Fungicides, when used properly, can be very useful. Deciding if you need to apply a fungicide and if so, which one, when and how are tough decisions when dealing with infections. Always remember that the best strategy is to select a plant resistant to common fungal problems avoiding fungicide use altogether.
One of the big mistakes often made by users is treating an infection which has developed as a result of another problem. Almost all plants will develop some fungal leaf spots when undergoing environmental, insect or disease stress. To treat the fungus and ignore the rest is counterproductive.
The development of a fungal infections is dependent on the susceptibility of the plant and environmental factors. Often, warm moist conditions and lack of air circulation are contributing factors. One should also remember that infected plant material tends to be full of spores and is very contagious to uninvolved plants. Sanitation should be part of the treatment strategy.
It is often difficult to decide which fungicide to use. Fungicide preparations tend to be specific for certain fungi, not all of them. It is not practical to do a lab test to identify the fungus, so the decision of which fungicide to use is usually made on the basis of the diseased tissue appearance and species of involved plant.
It is important to note that no fungicide will ever make and existing infection go away. The best one can hope for is to prevent new infections.
These chemicals tend to be expensive, especially the commercial products which are available only to licensed applicators. The number and types of fungicides available to homeowners is limited but there are several broad spectrum chemicals useful for things such as black spot on roses.
Other than manufactured fungicides, there are some more simple and cheaper agents such as elemental sulphur, copper salts, horticultural oils and bicarbonates which have been used effectively to treat fungal infections for many years.
Master Gardeners with access to OSU's recommendations for treatment can help with the choice of the proper fungicide for a specific problem.