Powdery Mildew (fungus – Erysiphe cichoracearum): Whitish-to-grayish growth covering upper surface of leaves. Avoid shaded locations and crowding of plants.
Ring Spot and Mosaic (viruses): Leaves are light green with mottled areas. Veins of leaf may appear light. Plants may be dwarfed and bushy. Leaves may have light green ring spots. Remove and destroy all infected plants as soon as detected. Tubers from virus-infected plants should never be used for planting purposes. Control aphids, leafhoppers and thrips.
Other Disease: Dahlias are susceptible to Verticillium Wilt, Bacterial Wilt, Root Rots, Root Knot Nematodes, Charcoal Rot, and Southern Blight. In every case, diseased plants should not be used for propagation of new plants. (See the appropriate section for more information on these diseases.)
Tuber Rot (fungus – Botrytis cinerea): The most popular method of propagation is root division. To avoid root decay, dust fresh cuts with sulfur.
Flower Blight (fungus – Botrytis spp.): In moist weather the buds may be attacked by a soft rot and all infected parts may become covered with the grayish, powdery mold.
Dahlia Mosaic (virus): Infected plants are stunted, yellow and pale green bands will develop along the veins of affected leaves. The leaves may be smaller than normal, wrinkled or blistered.
Dahlia Ringspot (virus): Yellow circles, dime-sized, will develop on the leaves. As time passes these may merge into larger yellow to pale green areas and the centers of the rings may turn brown. This disease is carried in roots and cuttings from diseased plants. Dahlia mosaic and ringspot can also be transmitted by insects.