Barley diseases update: Pathotype surveys of powdery mildew and net form net blotch and implications for management

Author: Greg Platz and Ryan Fowler (DAF Queensland) | Date: 01 Mar 2016

Take home message

Disease populations are in a constant state of change – change that is often directed by resistance genes in commercial varieties.

Annual pathotype surveys are the best means of monitoring the presence of existing virulences and detecting the evolution of new virulences in disease populations.

Keep abreast of significant changes in disease populations that may alter the resistance ratings of commercial varieties to plan appropriate management strategies.

Introduction

Powdery mildew (Blumeria graminis f. sp. hordei) and net form of net blotch (Pyrenophora teres f. teres) of barley (Hordeum vulgare) are omnipresent throughout the Northern Region.  In susceptible varieties, losses of up to 13% and 47% respectively have been recorded. Varietal resistance remains the best means of control; however few Australian varieties have maintained their resistance to either disease for extended periods.  

One of the best examples of applied pathology in overcoming a major disease problem is our own Australian Cereal Rust Control Program and its predecessors in combatting the stem rust (Puccinia graminis f.sp tritici) scourge in northern Australia. Over the past 50 years, wheat stem rust has declined from a major disease to the point where it is difficult to find in the region. This has been achieved by:

  • Widespread adoption of resistant varieties - Resistant varieties deny the disease a host and inoculum levels plummet.
  • Reduction in inoculum levels - Whether through elimination of the green bridge or removal of susceptible varieties from the cropping system, inoculum survival and increase is greatly reduced.
  • The use of combinations of resistance genes - Most fungal diseases will mutate or recombine to develop new virulences. It is much easier for a fungus to overcome a single resistance gene than several genes in combination and
  • Annual pathotype surveys - By knowing what virulences are present in a disease population we are able to target resistances effective against those virulences and alert industry as to what resistances may be at risk.

If this strategy worked for wheat stem rust – one of the most voracious and damaging crop diseases - why then should it not prevail for other crop diseases?

Pathotype surveys

Pathotype surveys involve appropriate sampling of a disease population by the collection of individual isolates and then determining the virulence combinations of those isolates, by inoculating them onto a range of varieties or lines with known and /or different resistances (differential varieties). For example, the varieties Beecher, Prior, Skiff and Shepherd are key differentials in identifying pathotypes of net form net blotch (NFNB) in Australia. If any of these varieties are shown to be susceptible to an isolate collected in a survey, then we know the isolate has virulence for the resistance in that variety. It may be virulent on one or several varieties which not only defines pathotype but also tells us how virulent that particular isolate is.

Both powdery mildew (PM) and NFNB reproduce sexually on crop stubble, resulting in rearrangement of their genetic material and the potential for new virulences to evolve. It is therefore imperative that surveys are conducted annually and that differential varieties are reviewed regularly to reflect their relevance to commercial production.

Results of pathotype surveys

There have been a number of state based surveys of PM and NFNB in the past; but these now have a national focus as integral components of the GRDC funded Barley Foliar Pathogens Project.  Researchers at Hermitage Research Facility conduct surveys for both PM and NFNB.

The initial PM survey (Dreiseitl et al. 2013), conducted in  2010 and 2011, identified 27 pathotypes with virulence on just eight PM resistance genes – a relatively simple population.  Since then, virulences on Shepherd (Mla3 2012), Grout (Mla9 2012), Navigator (Mla12 2012) and Hindmarsh (MlLa 2014) have been detected in eastern Australia. The resistance gene MlLa contributes to PM resistance in several popular varieties.  The changes in resistance ratings of some of those varieties to the “new” virulence are given in Table 1.

Table 1. Comparison of resistance ratings of varieties carrying MlLa resistance before and after detection of MlLa virulence.

