Maybe this would be of interest to some people. Last summer some colleagues and I did a trial with some of the natural bio control and other natural control products for blackspot. Unfortunately, they didn’t control blackspot very well or at all. The summer before we tried a number of other products including Bordeaux mix, Cornell Mix (has baking soda and hort oil), hort oil, aspirin, dishsoap, chlorothalonil, and milk. Out of them all, chlorothalonil (the standard fungicide) had the best control and then Bordeaux mix and milk provided some control as well. The rest of the treatments did not provide control and were comparable to spraying water; even a ready to use spray bottle of sulfur labeled for control of blackspot. Surprisingly milk (one part whole milk, two parts water, and a drop per quart of mild dishsoap as a spreader sticker) provided some control. It went on white and dried clear. There are many proteins in milk and we tried one of then individually as a follow up, but that one did not offer noticable control.
We should probably get this information out more widely because there are people that probably will find the information useful.