A number of folks have reported having luck with copper-based products for other leaf molds (i.e. botrytis). Maybe that would help with Cladosporium also.

You could use a copper product as either a dust or a spray.

I'm not sure if compost tea would be a help; you may end up feeding the Cladosporium instead of eradicating it.

Carefully remove and dispose of affected leaves, so as not to spread the spores. Bag them up in a plastic bag. I'm not sure that anything will kill the existing disease on the leaves, but I'd use a copper-based product to prevent the spread of the disease.