Yellow spot still an agronomy puzzle

Photo of a man crouching among plants

Victorian Department of Primary Industries cereal
pathologist Dr Grant Hollaway is quantifying yield
losses in wheat caused by yellow leaf spot. The
disease is spread from wheat stubbles onto varieties
with weak resistance.

 PHOTO: Felicity Pritchard

Its name sums up its symptoms concisely, but researchers are finding the wheat disease yellow leaf spot (YLS) to be somewhat puzzling. In particular, scientists are yet to pinpoint the yield losses attributed to YLS, a disease that is widespread in medium to low-rainfall districts, particularly where stubbles are retained and wheat is grown in tight rotations.

Senior cereal pathologist with the Victorian Department of Primary Industries Dr Grant Hollaway says the incidence of the disease has increased in the long term due to stubble retention and the susceptibility of some dominant wheat varieties. “Yet the extent of yield loss due to this disease is still unclear in the southern grain-growing region,” he says.

For growers, the question of yield loss can be a barrier to making effective decisions to manage the disease. The best advice to reduce YLS is to avoid growing susceptible wheat cultivars where infected stubble is present.

Some growers are relying on fungicides for YLS but are unlikely to be achieving complete control of the disease. According to Dr Hollaway, fungicides only have a partial effect on the disease, whereas variety resistance is more effective, particularly where there is only a one or two-year break in wheat crops.

Management options

  • Growers can still take action this season to reduce the likelihood of yellow leaf spot (YLS).
  • When it is dry a two-year break is generally needed to reduce YLS. Most districts in the southern region have been dry and wheat straw from the 2011 crop still exists, carrying the disease.
  • Variety resistance is the most effective option. Any grower planning to sow wheat on wheat or wheat on two-year-old wheat stubble should check the latest variety resistance ratings first.
  • It is difficult to control YLS with foliar fungicides, and seed and fertiliser treatments are ineffective.

In response to the YLS challenge, the GRDC is investing in research to ultimately provide clearer guidelines and more resistant varieties to growers to reduce the damage from the disease. This includes:

  • a new project to more accurately quantify yield losses from YLS;
  • screening of National Variety Trials lines before commercial release;
  • identifying the optimal timing of fungicides – in a project close to completion; and
  • pre-breeding wheat with improved resistance to the disease.

Dr Manisha Shankar of the Department of Agriculture and Food, Western Australia (DAFWA), has reported that currently grown Australian wheat varieties carry only one known YLS resistance gene.

However, screening of new wheat populations developed by her team of researchers, along with breeding lines from wheat research centres across the globe, has recently revealed up to 32 wheat breeding lines containing good levels of broad-spectrum resistance to YLS.

Dr Shankar’s team has also sped up the process of screening for resistance through new glasshouse methods.

“The next steps include identifying additional YLS resistance genes as well as introducing resistance genes into elite Australian wheat breeding lines. The aim is to achieve better disease resistance in Australian varieties by ‘stacking’ genes in breeding lines for wheat breeders to use directly.”

If successful, the pre-breeding research could mean more Australian wheat varieties will have good YLS resistance in the long term.

For Victorian southern Mallee grower Cameron Barber, YLS is relatively low on his priority list. However, he selects varieties with resistance to the disease once other agronomic and disease ratings are met.

“Most varieties are susceptible to YLS. I’m growing MacePBR logo because it is very good in terms of its resistance to YLS and it is just as good as other varieties, agronomically,” Mr Barber says. “ScoutPBR logo is very susceptible to YLS and YitpiPBR logo is quite susceptible.”

Mr Barber will grow two to three successive wheat crops in some paddocks – particularly in dry seasons – to manage financial risk. While this raises the odds of YLS, he does use a range of strategies to reduce the disease likelihood.

“In addition to choosing a resistant variety, I have observed that sowing wheat between rows of standing stubble leads to less disease.

“I used to knock down the stubble to get through it to direct drill, but I found that the YLS is worse when the wheat has to grow through stubble that is lying down and completely covering the ground.”

He has also found that two-year-old stubble can spread YLS. “Even when two-year-old stubble is prickle-chained and brought to the surface, you can still get the disease,” he says.

Mr Barber says some of his crops near Rainbow “looked terrible” from YLS in 2012 and his agronomist advised him to spray fungicide, while his Birchip crops were less affected. However, he chose not to spray and the yield difference was around 150 kilograms per hectare, much of which he puts down to the drier finish at the Rainbow farm rather than YLS.

“Who knows how much damage it is doing. But from my experience, I think it is one of the lesser diseases to worry about.” If the researchers are successful, Mr Barber may be able to cross YLS off his priority list altogether.

 Wheat variety
YLS resistance rating
Table 1: Yellow leaf spot (YLS) resistance ratings for popular western region varieties
 Calingiri  MS
 Carnamah  MR-S
 EGA Bonnie RockPBR logo
 MR-MS
 MacePBR logo  MR-MS
 MagentaPBR logo  MR
 Stiletto  S-VS
 WyalkatchemPBR logo  MR
 YitpiPBR logo  S-VS
 Westonia  MS-S
  For more varieties, refer to DAFWA's Wheat Variety Guide for WA 2013.

S-VS = susceptible to very susceptible
MS-S = moderately susceptible
MR-MS = moderately resistance to moderately susceptible
MR = moderately resistant
    green = low risk
   orange = medium risk
   red = high risk
 Wheat variety
YLS resistance rating
Table 2: Yellow leaf spot (YLS) resistance ratings for popular southern region varieties
 AxePBR logo  S
 BolacPBR logo  S
 CorackPBR logo  MR#
 CorrellPBR logo
 S-VS
 DerrimutPBR logo  S
 EGA GregoryPBR logo
 MS-S
 Elmore CL PlusPBR logo
 S
 Grenade CL PlusPBR logo
 S
 Kord CL PlusPBR logo
 MS-S
 LongReach LincolnPBR logo
 MR-MS
 LivingstonPBR logo  MS
 MacePBR logo  MR-MS
 LongReach SpitfirePBR logo
 MS-S
 SuntopPBR logo (SUN595B)
 MS-S
 WallupPBR logo  MS-S
 YitpiPBR logo  S-VS
For more varieties, refer to your state's winter crop guide for 2013.

S= susceptible
S-VS = susceptibe to very susceptible
MS-S = moderately susceptible to susceptible
MS = moderately susceptible
MR-MS = moderately resistant to
moderately susceptible
MR = moderately resistant
# = variety may be more susceptible if alternative strains are present
   green = low risk
   orange = medium risk
   red = high risk

More information:

Dr Grant Hollaway
03 5362 2111

www.grdc.com.au/GCTV

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


Next:  Look beyond fungicides for yellow leaf spot control
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GRDC Project Code DAV000111, DAW00206

Region South