Yellow and brown rust control is set to become more important for winter wheat growers at the key flag leaf fungicide spray timing later this month as septoria is largely confined to the older lower leaves in this very dry spring.

Septoria remains the biggest threat to yield at the T2 stage.

However, the two main diseases being picked up at the Bayer Crop Doctor site in north-west Norfolk, close to The Wash, were yellow rust and mildew in untreated fungicide trial plots.

With dry weather predominating across most the UK, wheat growers could be tempted to cut back on T2 sprays.

Jonathan Blake, technical director for crop protection at consultant Adas, however, warns against this approach.

See also: April showers could change wheat disease outlook

“It is too big a risk to cut back too much at T2. This is the timing that can give the biggest yield response and also the biggest yield loss,” he tells Farmers Weekly.

Yellow and brown rusts

Jonathan was spotting yellow rust on a range of wheat varieties on the site including Dawsum, Champion, Typhoon and Bamford in untreated plots and some brown rust was starting to come into the plots, albeit much later than last year.

Yellow rust is being seen on more wheat varieties this season, which could be due to a new race of the disease being present or maybe it is being seen before true adult resistance starts to kick in.

“Crops look well with good yield potential, but growers need to consider more than septoria at the T2 spray and consider the risk from rusts,” he says.

Yellow rust was present in some of the untreated plots © David Jones

There was little septoria seen on the site with the plots late drilled in November and quickly followed by a very dry March and April, with the first day of May being the hottest ever recorded in the UK.

But disease risk could quickly change with a change in the weather.

Septoria tends to favour wet and windy weather with contact between leaves helping its spread, while yellow rust favours warm moist conditions and brown rust hot and moist weather.

Therefore, the T2 spray applied in late May to protect the all-important flag leaf from these diseases is key to preserving yield potential.

Mildew

Mildew was also being seen in the untreated plots, but many fungicides aimed at septoria, yellow rust and brown rust, such as the widely-used azole prothioconazole, give good control of mildew.

Although the weather has been dry, the site only a few miles from The Wash is seen as a yellow rust “hot spot” due to moisture-ladened sea mists.

Experts say that, although septoria is only being seen on the lower leaves, 20mm of rain could spark septoria into action.

Increased moisture will also encourage rusts which can cycle in about seven days, much quicker than septoria at 15 to 18 days.

Growers have a wide choice of products to use at T2 between specific strong septoria products such as Miravis Plus and Univoq.

These may need rust tops up in certain circumstances, and more broad-spectrum products such as Iblon-based ones, Revystar and Ascra.

With low current septoria levels, Jonathan says growers should be in a protective mode.

And with many planning to use SDHI-azole or Inatreq-azole products for a T2 spray he adds some may be able to cut rates if the weather remains dry.

However, some varieties such as Crusoe are susceptible to brown rust and Skyfall to weakness to yellow rust.

They may need a top up for the T2 spray, with a strobilurin such as Amistar or Comet and/or the azole tebuconazole to boost rust control.

Crop Doctor site

The Crop Doctor trial site on the Norfolk farm of father and son Clive and Andrew Melton was drilled on 8 November.

The surrounding field was sown with new feed variety Beowulf on 6 October into largely silty clay loamy soils.

The T1 on the later-drilled plots was applied on 2 May and the surrounding Beowulf field on 22 April.

The site is at Pear Tree Farm, Walpole Highway, just south-west of King’s Lynn.

The farm grows 400ha of winter wheat including milling varieties Crusoe and Loxton, and feed wheats Dawsum and Beowulf.

The Crop Doctor site showed low levels of foliar diseases with only fungicide in untreated areas © David Jones

Andrew’s fungicide strategy is based on variety and drilling date.

Varieties showing good disease resistance such as Dawsum and Loxton miss out on a T0 spray in late March and then have a T1 treatment of Revystar in late April.

Crusoe, with its known susceptibility to brown rust, had a T0 of azole tebuconazole plus strobilurin Amistar and then a T1 of Revystar plus tebuconazole.

New variety Beowulf had the same azole-strobilurin at T0 and then the new SDHI-azole combination Plaxium at T1.

Crusoe only scores a lowly 3 for brown rust in the AHDB’s Recommended List, which scores disease resistance for varieties on a 1-9 scale with 1 being very susceptible and 9 showing good resistance.

Beowulf also has a low score for brown rust at  4, while it scores better on yellow rust (9) and septoria (6.6).

Dawsum scores 9 for yellow rust, 7 for brown rust and 6.3 for septoria while Loxton is not on the Recommended List.

Andrew is generally seeing very low levels of disease in his crops, with septoria seen only on the lower leaves, and no rust diseases apart from some small spots of brown rust on the Crusoe.

Dawsum

He has resisted cutting back too much on T1s, although late-drilled Dawsum did see a cut in rates at T1 after such a dry spell.

He could take that approach at T2 which he sees occurring around 23 May on the farm.

“We will be looking for a strong product against septoria at T2 as we are looking for longevity, but we may be able to cut back a bit on rates,” he says.

Aleks Curcic, commercial technical manager in Norfolk for Bayer, says in a generally low-disease year she is seeing predominantly yellow rust with some brown rust and mildew on the lower leaves of crops in the county.

Therefore, she suggests that T2 spray will need to give good protection for septoria, but also yellow and brown rust as well.

“The dry weather has put septoria on the back foot, but it is still there in what has been very much a rust season so far,” she says.

Aleks suggests that when growers are selecting their T2 product they should see septoria as the main threat, but to make sure the rust activity of their sprays are adequate to control yellow and brown rust.

Farmers Weekly visited the Bayer’s Crop Doctor site in north-west Norfolk in early May