Apple scab is caused by a fungus, so 99.9% of the recommendations you will find involve a fungicide, most commonly captan. I don't go that route as i don't want to kill all of the good fungus I've worked do hard to promote in my soil.
A Google search for 'cobalt for apple scab' yields this AI summary. "According to some, foliar applications of cobalt can prevent and reverse apple scab infections. Apple scab is caused by a cobalt deficiency, which leads to a buildup of arginine, an amino acid that makes apples susceptible to a fungus that causes scab. When apple trees have enough cobalt, they metabolize arginine into peptides and proteins, which reduces arginine levels in the leaves and prevents them from feeding the scab organism."
I've purchased a product called Rebound Cobalt from Advancing Eco Agriculture and used it as a foliar fertilizer. I no longer have any cases of scab in my 160 tree orchard. A 1 gallon jug is like $35 and has lasted me several years.
