Profile of a misdiagnosed wheat disease: yellow spot

Yellow spot is prevalent in northern region wheat crops, yet it remains one of the most misdiagnosed wheat diseases.

Photo of diseased leaf

Small tan-brown spots with yellow margins that become more elongated with age indicate the presence of yellow leaf spot. Yellowing of the leaf without lesions is not a symptom.

PHOTO: Dr Hugh Wallwork

Yellow spot disease does not actually produce a yellow spot but rather a tan-coloured lesion that spreads across the wheat leaf, said Dr Steven Simpfendorfer from the New South Wales Department of Primary Industries at the recent GRDC Grains Research Update in Coonabarabran, NSW. 

“Arguably, tan spot is a better description of the disease, and it is known by that name around the rest of the world,” he said.

The disease is usually treated with fungicide during the early growth stages at mid-tillering, but Dr Simpfendorfer has urged growers to be vigilant where this approach is used. 

“Fungicides only treat the part of the plant that they come into contact with,” Dr Simpfendorfer said. “At mid-tillering, the key yielding leaves are not out, and therefore don’t receive protection from sprayed fungicides. The new leaves can then be rapidly infected from the stubble remaining in the paddock.

“You never actually kill the infection with a fungicide because inoculum remains on stubble throughout the year, and leaf lesions in the lower canopy can reinfect upper leaves later in the season.” 

Yellow spot diagnosis

When diagnosing yellow spot or tan spot disease, ask yourself the following key questions:

  • Is there wheat stubble in the paddock?
  • Are black fruiting structures visible on the wheat stubble?
  • Are the spots or lesions a tan colour with a thin, yellow margin?
  • Is the distribution of lesions on the leaves consistent with the disease, which tends to show more large lesions on the lowest leaf and fewer smaller lesions as you move up the leaves of the plant?
  • Has there been rain recently?

If the answer to any of these questions is ‘no’, it is quite likely that the problem is not a disease that can be controlled by applying a fungicide.

Yellow spot is distinctive because it initially produces small brown spots surrounded by a thin, yellow margin that progressively elongate into tan lesions, which kill the plant tissue as they develop. A common misdiagnosis occurs when a general widespread yellowing of the leaves is identified as yellow spot. “Unless tan lesions are present and surrounded by a discrete, tight yellow margin,” Dr Simpfendorfer said, “it could be a whole range of other problems.”

This general yellowing of leaves can be a symptom of herbicide coming into contact with the plant (phytotoxicity) or a result of frost or nitrogen deficiency, among other possible causes. Yellow spot is also a wet-season disease, and only moves up the plant to affect the top three yielding leaves in wet weather conditions. “If there has not been any rain, it’s unlikely that it’s yellow spot,” Dr Simpfendorfer said. 

“If misdiagnosed, growers can end up spending a lot of money on fungicides that won’t actually do anything,” he said. “Fungicides are not cheap insurance – think first and spray later.”

Key points

  • Diagnose yellow spot correctly and avoid confusing it with yellowing of the leaves
  • Burning autumn stubble can help control the inoculum
  • Avoid sowing wheat-on-wheat to reduce disease risk
  • Yellow-spot-resistant varieties are available

Measures to help disease risk

To help minimise the risk of yellow spot developing, Dr Simpfendorfer recommended autumn stubble burns to help destroy the inoculum and sowing yellow-spot-resistant varieties.

If planting wheat-on-wheat, choosing the right variety was the key weapon in a grower’s arsenal against yellow spot, he said. 

While bread wheat is the primary host of yellow spot, it can also infect durum and triticale. Dr Simpfendorfer said a lot more inoculum was generated on stubbles of more susceptible wheat varieties, such as EGA GregoryPBR logo and LongReach SpitfirePBR logo, so the risk of the disease developing was much greater where a susceptible wheat variety was sown on the stubble of a previously infected susceptible variety, such as EGA GregoryPBR logo on EGA GregoryPBR logo

“Check the suitability of resistant varieties, as there can be other problems, such as root lesion nematode susceptibility with some of these varieties,” he said. 

“Growers need to make management decisions before and at sowing because fungicides are a poor last resort.”

More information:

Dr Steven Simpfendorfer
0439 581 672,
steven.simpfendorfer@dpi.nsw.gov.au

www.grdc.com.au/GCTV

Dr Simpfendorfer’s full paper, presented at the GRDC Research Update, is available on the GRDC website:
 www.grdc.com.au/updates

A fact sheet on yellow spot is available at: www.grdc.com.au/GRDC-FS-YellowSpotNorth

End of Ground Cover issue #105 Northern edition
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Region South, North