Leaves on Berberis darwinii seem to have a red/yellow discolouration
By Brassica
United Kingdom 
Thanks Bamboo for reply but apparently the problem gets worse with moisture hence suggestion of fungal disease. Don't really want to treat chemically so any suggestions would be appreciated.
On plant
Berberis darwinii
9 Jul, 2009
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That looks more like autumning to me - if the plant dries out too much, it thinks its autumn, the leaves colour and fall, or the leaves lose their chlorophyll and drop off because of drought, so I'd say its a result of the heatwave and lack of water.
9 Jul, 2009