Increased levels of net form of net blotch in Commander and Compass

Take home message

• Pathotypes able to infect barley varieties Commander and Compass have increased in recent years.
• Prolonged and widespread cultivation of any barley variety will allow net form net blotch (NFNB) to adapt to that variety.
• Sowing barley on barley, especially successive crops of the same variety, increases the risk of NFNB and favours the selection of virulence in the pathogen for that variety.
• Environmental conditions play a major role in the development of NFNB, with wet conditions favouring infection and spread of the disease.
• NFNB can be spread via the sowing of infected seed that has not been treated.
• NFNB is best controlled by crop rotation, sowing of varieties with a disease rating of MS or better, treating seed prior to sowing, regular monitoring crops for NFNB and timely application of a registered foliar fungicide before disease becomes well established.

Background

In 2017 Fowler and Platz (Grains Research Update, Goondiwindi) provided a summary of the net form net blotch (NFNB) situation in barely in the Northern Region. They noted the increased prevalence of the disease on varieties such as Commander and Shepherd and attributed this increase to a selective advantage of the pathotypes occurring on those varieties. Widespread and long term cultivation of Commander and Shepherd had selected pathotypes virulent on those varieties and provided plentiful substrate for them to increase and spread. In seasons favourable for infection, net blotch was becoming increasingly conspicuous in many crops.

In the vegetative stages Commander is very susceptible (VS) to a range of NFNB pathotypes, yet the variety develops an apparent Adult Plant Resistance (APR) as it approaches heading. This is quite conspicuous with the common Skiff virulent isolate where the variety changes from VS as a seedling to MRMS (moderately resistant – moderately susceptible) as an adult. Compass, a derivative of Commander, demonstrates a similar response but is even more resistant to this pathotype as an adult.

DAFQ is funded by GRDC to conduct annual pathotype surveys to monitor virulences and changes in pathotypes occurring in the Australian NFNB population.

Pathotype determination

A pathotype is defined by the range of virulences that an isolate of a patghogen might possess and the range of virulences is identified by the responses of a suite of varieties to infection by that isolate. Currently we are using a suite of 30 varieties to differentiate virulence patterns (pathotypes) among isolates.

With the interest in NFNB from Commander, we have processed 15 isolates from 2017, 12 of which were collected from Commander (Table 1). From these 12 isolates we have identified at least 6 pathotypes. These all have virulence on Corvette, Compass, Commander, Harrington and Navigator in common, but vary in their virulence to Tallon and Maritime. Virulence on Kombar, Prior, Oxford and Urambie further differentiate among these isolates.

Table 1. Summary of net form net blotch infection types (IT) on seedlings of differential varieties. Shaded cells indicate susceptible response.

*IT = infection type

What does Table 1 tell us?

This data tells us that all NFNB isolates are well adapted to Commander and Compass at the seedling stage. It also tells us through the virulence on Corvette, that Commander would be susceptible as an adult – Compass less so. We know this from regular screening of those varieties with a Corvette virulent pathotype. Virulence on Corvette is not new and has been present in the population since at least 1995. The extensive cultivation of Commander has selected virulence from the population and then provided a compatible host for that virulence to increase.

Our pathotyping identified at least 6 pathotypes in 12 isolates from Commander. This demonstrates the variability that exists in the population. NFNB has a sexual reproduction stage; so there is continuous recombination occurring within the population. Should a popular variety have a different resistance, it is likely that over time that variety will select an isolate virulent on that resistance which will then increase in prevalence in the population.

From Table 1, isolate HRS17087 demonstrated virulence on Urambie which is rare; however the consequence of this virulence is minimal, as Urambie and its resistance are not common in Northern farming systems.

Virulence on Maritime is also of interest as it was not present in the virulence profiles of the 10 isolates reported in the 2017 Updates. This virulence is rare in Queensland but poses no immediate risk to our current varieties.

Conclusion

Commander must now be regarded as susceptible to NFNB. Pathotypes virulent on Commander have increased in the NFNB population to the point that they now dominate in the region. Compass is more resistant than Commander. Although Compass is quite susceptible prior to stem elongation, it develops APR which is more effective than Commander’s. Data from the DAFQ National Variety Trials confirm this. NFNB nurseries conducted from 2014 to 2017 across 3 pathotypes show Commander at an average rating of MSS (moderately susceptible to susceptible) and Compass at MRMS. When screened with the Corvette virulent pathotype, Commander returned an average rating of S (susceptible) and Compass MS (moderately susceptible).

This does not suggest that growers should sow Compass instead of Commander. It does mean that growers who plan on sowing Commander need to consider the risk of growing the variety and implement an integrated management strategy to minimize infection from NFNB.

• Treat seed if there is a risk of NFNB infection
• Avoid sowing into or near barley stubble
• Monitor the development of NFNB in crops and
• Apply fungicide before the disease becomes established.

Seed-borne net blotch

Infected seed is often overlooked as a medium for the initiation and spread of NFNB. During our presentations at the 2017 Update, we offered a seed testing service to determine the incidence of NFNB in seed for sowing. We received 20 samples from 3 agronomists. Results of 13 of those samples are given below. Seed infection by Drechslera teres (SFNB or NFNB) varied from 0 to 70% (Table 2).

NFNB is known to be transmitted to seedlings from infected seed. Where seed infection levels are high in Commander, one could expect high levels of net blotch in an emerging crop at a stage when the variety is very susceptible. SFNB can be seed-borne; yet it is reported not to infect emerging seedlings.

Table 2. Percentage Drechslera teres infection in grain samples from 2016 harvest.

*Dtm = spot form net blotch
^#Dtt = net form net blotch

From 13 samples listed in Table 2, Drechslera teres f. teres (Dtt = NFNB) was present in 5; D. t. f. maculata (Dtm) was detected in 4 samples, with both Dtt and Dtm present in one sample. Three samples showed no infection.

What does this mean?

The five samples that were positive for Dtt had average infection levels of just over 50%. Crops that produced this seed would have had conspicuous NFNB infection and wet periods at or around flowering. Given ideal conditions after planting, every second seed could potentially produce an infected seedling. Assuming a grower targets one million plants per hectare on a 40cm row spacing, potentially twenty seedlings per metre of row could emerge infected with NFNB. This would be more than adequate to initiate an epidemic.

As stated above, SFNB does infect seed but reportedly is not transmitted to the seedling. Our observations would confirm this; but it does need further verification.

Options for control

There are two options for control - source clean planting seed or treat infected seed. If planting seed was harvested from a crop that had conspicuous NFNB and there were rainfall periods around flowering, then it is likely to have high levels of infection. This can be determined in the laboratory any time after harvest. Where infection levels are at or above 50% alternative planting seed should be considered.

Several fungicides are registered for control of seed-borne NFNB – Systiva®, Vibrance®, Dividend® and Vitavax 200FF®. In instances where you suspect seed may be infected, treatment with any of these products should ensure that seedlings emerge free from NFNB.

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.

References

Fowler R and Platz G (2017) Increased levels of net form of net blotch in Commander and Shepherd.

Youcef-Benkada M, Bendahmane BS, SY AA, Barrault G and Albertini L (1994) Effects of inoculation of barley inflorescences with Drechslera teres upon the location of seed-borne inoculum and its transmission to seedlings as modified by temperature and moisture. Plant Pathology 43, 350-355.

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

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