Pat,
If you notice old composition shingle roofs, beneath each vent pipe
there usually is a long teardrop shaped clean area going down the roof,
the rest of the roof will be darker in color, that is black algae on
the slate pebbles. That is where Zinc or Lead Oxide washes off the pipe
and kills or prevents black algae. A strip of the metal fixed to the
top rail of your basket should solve your problem.:-))

Joe T