Beallara "Tahoma Glacier" with brown spots on leaves

[Gregor]

The hybrid described in title is affected by a desease showed in the below posted pictures. The problem has started like a small circular brown spots on leaves. These brown spots have progressed increasing in size along the days. At the edge of these spots sometimes occurs a watery and transparent aspect especially when the spots are observed on the leaf underside.
I have treated the plant with copper oxychloride with the intent to slow down the disease progress.
Somebody can explain me the best way to deal with this problem and the disease cause? Thank you in advance