Ramularia is a disease of barley that manifests itself as reddish brown lesions on the leaves, awns and stems of the plant.
Until the mid-1990s it had not been a known concern for Irish barley crops.
Since then, it has been ever present and it has the potential to cause significant reductions in both grain yield and quality.
Fortunately, apart from a small number of cases, its destructive potential has rarely been realised in Irish crops. This was largely thanks to the availability of effective fungicides, and chlorothalonil in particular.
However, the use of chlorothalonil has been prohibited in the EU since 20 May last year and we now enter a new era without this fungicide. So, there are many questions as to how we will continue to control ramularia, or indeed if we can.
Alternatives to chemistry
The simplest response is to ask what chemistry or fungicide we can replace chlorothalonil with. However, as with every decision relating to pesticide usage, we should first ask what tools should be considered to eliminate, reduce or alleviate its impact before we take the decision to apply a fungicide.
In simplest terms we must take an integrated or integrated pest management (IPM) approach. So, what does this look like and how effective will it be?
Firstly, IPM is a knowledge-based process. The more we know about a pest the greater our capacity to put measures in place that limit its impact prior to chemical intervention. But ramularia presents many challenges in this regard.
The more we know about a pest the greater our capacity to put measures in place that limit its impact prior to chemical intervention
Given that it is a relatively new disease and generally only significant in wetter climates, such as Ireland, the overall knowledge base for ramularia is limited.
Understanding the disease
We have some ideas as to inoculum sources, triggers of disease development and potential impacts, each of which are described later, but this is only scratching the surface relative to what we know about our other major diseases.
In the chlorothalonil era, the disease was only seen if a breakdown in control occurred. Pinpointing the exact reason for this is difficult and can often be related to either the incorrect timing or application of chlorothalonil.
However, there were plenty of barley crops over the years in similar situations which did not suffer the devastating effects of ramularia.
Being able to understand these would be even more informative. However, the weather between stem extension and flowering was probably the biggest single factor in these crops and we can’t change that.
However, we can do things that can exacerbate the disease and these are the things we can manage.
Act to prevent or minimise
The first principle of IPM is to prevent the pest – ramularia. This means preventing the initial infection or preventing the disease from taking hold. The latter mainly relates to varietal resistance, but the science of this is only in its infancy with ramularia.
The evaluation of new varieties for recommendation aims to ascertain which weakness and strengths are present. Because local environment is highly influential in such pressures, varieties are trialled in different locations. Because ramularia is extremely dependent on local environment, plant breeders are uncertain as to whether a lack of disease is due to resistance, or an artefact of the trialling process.
In recent years, the Department of Agriculture started including ramularia as a key disease in its barley variety evaluation trials and these, in time, will provide an extremely valuable resource for Irish growers.
Because of the influence of location on disease development, these trials may struggle to separate varieties that have a five, six or even a seven for resistance, but we can have confidence that extremely weak varieties will be identified early in the process.
If varieties remain limited in their capacity to resist the disease in the short term, can we prevent it from getting into the crop? Short answer – probably not (at least for the moment).
The potential to use disease-free seed has been suggested, as we can readily detect the DNA of ramularia in seed. In a recent experiment we screened a sample of barley from all seed batches submitted for seed certification.
The amounts of DNA in the various batches differed, but most samples had detectable levels of the disease in the three assessment seasons.
However, it is thought that seed is unlikely to be the only entry pathway for this disease. The pathogen can be readily detected in debris and stubble and is also known to infect numerous grasses.
Further research is needed to help identify the specific role of seed in disease development. The real question is, “can we actually produce ramularia-free seed”? A chicken-and-egg type of question.
One thing we do know about ramularia is that stressed crops get more disease. Stressed crops can arise through water logging, drought, fluctuating temperatures, prolonged periods of leaf wetness post stem extension, nutrient deficiencies, other diseases, “hot” chemical mixes, etc.
So can we prevent these from happening?
Clearly, we cannot prevent those that relate specifically to weather. However, you will know the parts of your farm that are more likely to carry surplus water during the winter months and this is a risk factor for winter barley.
Likewise, for droughts in early April (they can happen) – a period of no rain followed by a prolonged wet spell can lead to serious levels of infection. Avoid “hot” tank mixes as this is a massive stress point on the crop, especially in fluctuating temperatures in early April.
The final aspect of IPM is chemical intervention – is it required? Again, ramularia makes decisions difficult. The disease often only develops symptoms late in the season and treatment after you see disease will be of limited benefit. Treatment needs to be preventative – before disease onset.
This is where disease forecasting would help but this is only in its infancy. So, until we can accurately forecast the disease, we are left with the simple strategy of applying a fungicide during booting to ensure we achieve upper canopy protection during grain fill.
Having developed resistance to the strobilurins relatively quickly following their introduction, it has since developed resistance to the SDHIs and most azoles, prothioconazole included.
Revysol, the new azole from BASF, is a step up on its azole cousins and provides the best disease control of all available fungicides at the moment. While folpet will provide moderate control, it requires help from a partner fungicide.
It is important to remember that grain filling is often over a shorter period in barley
Now that chlorothalonil is no longer an option for ramularia control, we must look at the entire fungicide programme (indeed, the entire disease control programme) and build it with ramularia in mind. What other diseases might need to be covered earlier in the programme, will they impact ramularia control, or can the fungicide(s) used add to ramularia control?
While the Irish ramularia population is dominated by types resistant to the various fungicide families, most fungicides still provide a nominal/moderate level of control and we should maximise this as much as possible. It is important to remember that grain filling is often over a shorter period in barley. So if we can maximise the grain filling period and limit brackling, ramularia at the end of the season will have limited impact on grain yield and quality.