Mr Bollebakker. Inspectors were assessing the situation – with further updates issued as soon as possible.

David King, of Syngenta, said: “It’s very much a case of being vigilant rather than panicking. However, in parts of the country, a lot of wheat wasn’t planted until November, and some earlier-drilled wheat struggled in poorer seedbeds.

“Both of these scenarios render crops more vulnerable to yellow rust.”

Help experts monitor rust

The next phase of the UK Cereal Pathogen Virulence Survey (UKCPVS) project got underway last month.

The survey supports breeding for durable resistance to yellow rust and brown rust in winter wheat – because the diversity and adaptability of these pathogen populations is relatively high compared to other foliar cereal diseases.

Brown rust was particularly interesting last year due to its relatively early appearance. This illustrates how rust pressures can swing between years and change rapidly within seasons, say researchers.

To monitor UK rust populations effectively, UKCPVS is asking growers to send in samples from infected leaves taken from geographically diverse areas and wheat varieties.  Anyone can submit a sample – in a paper envelope – via the to UKCPVS Freepost service.

This year, the UKCPVS team is prioritising the analysis of samples  from wheat varieties which are normally resistant to yellow rust but still showing unusual levels of disease in the field and in trial sites.