Betula pendula (Silver Birch) Yellow leaves + tiny black spots

Please see the attached photographs of my Betula pendula thier leaves have turned yellow with tiny black spots and eventually fall from the branches. It is early summer in South Africa and we've been experiencing extremely hot weather but all my trees are under shade cloth and are irrigated with an overhead sprinkler system. All the other trees in my nursery are looking very healthy Please can anyone advise what is the cause of this happening and what remedial treatment one may take. Cheers Terry Strong

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