Managing barley and wheat diseases priority issues and actions for 2015

Author: Greg Platz and Ryan Fowler, Department of Agriculture and Fisheries, Queensland | Date: 31 Jul 2015

Take home message

  • Sowing resistant varieties is the best means of avoiding serious yield losses to disease.
  • Timely application of fungicides can be very profitable in susceptible varieties.
  • Keep abreast of NVT disease resistance ratings.  Changes in pathotypes in one growing season can have major implications for varietal selection in the following season.

Priority issues

  1. A new pathotype of wheat leaf rust is likely to arrive in 2015.
  2. New pathotypes of powdery mildew of barley in the Northern Region
  3. Barley leaf rust will increase
  4. Spot form of net blotch and yellow spot -  to spray or not to spray

Introduction

Foliar diseases pose a constant threat to the productivity and profitability of winter cereals in Queensland. Given a combination of susceptible varieties, virulent pathogens and favourable weather, diseases will infect crops and may develop into widespread epidemics.

Seasons that are favourable for good crop growth are also favourable for the development of diseases like mildews, rusts and leaf spots.  Consequently, when crops have high yield potential they are often exposed to high disease risk.  The major effect of foliar diseases is a reduction in the effective photosynthetic leaf area; therefore to ensure crops are able to reach their potential, these diseases must be kept to a minimum.

Disease epidemics are more the exception than the rule and most can be controlled by in-crop application of fungicides; however an integrated approach of appropriate crop rotations; varietal resistance and fungicides will give the best control.    Disease control by the application of foliar fungicides is an economic decision and can only be accurately determined by the availability of reliable yield loss data.  The impact of diseases on yield and quality varies with the severity of disease, the duration of the disease epidemic, the levels of resistance in the host variety and environment.  Under funding from GRDC, we have been investigating the response of varieties with different levels of resistance to the application of fungicides and to epidemics of different severities. The results of these trials might help guide your decisions on whether or not to apply foliar fungicides for disease control.

Trial results

Barley leaf rust (Puccinia hordei)

In 2013 trials, an epidemic of leaf rust in the very susceptible (VS) variety Grout caused a loss in yield of 32%. Shepherd  (moderately resistant (MR)) subjected to the same levels of initial inoculum recorded a non-significant loss of 6.1%.

In 2014, Grout lost 25.1% of its yield under a heavy epidemic while once again there was no significant difference in yield between the sprayed and unsprayed Shepherd treatments.  To our surprise, the newly released variety Compass suffered a 38.5% loss in yield under the same epidemic conditions. 

Fortunately, current levels of leaf rust inoculum in the region are low; however the popularity of Compass in the 2015 sowing season and its apparent susceptibility to leaf rust is likely to lead to an increase in the disease by the end of the season.

Trials in both years reinforced the principle that the best way to avoid losses in yield from rust is to sow resistant varieties; yet the application of fungicides to a susceptible variety can still be very profitable.  In 2013, a single fungicide spray increased the yield in Grout by 29% resulting in a net return of around $250/ha.

Spot form net blotch (Pyrenophora teres f. maculata)

Shepherd  barley is rated SVS to spot form of net blotch and the disease is often present in crops of this variety.  In 2014, under a moderate to heavy epidemic “disease free” plots out yielded diseased plots by 18.7%.  Two applications of a popular commercial fungicide gave a yield increase of 10.2% returning an extra $162/Ha. Failure of fungicides to completely control the disease in nil disease plots suggests losses could be even higher than those recorded.

Yellow spot (Pyrenophora tritici repentis)

As with other diseases the yield impact of yellow spot varies with varietal susceptibility. An epidemic of yellow spot in Kidman (S) reduced potential yield by 28.7% yet in the MR variety Leichhardt by only 10.1%.  A two spray strategy on the same varieties increased yields by 21.6% and 14.2% giving net returns of $167 and $70 respectively.

Changes in pathotype

Most of our major foliar diseases are populations of different strains or pathotypes of that particular disease.  When a variety that was once resistant becomes susceptible to a disease, it is usually a result of the disease gaining virulence for the particular resistance gene in that variety and we say the resistance has broken down.  The strain (pathotype) that has developed additional virulence is a new pathotype.

In Australia, the compositions of our winter cereal rust populations are well documented; but less so for mildews and leaf spot diseases.  With funding from GRDC, annual pathotype surveys of net form net blotch, spot form net blotch, powdery mildew, scald (Rhynchosporium commune)  and spot blotch (Cochliobolus sativus) are being conducted across Australia.  This allows early detection of new virulences in the disease populations and reporting of any changes in pathotypes before the new planting season.  In Queensland, we are focussing on net form net blotch and powdery mildew of barley and have detected several shifts in virulence in recent years.

