Question
The english ivy under my walnut tree has done well for years.  Now the leaves have black spots and growth is getting sparce.

Answer
you probably need to apply an organic fertilizer over the ivy. buy & apply a good granular fertilizer(espoma plantone, hollytone, gardentone, etc.) at your local garden center/nursery. quiz them on plants in your area.
i'm in southern NJ zone6. check to make sure they are not infested w/spider mites. you'll see fine webbing over the leaves and little specks of white to yellow dots(these are the mites). ask the people at the gerden center what they sell to spry the ivy with.