In this video from Advancing Eco Agriculture it is claimed that apple scab is totally arginine-dependent for energy, and historically sprays for apple scab suppressed arginine production. In some trees prone to apple scab, their cobalt levels are lower in sap than those relatively resistant to scab. They treated affected trees with cobalt (sulphate I presume) to try and speed up the metabolism of arginine to other peptides and proteins to deprive the fungus of an arginine source curing the scab.
This article talks about herbal leys to improve apple production. A herbal ley is a leafy green ground cover including amongst others Apiaceae, yarrow, chamomile, borage, clover, chicory, cornflower, clover, lupin and comfrey.
Comfrey (Symphytum Officinale) is a well known biodynamic accumulator that many have recommended growing in the garden (confined so that it doesn't take over the garden) for using its leaves to add nutrients to the compost pile.
comfrey’s thick and tuberous roots create an expansive root system, allowing the plant to “mine” compacted soils for minerals and other nutrients which are often difficult for other plants to obtain. It is this ability to help cycle nutrients through the soil that has given comfrey its designation as a dynamic accumulator plant.
I wonder whether therefore it's worthwhile to try growing comfrey alone under my apple tree to see if it can bring up cobalt to help the tree fight the scab. I'm hoping a ground cover might also stop the spores from being blown back up into the canopy.