| Disease (causal agent) | Symptoms | Survival of pathogen and effect of environment | Comments on control |
|---|---|---|---|
| Black leaf spot * (Pseudomonas syringae pv. delphinii) |
Irregular, tarlike, black spots in leaves. Petioles and stems also infected. Spots viewed from lower leaf surface appear brown. | Bacteria survive in plant debris from previous delphinium crop. Disease is favored by cool, wet weather. Bacteria are spread in splashing water. | Rotate field to a different crop. In perennial plantings, remove old leaves and stems from field. Avoid overhead irrigation. |
| Diplodina disease (Diplodina delphinii) |
Brown to black stem cankers often at stem bases of older plants. Basal canker may girdle the stem, causing the tops to die and break over. Tiny, black fungus fruiting structures (pycnidia) may be visible in necrotic tissues. Uncommon. | Fungus survives on delphinium debris and in crowns of living plants. Spores are spread in splashing water. | Avoid overhead irrigation. Plant on raised beds. Do not replant fields for 2 or more years. In perennial plantings remove old stems and plant debris. Protective fungicidal sprays would probably be effective. |
| Downy mildew * (Peronospora spp., Plasmopara spp.) |
Purplish red to dark brown, irregular angular spots on leaves. Lower surface covered with sparse, downy fungal growth that may be hard to see. Leaves turn yellow and fall. | Spores produced only on living plants. Resistant spores (oospores) carry fungus over unfavorable periods. Moist, humid conditions. | Protect foliage with mancozeb, fosetyl-Al, dimethomorph, or mefenoxam. |
| Powdery mildew * (Erysiphe aquilegiae and Podosphaera delphinii) |
White powdery patches on surface of leaves and stems. Basal leaves yellow, then brown and die. Flowers may be deformed. Larkspur is particularly susceptible. | Fungus survives on living plants. Spores are airborne. Disease is favored by moderate temperatures, shade, crowding, and dry foliage. | Avoid overcrowding. Protect foliage with a powdery mildew fungicide. |
| Soft crown rot and black leg (Pectobacterium (=Erwinia) atrosepticum) |
Stem bases are blackened and rotted, causing stems to fall over. New shoots may develop disease free under drying conditions or entire crown may rot. Rotted tissues usually have an offensive odor. Disease often appears at time of flowering. | Favored by warm, wet conditions. Bacteria survive in plant debris. Seeds may be contaminated by bacteria. Infection is through normal stem cracks and wounds. Bacteria are spread in water. | Avoid overhead irrigation, especially after flower spikes begin to elongate. Plant on raised beds and avoid wetting crowns. Heat treat or fumigate soil used to produce seedlings. |
| Water mold root rots (Pythium * and Phytophthora spp. *) |
Plants are stunted and yellow. Roots rotted. When Phytophthora spp. are involved, crown tissues may be rotted. Lower stems are sometimes infected. | Pathogens are soilborne and present in most agricultural soils. Spores (zoospores) are spread in water. Favored by wet weather, poor drainage, and overwatering. | Improve drainage. Grow on raised beds. Do not overirrigate. Fungicides used to control Pythium and Phytophthora should be useful. |
| Virus or viruslike disease | Symptoms | Host range and natural spread | Comments on control |
| Aster yellows * (Aster yellows phytoplasma) |
Flowers are converted to green leafy structures. Plants infected the preceding year produce many spindly, upright yellow shoots and no flowers. | Phytoplasma that has a wide host range and is spread by leafhoppers. Not spread by seed, other insects, or handling. | Do not plant seed beds downwind from delphinium, carrot, or celery fields. Control leafhoppers. Eliminate nearby weeds. Destroy infected plants. |
| Delphiniums are also susceptible to gray mold * (Botrytis cinerea), southern blight * (Sclerotium rolfsii), cottony rot * (Sclerotinia sclerotiorum), root knot nematode** (Meloidogyne spp.), Verticillium wilt * (Verticillium dahliae), damping-off * (Rhizoctonia solani, Pythium spp.), curly top (Beet curly top virus), various virus diseases (Tobacco mosaic virus, Radish mosaic virus, Cucumber mosaic virus), leaf spots (Ascochyta aquilegiae, Cercospora delphinii, Ramularia delphinii), white smut (Entyloma winteri), rust * (Puccinia delphinii), and Fusarium wilt * (Fusarium oxysporum f. sp. delphinii). | |||
| * For additional information, see section on Key Diseases. |
| ** For additional information, see section on Nematodes. |