Powdery mildew – pathotypes and management

Take home messages

• Powdery mildew is a highly variable and ubiquitous pathogen of barley.
• Powdery mildew is essentially a disease of the vegetative stage in the Northern Region and seldom causes losses greater than 15%.
• Historically single gene resistances are readily overcome.
• The resistance gene mlo has remained effective for over 50 years in Europe but has not been widely adopted in Australian breeding programs.
• Fungicides usually provide very effective control.
• Resistance to fungicides in powdery mildew is well documented in Western Australia and recorded in eastern Australia.
• Best management fungicide practices are required to preserve their efficacy.

Epidemiology

Powdery mildew (Blumeria graminis f.sp. hordei) is a disease synonymous with barley cultivation world-wide. Under mild humid conditions, it will infect leaves and leaf sheaths of plants eventually covering them with white, fluffy mycelium and conidia. Colonies of mildew on the leaf extract nutrients from the plant and reduce the area of tissue contributing to photosynthesis. Seldom does the disease persist to adult stages in our environment and consequently losses in yield are usually less than 15%.

Powdery mildew survives between seasons on barley stubble and to a lesser extent on volunteer plants. In the latter part of the growing season as conditions become less favourable for the mildew, the pathogen undergoes sexual reproduction and forms fruiting bodies (cleistothecia) in existing colonies. These can be seen as small brown and black spheres which persist until the new growing season. When cleistothecia mature and under favourable conditions of temperature and moisture they release ascospores to infect the new crop. It takes less than a week from initial infection to the appearance of mildew colonies. These soon produce conidia (asexual spores) that spread the disease within and between crops.

Control

Unless a variety is very susceptible to powdery mildew it is unlikely that the disease will progress to upper leaves of adult plants. Nevertheless potential losses in yield from infection during early stem elongation may reach 15%. In susceptible varieties where yield potential is high, fungicidal control can be justified.

There are two options for control of powdery mildew – fungicides and host resistance. Fungicides can be applied as seed treatments, in-furrow treatments and/or as foliar sprays. Up The registered “up front” treatments will cost between $5 and $15 /ha depending on product and rate while a single foliar spray might cost $15 - $20 /ha. If fungicide is applied with a herbicide spray this would be considerably cheaper. Mildew is quite sensitive to most fungicides registered for control but there is a real risk of resistance developing in the disease if repeated applications of fungicides with the same mode of action are practised. This has already been demonstrated in Western Australia.

The better option is host resistance. Breeding for resistance to powdery mildew in Australia has a history of resistance breakdown. For example, Commander, Compass, La Trobe and Shepherd were all resistant when released; but changes in the powdery mildew population in recent years have rendered these susceptible. Chances are that the virulences responsible (MlLa and Mla3) were already present in the population at release and the widespread cultivation of these varieties has selected pathotypes carrying the specific virulence from the population. Genetic resistances need to be durable to provide long-term protection.

Current commercial varieties resistant to powdery mildew and their effective resistance genes follow - Fairview (Mla13), Flinders (Mla1), Granger (mlo), Oxford (St), RGT Planet (mlo), Scope (Mla7) and Westminster (mlo). Four of these are direct introductions from Europe and three of the four rely on the durable, single resistance gene mlo. This gene has remained effective in Europe for over 50 years and is the mainstay of resistance to powdery mildew there.

Australian breeders have been reluctant to use this gene as it has been associated with lower yields in the Australian environment. DAFQ is looking at a number of sources of mlo to try to detect lines where the gene does not have a negative impact on adaptation to our environment.

Pathotypes

A survey of Australia’s barley powdery mildew population was conducted by Dreiseitl et al in 2010 and 2011. They identified 27 pathotypes and noted that virulence for 16 major genes was not detected. Since then virulences on Mla3, Mla9, Mla12 and MlLa have been identified reducing the number of effective major resistance genes to 12.

As part of our commitment to the National Barley Foliar Pathogens Program (NBFPVIP), DAFQ conduct annual pathotype surveys of powdery mildew on barley. In the 2017 season, we pathotyped 84 isolates on 38 differential genotypes. A selection of the results are presented in Table 1. The isolates tested were virulent on up to 16 genotypes of the differential set. The most virulent pathotype was collected from the Lockyer Valley. Virulence for MlLa was collected from Gatton, Toowoomba and Jondaryan and is responsible for the increased levels of mildew occurring in varieties like Commander, Compass and La Trobe. As expected mlo (differential P22) was resistant to all isolates. No new virulences were detected in 2017.

Table 1. Infection types of selected isolates of powdery mildew on differential genotype sets. Shaded cells indicate virulence for the particular resistance gene.

We were able to source four isolates from Mt Pleasant, Tasmania (Isolates 32 and 33). The virulence profiles of all isolates contrasted markedly with most of the Queensland pathotypes. Where local isolates usually showed virulence on 12 to 16 differentials, the Tasmanian isolate was virulent on just three to four. This probably reflects the difference in the genetic diversity of barley varieties grown between the two states.

Conclusion

Continuous monitoring of the powdery mildew population provides intelligence on the virulences in the Australian powdery mildew population. This information guides the breeders in choice of sources of resistances and facilitates screening of breeding material with relevant virulences. There are still a range of major resistance genes effective against our powdery mildew population and increasing evidence of useful adult plant resistance genes. These need to be used in a manner that ensures the resistances will be durable. We know that mlo is both effective and durable and we encourage Australian breeders and pre-breeders to make it their prime source of resistance.

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.

Reference

Dreiseitl A and Platz G (2012) Powdery mildew resistance genes in barley varieties grown in Australia. Crop and Pasture Science 63, 997 – 1006.

Contact details

Greg Platz
Department of Agriculture and Fisheries
Hermitage Research Facility, 604 Yangan Road, Warwick QLD 4370
(07) 4542 6733 / 0408 733 055
Greg.Platz@daf.qld.gov.au

Lisle Snyman
Department of Agriculture and Fisheries
Hermitage Research Facility, 604 Yangan Road, Warwick QLD 4370
(07) 4542 6761 / 0408 733 055
Lisle.Snyman@daf.qld.gov.au

Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994