There is rarely a one-size-fits-all solution to plant disease problems. Many diseases are species-specific-meaning that a wilt, spot, or rot on one plant can rarely spread to plants that are not closely related. Also, be sure the problem is a fungus-fungi cause most mildews and blights, but bacteria cause some diseases. Then again, some plant problems that look like diseases are really nutritional disorders. Regardless of the cause, all plant diseases must be treated early to get good results. As for your desire for an all-natural solution, there are products containing neem oil and neem extracts that often effectively prevent outbreaks of fungal diseases such as rose black spot or powdery mildew. Neem oil is derived from a tropical tree; it works as a fungicide and an insecticide. To prevent fungal diseases, spray neem oil on plants at risk every 7-10 days. Remember: It prevents fungus spores from germinating and penetrating plant tissue. It cannot cure diseases that are already established.