Postulated resistance gene(s)

Field 2013

 MlLa avirulent

Field 2014

MlLa virulent

Field 2015

MlLa virulent

Mid-elongation

Tillering

Adult

Baudin

Mla8

VS

VS

VS

SVS

Commander

Mlg, MlLa

MRMS

MSS

MSS

MS

Compass

Mlg, MlLa

MS

S

MSS

MRMS

Hindmarsh

Mla8, MlLa

MRMS

SVS

MSS

MSS

La Trobe

Mla8, MlLa

MS

S

MSS

S

Mackay

Mlg, MlLa, +

MR

MR

MR

R

Oxford

Ml(St)

R

R

R

R

Pathotype determinations are usually made using seedlings. It is important to highlight that virulence on seedling resistances does not automatically imply that a variety carrying that resistance will be susceptible at adult stages. Our program now screens lines at both the vegetative and adult stages to determine resistance status at both growth stages.

The NFNB survey identified at least 15 significant pathotypes which grouped into families aligned to virulence on the varieties Beecher, Prior or Skiff.  Skiff virulence is dominant in the Northern region; yet virulence on Shepherd and Prior is frequently detected. Virulence on Beecher has not been detected in this region; but is common in Western Australia.

Implications for management

Both diseases are best managed through an integrated approach of sowing resistant varieties; adopting rotations that avoid sowing barley on barley and the use of fungicides if resistance is not available.

Powdery mildew

PM is mostly a disease of the vegetative stage occurring between early tillering and mid elongation. Few varieties appear susceptible after flowering. This suggests that yield should not be seriously impacted by the disease and we have been unable to demonstrate yield losses in excess of 13.4%; yet in high yielding crops this can be significant. It also warns that powdery mildew control should be implemented early in crop development.

Fungicides can be applied up-front as seed treatments or on fertilizer in furrow. This will provide up to 8 weeks protection around which time crops should be breaking into head.

Foliar fungicides are very effective; but need to be applied early in epidemic development as PM can increase rapidly. In most cases a single application should be sufficient; but in favourable conditions a second spray may be warranted.

Net form net blotch

Both PM and NFNB are airborne diseases but PM is much more mobile and a more prolific sporulator than NFNB; therefore PM can spread farther, quicker than NFNB. However, NFNB can be seed-borne which has major implications for disease management.

Skiff virulent pathotypes dominate in the northern region and have done so since the late 1990’s. The cultivation of varieties susceptible to these pathotypes allows them to survive and increase. Varieties resistant to the Skiff virulence have been released in recent years; yet some of these are susceptible to other virulences that are already present in the population. 

One such variety is Shepherd which is resistant to the dominant Skiff pathotype but very susceptible to pathotypes with the Shepherd virulence. In a recent survey, Shepherd virulence was present in about 10% of isolates; but it is an increasing component of the NFNB population. In 2015, we detected two crops of Shepherd on the Darling Downs that had severe NFNB and the disease was present at much lower levels in many other crops of this variety.

Why were these crops heavily infected? It would appear that grower retained seed infected with the disease was sown early providing conditions very favourable for disease development.

This example is not a recommendation NOT to sow Shepherd. Shepherd can continue to be grown successfully with appropriate management.

  1. Use clean seed

If in doubt treat seed with registered seed treatment.

  1. Do not sow Shepherd on Shepherd
  2. Monitor crops at risk
  3. Apply fungicides before the epidemic is well established.

Similarly, our most popular malting variety Commander is resistant to the Skiff pathotypes, but susceptible to the Prior virulence. If these and similar varieties remain in our cropping system for an extended period it is likely that the minor pathotypes will increase and the current dominant pathotypes will decrease.

Conclusion

Pathotype surveys provide the intelligence necessary to make reliable varietal recommendations, to select effective resistances in breeding new varieties, to choose disease isolates appropriate for effective screening and to assess risk in the management of plant disease.

Surveys must be at least regionally based and ongoing to keep abreast of changes in disease populations as the evolution of new virulences can render resistant varieties susceptible and recent information obsolete.

Reference

Dreiseitl A, Fowler RA and Platz GJ (2013)  Pathogenicity of Blumeria graminis f. sp. hordei in Australia in 2010 and 2011. Australasian Plant Pathol. 42:713–721.

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 authors would like to thank them for their continued support.

Contact details

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

Ryan Fowler
Department of Agriculture and Fisheries Queensland
Hermitage Research Facility
604 Yangan Road, Warwick Qld 4370
Ph: 07 4660 3665
Fx: 07 4660 3600
Email: Ryan.Fowler@daf.qld.gov.au

GRDC Project code: DAQ00187, DAQ00189