Before purchasing a product, check the container to be sure it is approved for the plant and that it will control the disease problem observed. Effective control is dependent upon applying the proper rate at the proper time and providing good coverage on the plant. Most fungicides and bactericides are protectants, meaning that they must be present on the plant part before the pathogen arrives. Successful control of diseases can be achieved by using a combination of the methods described. However, don't expect a perfect garden.
Plant pathogens are a part of nature.
Some disease loss may be acceptable, especially when only minor blemishes result.