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Dahlia Diseases and Viruses
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.
Mosaic also is spread in the field by aphids feeding on diseased plants and then working on healthy dahlias.
There are three possible controls of dahlia mosaic: (1) grow only tolerant varieties, (2) control aphids, and (3) propagate only from selected healthy plants and grow them away from diseased dahlias
Bacterial wilt - Symptoms are almost identical to those of Verticillium wilt.
Crown gall - Overgrowths or galls occur on stems and roots.
Flower blight/Gray mold - Brown, water-soaked spots appear on petals. Woolly gray fungus spores form on soft, brown, decayed tissues; fungus may invade plant tissue that touches infected petals
Powdery mildew - White, powdery fungus principally grows on older leaves and stems. Severely affected leaves dry up and may fall.
Soft rot - Southern blight/Cottony rot - Plants wilt and die suddenly. Water-soaked stem cankers appear near the soil line. Cottony, white fungal growth; later, large black sclerotia are found on insides and outsides of stems.
Smut - Yellowish, circular to irregular spots appear on leaves. Leaves later become brown and dry.
Stem and tuber rot
Cottony stem rot
Vascular wilt - Basal leaves wilt and turn yellow. Frequently, only one branch is affected at first. Later, the entire plant dies. Dark discoloration of the vascular system occurs.
Cucumber mosaic virus - Mild leaf mottle accompanied by little or no leaf distortion. Some varieties are symptomless carriers of the virus. Not common in California
Dahlia mosaic virus - The normal green color of leaves develops irregularly. Bands adjacent to the veins remain pale green (vein-clearing). Leaves may be distorted. Shortening of internodes (stunt) occurs in some cultivars. Flower color is usually normal
Tomato spotted wilt virus - A well-defined mosaic mottle or irregular concentric rings or wavy lines in leaves. No leaf distortion or stunting occurs. Symptoms seen on older leaves
Ringspot - Impatiens necrotic spot virus