Look beyond fungicides for yellow leaf spot control

Key points

  • Control yellow leaf spot (YLS) inoculum levels to control the disease
  • Choose more resistant varieties if it is practical to do so
  • Do not sow susceptible varieties into stubbles containing moderate to high levels of inoculum
  • The disease is frequently misdiagnosed. Ensure that it is actually YLS, not herbicide damage or some other form of plant stress
  • Fungicide results against YLS are inconsistent and should not be the only strategy for managing the disease
  • Rotation with non-host species is an effective way to manage inoculum levels

Yellow leaf spot costs Australian grain growers more than $200 million a year in lost yield. Returns on fungicide inputs are inconsistent with this disease and any decision to spray should be carefully weighed
 

Yellow leaf spot (YLS) is an important disease of Australian wheat crops, yet there is still confusion about how best to manage it. Specifically, whether fungicides will be effective in controlling the disease.

New South Wales Department of Primary Industries plant pathologist Dr Andrew Milgate advises growers who wish to avoid YLS epidemics in their crops to look carefully at crop growth stage and seasonal conditions before spraying and to adopt a range of non-chemical strategies to control inoculum levels.

Identifying yellow leaf spot

Yellowing of the leaf tissue can have many causes, and YLS is frequently confused with herbicide phytotoxicity, moisture or nutrient stress, or some other cause.

There are several things to look for to correctly identify the disease.

 Photo of a 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

  • Is there wheat stubble visible in the paddock?
  • Are black fruiting bodies visible on the stubble?
  • Do the lesions (spots) look right? Small brown spots with yellow margins that become more elongated with age indicate the presence of YLS.
  • Seek advice if unsure of the symptoms.

Source of inoculum

The ascospores of YLS do not travel far from their source, and identifying the source of the inoculum – from within the paddock or from a neighbouring paddock – may influence the decision to spray.

Stubble carrying high inoculum loads from within the same paddock is more likely to cause repeated infection than stubbles further away.

Should I spray?

Once the disease is positively identified and the source of the inoculum determined, consider the crop growth stage, coupled with the seasonal forecast and the cropping region.

Reducing the risk of yellow leaf spot

  1. Avoid sowing wheat-on-wheat.
  2. Choose varieties with higher levels of resistance.
  3. If sowing a susceptible variety, make sure YLS is not present in significant levels.
  4. Manage the stubble loads by burning, grazing or stubble incorporation.
  5. Rotating with non-host species can reduce the inoculum loads by up to 95 per cent in a single year where stubble breakdown occurs.

Early season infection

In southern NSW, it is common to see YLS develop during May to August, particularly where wheat-on-wheat is grown and a susceptible variety is sown. However, trials in this region have indicated that despite high levels of infection occurring in early growth stages (before GS31 – first node on main stem) no significant yield loss was recorded. This is due to a lack of frequent rainfall events in spring which constrains the ability of the disease to establish on the flag leaf and flag-1 leaf.

However, Queensland trials with high levels of infection at the same growth stage saw yield losses of up to 13 per cent.

In wet years or in high-rainfall regions, early infection establishes the platform for disease to cause yield losses. Trials in the western region demonstrated that a program of early and repeated spraying was necessary to minimise potential losses in susceptible varieties in these conditions.

In seasons with frequent rainfall, the ability to control the disease with repeated spray applications is limited because of the continual release of spores from primary and secondary infections on stubble and on the lower leaves of the plant.

Photo of a plant. Green with yellow edges

Tan-coloured lesions surrounded by yellow margins are a clear indication of yellow leaf spot infection. Under favourable conditions the lesions will expand and join together, producing large areas of diseased yellow plant tissue.

PHOTO: Manisha Shankar

Mid-season infection

Epidemic events commencing in growth stages GS31 (first node on main stem) to GS55 (early ear emergence) around August to September are likely to be caused by weather events triggering mass spore release and infection.

The outlook for spring rain will determine whether spraying will be economical. If significant infection has occurred on the flag leaf and flag-1 leaf, which are critical for yield, spraying may prevent further loss of green leaf area, but only for about three to four weeks. Infection that has already occurred remains active, with the potential to provide spores for new infections once the fungicide protection ceases.

Late-season infection

In low-yielding Australian environments, it is generally accepted that there is little benefit in applying fungicides after GS55 (early ear emergence).

Which fungicide?

Seed or fertiliser treatments are ineffective against YLS. Fungicides containing propiconazole or tebuconazole have been demonstrated to be effective. However, because YLS is a necrotrophic pathogen (feeding on dead plant tissue), fungicides will only protect uninfected green leaf, and will not kill the fungus living on dead leaf or stem tissue.Consequently, when the fungicide wears off, reinfection can occur rapidly, particularly if weather conditions are conducive.

More information:

Dr Andrew Milgate
02 6938 1990
andrew.milgate@dpi.nsw.gov.au

www.grdc.com.au/GCTV

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

 

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GRDC Project Code DAN00147

Region North, West, South