Virulence in net form of net blotch in barley

Take home messages

• Continuous barley cropping increases the risk of net form of net blotch (NFNB)
• The NFNB pathogen is seed-and stubble-borne
• NFNB is best controlled by using resistant varieties (rated MS or better), crop rotation, seed treatment, regular crop monitoring and timely fungicide application
• Virulences are dynamic and fluctuate in response to available host genotypes
• No new virulences were identified in 2018.

Background

Net blotch in barley is caused by one of two forms of Pyrenophora teres (P. teres). Net form of net blotch (NFNB) is caused by P. teres f. teres (Ptt) and spot form of net blotch (SFNB) is caused by P. teres f. maculata (Ptm). The two forms are morphologically identical and can only be distinguished by symptoms and molecular characterisation (McLean et al. 2009). Symptoms of NFNB are characterised by net like dark brown necrotic lesions, whereas SFNB symptoms are characterised by dark circular or elliptic brown spots surrounded by a yellow chlorotic area.

NFNB occurs regularly in the northern region (NR) and samples are collected from crops on an annual basis. Thirty-three NFNB samples were collected and pathotyped in QLD and NSW in 2016 and 35 in 2017. However, with the dry conditions, only a single NFNB isolate was submitted for pathotyping in 2018. That isolate was collected from a research trial with no samples being received from commercial crops during that year.

DAF is funded by GRDC through the national barley foliar pathogen variety improvement program to conduct annual pathotype surveys to monitor pathotypes and pathotype changes in the NFNB population in Australia. Results have been reported at the GRDC updates in both 2017 and 2018.

Pathotype determination

The virulence of a pathotype is determined by its responses on a suite of varieties. Currently a set of 30 varieties is used to differentiate pathotypes.

Previous research has identified four distinct groups of isolates across Australia. Collectively these isolates have the ability to infect most varieties. The level of disease development will however depend on the isolate/variety combination. The four groups are assigned on the basis of varieties that they are virulent on, resulting in a susceptible infection type.

Group 1: Binalong, Cowabbie, Fairview, Grimmet, Skiff, Tantangara, Yambla

Group 2: Gilbert, Grimmet

Group 3: Commander, Grout, Keel, Mackay, Navigator, Prior

Group 4: Beecher, Maritime, Roe.

Pathotypes from Isolate Groups 1, 2 and 3 are commonly present in the northern region.

Pathotype survey results suggest that the cultivation of regionally adapted varieties result in the evolution of P. teres f. teres, with increased virulence for the resistance profiles in the particular varieties, which in turn lead to higher incidence and severity of NFNB infection. This was clearly demonstrated with an increased prevalence of pathotypes virulent on the varieties Commander and Shepherd. Pathotypes able to infect these varieties were earlier considered to be rare. Compass, a derivative of Commander demonstrates a similar response to NFNB infection at the seedling stage, however develops adult plant resistance (APR) after stem elongation. Commander is now regarded as susceptible. This was communicated at the Grains Research Updates in Goondiwindi in 2017 and 2018. The continued cultivation of these varieties provides a selective host for virulent pathotypes to increase.

In 2017 isolates virulent on Maritime and on the variety Urambie were identified. These isolates are not expected to pose a threat to northern region varieties. Virulence was detected to all genotypes tested, except Clho 5791 indicating that this genotype carries effective resistance against all current Australian isolates. No isolates virulent on Algerian and Vlamingh were detected in the northern region in 2017. All isolates were virulent on the genotypes Corvette, Gilbert, Harrington, Commander, Compass, Bass and Navigator. Isolates virulent on Oxford were identified from both the northern region and WA in 2017. Out of 16 isolates from WA pathotyped in 2018, five were highly virulent on Oxford. This variety is not widely grown in the northern region. Virulence on Beecher, common in WA (not present in the northern region), was detected in a single WA isolate in 2017 and none in 2018.

Environmental conditions play a major role in the development of NFNB. Disease development and infection is favoured by frequent wet periods and mild temperatures. As mentioned before, no isolates were collected in the NR in the 2018 season due to the dry conditions.

Disease management

NFNB is best controlled by sowing varieties rated MS or better to NFNB and a combination of cultural practices.

Resistant varieties

All current barley varieties and varieties considered for release are rated for resistance to a suite of diseases and pathogens through the National Variety Trial disease screening process. They are categorised in 9 resistance categories rating from resistant (R) to very susceptible (VS). These genotypes are screened annually in nationwide disease nurseries, with disease ratings assigned and reviewed on a yearly basis. Growing a high yielding well adapted resistant variety provides the most economic and environmentally friendly means of disease control. Information on resistance ratings are available in the crop variety sowing guides for QLD and NSW.

Stubble management and crop rotation

The NFNB pathogen persists on plant residue.  The adoption of stubble retention practices has led to an increase in the incidence of NFNB. Planting successive barley crops in the same paddock increases the incidence of NFNB and cultivation of the same variety will lead to an increase in the presence of pathotypes virulent on that particular variety and put increased pressure on effective resistance genes. Best practice includes crop rotation with non-host crops such as wheat, canola and chickpea.

Seed treatment

The NFNB pathogen is seed-borne and can spread with infected seed. Ensure seed is treated adequately. Various seed treatment products are registered for the control of NFNB.

Fungicide application

Foliar fungicides are applied routinely in most barley crops and should be aimed at protecting the top two leaf layers. NFNB isolates with reduced sensitivity to demethylase (DMI) fungicides have been identified in WA. All isolates tested from NSW and QLD thus far were sensitive to fungicides. To ensure that fungicides remain effective, it is important to limit fungicide application by spraying only when necessary, rotate fungicides with different modes of action and use fungicides at recommended rates. Avoid using tebuconazole as a stand-alone product in barley for scald or powdery mildew to avoid indirect fungicide resistance selection. By applying it for control of one of these diseases, you can indirectly select for NFNB or SFNB isolates resistant to tebuconazole without the intention of controlling those diseases. Isolates resistant to fungicides can be spread through infected seed. It is beneficial to all to ensure that we use fungicides in such a way that we protect their longevity.

Crop monitoring

Fungicide applications are more effective if applied before NFNB becomes established in the crop. This requires regular monitoring to ensure crops can be sprayed at the first sign of disease. When conditions are favourable for disease development, more frequent crop inspections will be needed and repeat fungicide applications may be necessary.

The absence of NFNB in 2018 in the northern region does not mean that we can get complacent. With the right environmental conditions, the pathogen will continue to cause yield and quality loss and we have to make the right decisions to ensure that we can stay ahead of disease development and the evolution of the pathogen.

References

2019 Queensland winter crop variety sowing guide

2018 Winter crop variety sowing guide NSW

McLean MS, Howlett BJ & Hollaway GJ, (2009) Epidemiology and control of spot form of net blotch (Pyrenophora teres f. maculata) on barley: a review. Crop and Pasture Science 60,303-315

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.

Contact details

Lislé Snyman
DAF QLD
Hermitage Research Facility, 604 Yangan Rd, Warwick, Qld
Ph: 07 4542 6761
Email: lisle.snyman@daf.qld.gov.au

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

GRDC code: DAQ00187