The most serious diseases of dahlias are caused by viruses. Mosaic disease is a virus typically characterized by yellowish or pale-green bands along the midveins and branch veins of affected leaves. Certain varieties of dahlias develop dead streaks on the midvein and a distortion of the leaf.
Infected plants often are stunted, with many lateral shoots, short flower stems, and poor flowers. Cuttings and root divisions from diseased plants will be diseased.
Bacterial wilt - this makes the stem droop. Stems that are affected will be wet and subject to rot.