The disease only affects barley.
Halo spot is found mainly in western coastal areas where outbreaks occur in wet summers after flag leaf emergence. The disease appears as small leaf spots (1-3mm long) often square or rectangular in shape, pale brown in the centre with dark purple/brown well defined margins. Pycnidia occur in lines along the veins within the central area of a lesion. Spots generally occur towards the tips and along the edges of leaves. They also affect the leaf sheath and ear (especially the awns). This disease often occurs with Rhynchosporium but can be distinguished from the latter by the smaller size of the spots and the presence of pycnidia within the lesions. Also, halo spot tends to occur most frequently on the upper leaves whilst Rhynchosporium is often more common on the older foliage.
The disease originates from infected seed, stubble and volunteer barley plants. Symptoms on lower leaves which arise from seed infection or from stubble contact or splash are indistinct but spread up the plant in rain-splash, usually in warm conditions later in the season. The disease rarely becomes important until after flag leaf emergence when it can develop rapidly
in wet weather.
The disease occurs sporadically, usually in wet seasons. Traditionally the disease occurs mainly in the south west of England where rainfall is high but is rare elsewhere in the UK - it generally does not cause significant yield loss.