Organic fertilisers work on soil health in order to enhance plant health.
Before apple scab was sprayed for, mildew was a little known disease - it is now a major one due to the balance of nature being upset. Chemical sprays change the internal composition of plants making them more liable to pest and disease attack and wipe out soil organisms which provide disease controlling anti -biotics and build up plant resistance. Organic foliar sprays work on strenghtening the plant at a cellular level making them healthy and therefore more able to resist disease.