Bacteria Wilt (bacterium – Pseudomonas solanacearum): This bacterium has been shown to cause a wilt of zinnias in Florida.
Blight (fungus – Alternaria zinniae): Small, reddish-brown spots with grayish centers are the first symptoms of this widespread and highly troublesome disease. Dark brown cankers also occur on the stems. Flowers are spotted and at times completely blighted. Because the causal fungus may be carried with the seed, treat the seed before planting with hot water at 125 degrees F for 30 minutes. Then cool and dry. Older seeds are more easily injured by this treatment. Use recommended protectant fungicide.
Powdery Mildew (fungus – Erysiphe cichoracearum): Whitish powdery growth on leaves and stems. Leaves die from base of plant. Affected plant lose vigor and cease growth. Spray with recommended fungicide as soon as mildew appears.
Leaf Spot (fungus – Cercospora zinniae): Small, roundish spots with gray or whitish centers. Several spots may run together, killing the entire leaf. Spray with recommended fungicides two or three times during the season.
Viruses: Many virus diseases affect zinnias. Among the most prevalent are curly top, caused by curly top virus; mosaic, caused by the cucumber mosaic virus; and spotted wilt, cause by the tomato spotted wilt virus. Remove and destroy infected plants. Spray with insecticides every few weeks to control sucking insects which spread the viruses.
Stem and Root Rot: (See the section on Stem and Root Rot)
Charcoal Rot: (See the section on Charcoal Rot)
Nematodes: Two species of nematodes infest this host: Meloidogyne incognita, which causes root knot (See the section on Root Knot Nematodes); and Aphelenchoides ritzema-bosi, which causes angular spots on the leaves. For control see the section on Other Nematodes.
Other Diseases: Zinnias are subject to other diseases including a blossom blight caused by a fungus, Choanephora spp., stem canker by Botrytis cinerea and damping-off by Rhizoctonia solani.
Consult the Chemical Control Supplement (B-1140A) for specific chemical control suggestions.