Common fungal problems include powdery mildew (Erysiphe polygoni or E. cichoracearum), signaled by powdery white patches on leaves; gray mold (Botrytis cinerea) which attacks roots, rots like Pythium, Rhozoctonia solani or Sclerotinia sclerotiorum, or smut (Entyloma calendulae f. sp. dahliae). Some symptoms of fungal infection are easy to overlook; sometimes, however, a plant will simply fall over and die.
The best solutions for fungal problems are preventive in nature, according to Bob Romano of the American Dahlia Society. Treating tubers for winter storage with an anti-fungal preparation like Terrachlor can not only prevent problems but result in healthier, more vigorous plants, he says.
Among the causes of fungal infections in the summer growing season are excessive soil moisture and, in the case of greenhouses, overhead misting; overfertilization and the buildup around plants of insoluble salts; low soil temperature (below 68° Fahrenheit) before germination; high soil temperatures after emergence (above 77°F) and overcrowded flats or seedbeds for cuttings or young plants. Plant pathologists at Oregon State University note that plant debris in gardens should be removed and destroyed in the fall to minimize fungal growth. Good gardening practices will help you grow healthy dahlias.
Fungicides, Romano says, are of two types: contact or protectants, which are applied to healthy dahlias prior to infection, and systemic fungicides, which can not only protect the plant from infection but can actually cure and eradicate fungus present in plant tissue. A recently developed class of fungicides, the Strobulurin group, which are considered environmentally safe. They work by inhibiting the transport system of mitochondrial electrons in plant pathogens but not in other living entities.
Most systemic fungicides are specifically for particular types of fungi, so it's important to diagnose the fungal infection. Your Cooperative Extension office is a good resource. Garden experts there can help you identify the specific type of fungal problem that's affecting your dahlias and choosing the best and safest fungicide with which you can deal with it.