which fungicicide to use?
Fungicides can be useful in preventing black knot fungus, but they can't cure the fungus. In addition, fungicides are unlikely to be effective as a preventive measure if proper pruning and sanitation steps are not followed.
You can read more on controlling the fungus in the OSU Extension's fact sheet on the subject, which you can find here: http://ohioline.osu.edu/hyg-fact/3000/pdf/HYG_3011_08.pdf
As a Master Gardener Volunteer, I'm not permitted to make pesticide recommendations. (Fungicides are a type of pesticide.) However, you can find current fungicide recommendations in
OSU Extension Bulletin 780, "Controlling Diseases and Insects in Home Fruit Plantings." It is availble for $10 in the OSU Extension eStore. Here's a link: http://estore.osu-extension.org/searchproducts.cfm