Course: Cereal diseases | Last Updates: 1st June 2017
Ramularia is a fungal disease that can affect barley across much of the UK, causing damage particularly to the upper leaves after flowering, leading up to 1t/ha yield loss.
It has only been officially recognised as a disease in the UK for 19 years and so research into risk factors and resistance is still very much ongoing.
As the pathogen is largely seed-borne, there are concerns that the disease could not only become more widespread in the UK, but also in the major barley growing regions of the world.
Ramularia is caused by the fungal pathogen, Ramularia collo-cygni – so named because of its distinctive swan neck shape – but is more commonly known as ramularia leaf spot or RLS.
Though the pathogen was not positively identified in the UK until 1998, there is DNA and anecdotal evidence that suggests it has been around a lot longer.
Originally recorded in 1893 in Italy, the disease has since been recognised as an important issue across Europe, North and South America, New Zealand and Australia.
Ramularia collo-cygni has a complex lifecycle, but is largely seed-borne, while spore movement is detected in late summer and into autumn in the UK.
The pathogen also produces air-borne spores from volunteer cereals and grasses.
A second fungal body, asteromella, exists in the straw stubble. Although the importance of this on subsequent infection and epidemics is still under investigation, it could potentially infect a following barley crop.
Ramularia is a polycyclic disease, with seed acting as the main source of infection, while air-borne spores can cause a secondary infection.
The pathogen grows with the plant asymptomatically – or invisibly – as an endophyte until the plant becomes stressed or until flowering growth stages, around growth stage 60-69.
Stresses that may cause an outbreak include nutritional deficiencies, adverse weather such as waterlogging and leaf scorch. Spores may be observed on the lower leaves as they die back at these early growth stages.
After growth stages 69-72 – the end of the flowering stage – ramularia can appear on the top leaves, with exposure to sunlight a major influence on symptoms developing.
Therefore, more prostrate varieties with a thicker canopy can show fewer symptoms on lower leaves. Flowering acts as a natural stress factor, which contributes to the appearance of the disease.
The first visible stage of the disease appears as irregular pepper spots on both sides of the leaves. These then develop into rectangular, reddish-brown lesions (2×0.5mm) surrounded by a yellow (chlorotic) ring.
Necorisis coalesces until leaves die back, losing green area and turning brown/yellow. Symptoms can also appear on the awns and leaf sheaths.
The fungal infection within the leaf exudes a number of toxins, including rubellin D, which damages the plants cells, reducing both photosynthetic area, yield and quality.
Though the fungus is more prevalent throughout the lower leaves of the plant, rubellin D is triggered by light, meaning the upper leaves are more likely be damaged by the pathogen.
There have been issues with the correct identification of ramularia as it can often be mistaken for net blotch – which differs from ramularia as it extends down the length of the leaf blade.
The four Rs, as outlined by James Brown of the John Innes Institute will help with the correct identification of ramularia in a barley crop:
- Rectangular – The spots form a rectangular shape that do not cross over the veins of the leaf.
- Ring – There is a ring of yellowing around the spot.
- Reddish – The spots themselves are a reddish-brown colour.
- Right – The disease goes right through to the other side of the leaf. So, turning the leaf over, the lesions will appear on both sides rather than just the upper.
Symptoms of ramularia infection do not appear on the seed, making identifying infected seed impossible without the use of a molecular diagnostic test.
The reason that cases of ramularia have been on the increase is likely to be a combination of factors.
It could be that, inadvertently, varieties which are more susceptible to it have been grown, and resistance is only now being factored into breeding.
The barley growing season has also extended, with barley being planted earlier than when ramularia was first identified in crops 20 years ago.
On top of this, tighter control and restrictions on fungicides have added to the situation to create the perfect storm.
Ramularia’s appearance in the UK was first recognised in Scotland and was initially considered to be a problem of Scottish growing conditions.
However, the disease is now common in East Yorkshire, the South West – where it is particularly wet – and Ireland. It has also been anecdotally identified in barley crops along the south coast of England.
Seed distribution round the country contributes to the spread of the disease, alongside spore movement late in the season.
Widespread movement of spores from heavily infected crops via winds that can affect the following seed crop. The spores are very small and have the potential to move easily on the winds throughout July and August.
Leaf wetness is also linked with spore dispersal. Increased wetness, particularly at crop stem extension growth stages has been implicated in more severe ramularia outbreaks.
Leaf wetness later in crop development can lead to secondary infection, as spores are dispersed after prolonged periods of leaf wetness and may then infect the barley through the leaf pores (stomata).
A heavy infection of ramularia in the UK could cost a grower 0.5t/ha in lost yield, but in Europe due to a combination of factors which have yet to be unravelled, disease losses have been estimated to be as high as 20% in some crops. Reports from South America suggest yield losses of up to 70% due to ramularia.
The early infection of a crop could also be influenced by the cultivation system, as secondary spores may be present on the crop debris.
This means that ramularia is a big problem on minimum tillage systems, especially in the South West due to its wetter climate and use of non conventional drilling techniques.
Leaf wetness at stem extension is also implicated with contributing to larger ramularia outbreaks – which is why certain areas have greater issues with the disease.
In addition, crop temperature and growth habit are likely to impact the disease, as they affect crop development and senescence, but more research is being developed in this area to build a risk forecast for AHDB, similar to that for eyespot. This can then be used to help growers manage the disease.
Varietal resistance on the AHDB spring barley Recommended List 2017-18 ranges from scores of 5 to scores of 8, on 1 to 9 scale with 9 being most resistant.
Planet, Dioptric, Sienna and Scholar score 8, while a number of other varieties score 7 and 6. Ovation is the only variety on the list to score 5 for ramularia.
However, growers may not choose a variety based on its resistance to ramularia, as this can be of a lower priority than out-and-out yield.
Using fungicides to manage the disease is currently the most effective method to control the pathogen. The best response will be achieved by applying a sensible combination of fungicides from growth stage 45 to 49 onwards to protect the upper canopy.
If fungicides are applied earlier than this, there would be some response in terms of ramularia control, but it would not be optimum and disease levels could still be damaging.
This is where the big discussion about profit and yield takes place, so when integrating crop protection it is important to think about margins.
Azole fungicide prothioconazole and to a lesser extent epoxyconazole are effective against ramularia. The succinate dehydrogenase inhibitor fungicides (SDHIs) are also particularly effective, but there are limitations on their use.
There is a limit of two applications of fungicides containing SDHIs per crop and there have been resistance issues.
Multisite chlorothalonil is also useful for resistance management of ramularia and can prolong the green leaf area, but there is a significant threat to its usage from increased EU legislation.
To add to concerns over resistance and increased legislation, there are reports from Germany of mutations within the pathogen, which could raise concerns about fungicide control.
Growers should avoid farm-saved seed from heavily infected crops, as ramularia is not effectively controlled by seed treatments.
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