Barley leaf blotch - impact
- Leaf blotch (also known as scald) is a globally important pathogen of barley (Hordeum vulgare) crops, especially in cool temperate regions which favour disease development.
- Yield losses of up to 40% have been reported in the field, although losses of 1 – 10% are more typical.
- In the UK alone, it causes an estimated national yield loss of 10.8 M (assuming a price of £225 / tonne) despite the use of chemical fungicides.
- Visible disease symptoms occur on the ears, leaf sheaths and most noticeably on the leaves.
- Young lesions are usually a distinctive water-soaked blue colour, developing later into mature lesions with tan centres with dark margins (see below image).
- As the disease progresses, foliar lesions can coalesce together and total leaf death can occur.
Key weather / geographical risk factors
- Epidemic severity mainly influenced by Spring rainfall – more rain, more disease.
- The disease is favoured by cooler and wetter weather conditions – more common in northern and western parts of the UK.
- Barley leaf blotch caused by the fungus Rhynchosporium commune (formerly R. secalis).
- The spores of the pathogen are spread within the crop canopy by rain-splash. This allows spread of the pathogen into the upper parts of the crop – severity of epidemics is increased by wet spring weather, while dry weather enables disease escape.
- Disease symptoms mainly occur in late summer / autumn (although can occur at other times of year as well). Note that R. commune can colonize barley crops without causing visual disease symptoms (i.e. transmit from seed to seed without visible infection).
- R. commune overwinters via infected barley stubble / volunteers and infected seed (also possibly by some infected grass species) – these then infect the developing seedlings.
- There is no evidence that sexual wind-dispersed ascospores play an important role in initiating disease epidemics.
Select a resistant variety such as 'Surge' or 'Funky' in areas of high disease risk – the 2017/2018 recommend cultivar list for winter barley is now available at: https://cereals.ahdb.org.uk/va...
- Use clean seed stocks in drilling – avoid use of ‘dirty’ seed from heavily diseased crops.
- Restrict possible sources of the pathogen in the field. This includes barley debris, barley volunteers, and other wild grasses known to harbour the disease (brome-grass, ryegrass).
- Avoid excessive nitrogen uptake.
- Control of barley leaf blotch is mainly through use of fungicides with different modes of action.
- Early application of fungicides is only necessary in Autumn if early disease symptoms are severe. If necessary, disease control in early Spring (GS30) can be helped by application of an early fungicide.
- Use of an effective azole with a strobilurin or SDHI fungicide at GS31 is a good foundation for disease management and will be the first treatment for most crops (a morpholine can be included for additional control).
- Later protection will be needed if cool wet weather favourable to the pathogen occurs between flag leaf emergence (GS39) and ear emergence (GS59), – this can be with an SDHI and chlorothalonil, or alternatively an azole with strobilurin and chlorothalonil.
- The current status of different chemical classes for barley leaf blotch control are:
- Anilinopyrimides (cyprodinil): resistance not yet reported, medium risk.
- Azoles: shifts in sensitivity, older azoles affected more, use newer azoles (epoxiconazole / prothioconazole) at robust doses in mixtures with other modes of action.
- Methyl benzimidazole carbamates (MBCs): resistance widespread, no longer used.
- Morpholines: no resistance reported, medium risk.
- Multisites: low resistance risk but protectant only so spray timing important.
- Succinate dehydrogenase inhibitor (SDHIs): remain effective, but high resistance risk - should only be used in mixtures.
- Quinone outside inhibitors (QoIs) including strobilurins: isolated cases of resistance but not yet widespread - consider effective but high-risk in the meantime – use in mixtures.