What we learnt in 2016 about scald and other wet season diseases

Take home messages

  • Disease epidemics are a function of presence of a virulent pathogen, availability of a susceptible host and environmental conditions favourable to the pathogen.
  • Environmental conditions that are optimal for a particular pathogen signal potential for that pathogen to develop into an epidemic.
  • Foliar diseases have a propensity for rapid increase where there are widespread sowings of susceptible varieties and environmental conditions are favourable for pathogen infection, sporulation and dissemination.
  • Mild, wet seasons should trigger more frequent crop monitoring to detect unexpected increases in disease.
  • Good disease control can usually be achieved by application of an appropriate fungicide before diseases become well established.

Background

Scald (Rhynchosporium commune) and other wet weather diseases like Fusarium head blight (Fusarium spp.) and white grain (Eutariosporella spp.) are considered minor diseases of the traditional GRDC northern region (north of Dubbo). Minor because the annual economic impacts of these diseases when averaged over time, is relatively minor. However in seasons when environmental and epidemic conditions are particularly favourable for these diseases, their economic impact can be quite serious.

Scald has been detected at levels that reduce yield as far north as Toowoomba but this is a rare occurrence. It was present at damaging levels in several crops in the Tamworth area in 2003 and at interest levels in 2005 and 2006. It is essentially a disease of barley in southern and Western Australia where ambient temperatures and frequent rainfall events favour the disease.

Both head blights were recorded but in Queensland were mostly confined to the western Downs.  They did not reach epidemic proportions in 2016.

As scald was the most conspicuous of the minor diseases to pose a problem in 2016, this paper will focus on that disease to demonstrate the principles of why a minor disease can become a major disease in some areas and seasons and what can be done to prevent such diseases becoming a similar problem in future seasons.

Why did scald become a problem in 2016?

Three factors are essential for any crop disease to occur:

  1. A susceptible host
  2. A virulent pathogen and
  3. A favourable environment.

In plant pathology this trinity is often referred to as the disease triangle which encapsulates the basics of disease epidemiology. If we dissect the scald “epidemic” in 2016 in terms of the disease triangle, it will provide a better understanding of why it reappeared as a problem last season and forewarn us of the potential for it and other diseases to create epidemics in future seasons.

Susceptible host

Most varieties of barley developed for the northern region are susceptible (S) to very susceptible (VS) to scald, because resistance to the disease in this region is not seen as a breeding priority.  This not only provides a susceptible host in areas where scald might over-season and infect any one variety; but it also provides large areas of other susceptible varieties, making an easy target for spores to be deposited and infect. Therefore a high proportion of spores released by the pathogen will find a target host, infect and establish the disease in new areas.

Virulent pathogen

Scald is a highly variable fungus and many pathotypes have been identified. It is claimed that numerous pathotypes can be isolated from just one square metre of infected crop.  Consequently, virulent pathotypes are omnipresent.

The pathogen survives on barley stubble, on barley grass (Hordeum leporinum, H. glaucum) and can be seed-borne. Barley stubble is most likely the major source of inoculum in 2016. The disease has probably been present in crops at low levels in most seasons and has persisted over summers on crop residues.  Winter environments in recent years have not suited the development and spread of scald in crop; so it has persisted at only low levels. It is unlikely that seed-borne infection played a major role in the recent epidemic.

Once infection occurs scald can proliferate at an alarming rate. A single scald lesion can produce up to 1 million conidia (Mathre 1997).

The over-seasoning phases have implications for future disease management. Stubble from crops of barley, heavily infected in 2016, will be a major source of inoculum in 2017. Stubble could be removed but if this is not an option, do not sow barley back into those paddocks for at least 2 seasons. Stubble has been shown to support sporulation over a 10 month period.

Furthermore if growers harvested seed from heavily infected crops in 2016 intending to save some for planting seed in 2017, it is recommended that they either buy in seed from non-infected crops or alternatively treat seed with a recommended fungicide. Transmission of scald from infected seed to seedlings can be as high as 86%.

Favourable environment

The authors believe that this was the key to the 2016 scald epidemic. Scald requires free moisture for sporulation and infection and relies on rain splash to move spores up the plant and within the crop. Frequent rain periods therefore promote sporulation, disseminate conidia and favour infection. These conditions also promote crop growth; so that in dense crops leaf tissue can remain wet for 24 hours per day. Serious losses to scald occur in seasons with frequent rain (Wallwork 2000).

The optimal temperature for spore production is 15 – 20°C which also favours infection. No doubt environmental conditions that promoted spore production, spore dissemination and infection occurred repeatedly in the problem areas in 2016 and played a major role in the development of epidemics.

Control

Unusually wet weather should trigger alarm bells for disease control. If you are in an area that has experienced diseases like scald before, then be suspicious that these diseases may reappear. Monitor crops for the presence of the regular diseases but also with a purpose to detect other diseases that have appeared in previous wet seasons.

Be proactive! Usually minor diseases do not command routine procedures to control the disease. Although once detected in a favourable season, they demand close monitoring and timely fungicide intervention to minimize yield losses. Foliar fungicides are quite effective on scald and will give protection for 3-4 weeks depending on product and rate applied. In a season like 2016, application of fungicide at GS31-32 and again at GS39-41 may have been required to give an adequate level of control.

Conclusion

It has been over a decade since scald occurred at damaging levels in the northern region, north of Dubbo. While the 2016 experience is unlikely to be a frequent occurrence, it is a reminder that given the right conditions, minor diseases can rapidly increase and cause significant yield loss.

The reappearance of scald was a “new” experience for many growers and agronomists. Consequently, it was easy to overlook the disease early in the season and later to underestimate the potential for epidemic increase. By the time scald was recognized as an issue, the optimal time for fungicide application had passed. No doubt the disease reduced yield in heavily infected crops.

So what did we learn in 2016?

Minor diseases can become major diseases when very susceptible varieties are grown under environmental conditions very favourable to the disease. Unseasonal wet weather should signal “look out” warnings. Look out for the common diseases favoured by wet weather but also look out for diseases that are less common and have been a problem in the past. This demands more frequent - and more careful monitoring to detect the diseases less encountered. If in doubt, consult your agronomist or regional pathologist for assistance in the identification of “different” leaf, head or stem symptoms and what options are available should control be warranted. Where application of foliar fungicides is one of those options, spraying too early is much better than spraying too late. 

References

Mathre DE (1997) Compendium of barley diseases pp 45-47. American Phytopathological Society, St. Paul, Minnesota.

Thynne, E., McDonald, M.C., Evans, M. et al. (2015) Re-classification of the causal agent of white grain disorder on wheat as three separate species of Eutiarosporella.

Australasian Plant Pathol. (2015) 44: 527

Wallwork, Hugh (2000) Cereal leaf and stem diseases pp70 -73.

Acknowledgements

The authors thank Dr Steve Simpfendorfer (NSW DPI) for his advice and Dr Dante Adorada (USQ) for information on head blight diagnoses.

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 thank them for their continued support.

Contact details

Greg Platz, Lisle Snyman & Ryan Fowler
Dept of Agriculture and Fisheries
604 Yangan Rd, Warwick QLD

Greg Platz
Ph: 0408 733 055
Email: Greg.Platz@daf.qld.gov.au

Lisle Snyman
Ph: 0428 324 932
Email: Lisle.Snyman@daf.qld.gov.au

Ryan Fowler

Ph: 0433 406 669
Email: Ryan.Fowler@daf.qld.gov.au

GRDC Project code: DAQ00187