Wheat leaf rust

In August 2014, a new pathotype of wheat leaf rust was detected in South Australia. This subsequently spread to Victoria and into New South Wales. It has been isolated from as far north as Narrabri, but as yet, not in Queensland. It is logical to assume it will arrive soon. The Australian Cereal Rust Control program has determined that this isolate is an exotic incursion and is virulent on several varieties that were resistant in 2014.  The varieties Baxter, Sunvale, Mitch  and Viking  are either S or SVS to this new pathotype and should be monitored for the disease.

Net form net blotch

Currently, our leading barley variety is Shepherd which was released in 2009 when it was resistant to powdery mildew and to the net form of net blotch.  In 2012, DAF detected virulence in powdery mildew for the resistance gene in Shepherd (Mla3) rendering the variety MSS to that disease.  We have also detected virulence in the Queensland net form net blotch population on Shepherd.

This is not an immediate threat as the pathotype(s) that carry this virulence exist at only a low level in the net form net blotch population.  However it does serve as a warning that increased levels of net form net blotch may be detected in the current and future growing seasons.  This will be more conspicuous where environmental conditions favour infection events and where Shepherd  is sown on the previous year’s Shepherd stubble.

Powdery mildew

Nationwide surveys of powdery mildew of barley were conducted in 2010 and 2011.  Those surveys identified a relatively simple population structure with apparently more virulences in the eastern states than in Western Australia.  This was not surprising as there has been relatively little breeding for resistance to powdery mildew in Australian breeding programs so the disease has not been forced to develop new virulences to survive.

Within 12 months of these surveys, three previously undetected virulences ( Va3, Va9, Va12) that were specific to resistance genes in Shepherd, Grout and Navigator respectively , were isolated in the Northern Region.  In 2014, virulence for the resistance gene MlLa was detected in northern NSW and Queensland. Varieties such as Commander, Compass and Hindmarsh are now more susceptible to the disease than previously (Table 1) and may become infected in the 2015 season.

Table 1. Comparison of seedling and adult responses of commercial barley varieties to established and new pathotypes of powdery mildew.

Shepherd pathotype

Shepherd pathotype

New MlLa pathotype

New MlLa pathotype

Variety

Seedling

Adult

Seedling

Adult

Commander

MSS

MRMS

VS

S

Compass

MSS

MS

VS

S

Granger

R

R

R

R

Grout

R

R

R

R

Hindmarsh

MS

MSS

VS

SVS

La Trobe

MS

MSS

VS

S

Mackay

MRMS

RMR

VS

MR

Oxford

R

R

R

R

Shepherd

VS

MS

MR

R

What does this mean for our barley growers?

Powdery mildew disease needs to be considered in context.  In the Northern Region, powdery mildew is unlikely to cause losses in yield in excess of 10-15% in susceptible varieties.  In crops with high yield potential, this can be significant and would justify fungicidal control.  Despite having detected powdery mildew virulent on seedlings of Shepherd, Grout  and Navigator the commercial impact to date has been minor.

Shepherd  crops on the Downs are commonly infected with powdery mildew; yet severe infections are rare.  We believe this is due to other minor resistances providing some protection.  We are not aware that any commercial crops of Grout have suffered from significant infection of powdery mildew and the area sown to Navigator in the North is still very small.

A greater risk is posed by pathotypes that can attack varieties carrying the MlLa resistance gene.  Commander and Compass  are sown throughout the region and are considered susceptible to strains carrying virulence for this gene.  Increasing levels of powdery mildew in either of these varieties would signal the need for application of fungicide.

Application of fungicides to control foliar diseases

It is now early August and the potential for diseases to become a problem in crops should be apparent.  If you are contemplating application of foliar fungicides, there are a few rules of thumb to follow.

  1. Foliar fungicides are far more effective as protectants than eradicants.
  2. Do the maths. The economics of chemical control are a function of potential yield, potential loss, cost of application and commodity prices.
  3. Most current commercial fungicides are very effective against mildews and rusts; but less so against leaf spots.
  4. Good control of leaf spots is reliant on application pre-infection.
  5. Application of commercial fungicides at recommended “full” rates can be expected to provide 3 – 4 weeks protection.
  6. Two applications (one at GS31-32 and a second at GS49) will maximize control in barley.
  7. Heed label warnings on fungicide resistance management

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

We thank Clayton Forknall of DAFQ for analysis of the trial data used in this paper.

Contact details

Greg Platz
Department of Agriculture and Fisheries
Hermitage Research Facility,
604 Yangan Road
Warwick Q 4370
Ph: 07 4660 3633
Fx: 07 4660 3600
Email: Greg.Platz@daf.qld.gov.au

GRDC Project code: DAQ00245; DUS0053