Black Spots On Leaves, Stems -Cyclamen Mites
Cyclamen mites are about 1/50 inch long, barely visible to the unaided eye. They have four pairs of legs, piercing-sucking mouth parts, and very compact bodies. They cause so-called "blacks", in which dark brown or blackish spots, streaks or blotches appear on delphinium stems and leaves. As soon as you notice the first stippling of the leaves, spray the pests with insecticidal soap. Repeat the spray every 3 to 5 days for 2 weeks. For more information see the file on Controlling Mites
Leaves Curled And Distorted - Aphids
Aphids, also called "plant lice," are soft-bodied, pear-shaped sucking insects about the size of the head of a pin. They often gather in large groups on delphinium leaves and tender buds. Their feeding retards plant growth. Leaves may turn yellow or brown. They wilt under bright sunlight, or sometimes curl and pucker. Spray the insects with insecticidal soap every 3 to 5 days. If that doesn't work, try pyrethrum every 5 to 7 days. For more information see the file on Dealing with Aphids.
Plants Stunted, Yellowed; Root Lesions - Nematodes
Nematodes are not insects, but slender, unsegmented roundworms. Most are microscopic-sized soil-dwellers. They have piercing-sucking mouthparts and feed on cell contents. Infested delphiniums look sickly, wilted and stunted. Their foliage yellows; they decline slowly and die. Underground, their root systems are poorly developed, even partially decayed. Dig up damaged plants and trash them. Add lots of compost (especially leaf mold), if it is available, to the soil to encourage beneficial fungi that attack nematodes. Drench the soil with a solution of liquid fish emulsion and water to discourage nematodes. For more information see the file on Controlling Nematodes
Ragged Holes In Leaves - Slugs and Snails
Slug s are essentially snails without shells. They are usually 1 to 2 inches long (some species grow up to 8 inches). Their coloration may be white, gray, yellow, brown or black. Slugs and snails leave large ragged holes in delphinium leaves and stems from their file-like tongues. They are attracted to moist, well-mulched gardens and acidic soil. Active at night, they hide under boards or leaf litter during the day. Although these pests are always most destructive in shaded gardens and during rainy spells, they may attack delphiniums as well. Trap them in a shallow plate baited with beer and set on the soil near the delphiniums. Slugs, attracted to the yeast in the beer, will climb in and drown. Begin trapping within the first three to four weeks after the last frost. The more slugs trapped, the fewer are left to reproduce. For more information see the file on Controlling Slugs
Leaves Become Discolored - Leafminers
Larkspur leaf miner larvae feed in small groups in the tissues between the upper and lower surfaces of delphinium leaves. Large areas of leaf blades become discolored and collapse as though blighted. Leafminer larvae pupate just outside the leaves near the mine, or tunnel, in brown seed-like cases, that are often attached to the leaf. Pick off and destroy all infested leaves. Larvae can sometimes be repelled by spraying plants with a soap solution in late June or early July. For more information see the file on Controlling Leafminers.
Crowns and Roots Rot; Odor Present - Crown Rot
Crown and root rot, caused by soil-dwelling bacteria and fungi, is the most common delphinium disease. If it is present, there may be no new shoots in the spring. A bacteria turns the crown (the thick base of the plant from which the roots grow) to mush. Often accompanied by an odor, the disease attacks a few spots on the crown at first, and then spreads to the entire crown. The lower leaves of a plant in leaf become discolored and its young shoots begin to wilt. The roots are blackened, rotten and covered with white fungal threads. Usually the whole plant dies in a few days.
Remove and discard in the trash any infected plants and the soil immediately surrounding them. Do not replant delphiniums in the same place next season. Allow the soil around the crowns to dry by removing any mulch and loosening it gently with a trowel. If you catch the rot early, spray the crowns with ferbam or captan fungicides. The disease is most likely to occur in the late winter thaw when dead leaves decompose on ground and harbor bacteria and fungi that spread to healthy tissue. Clean up garden debris promptly. Keep winter mulch away from the crowns to reduce problems. For more information see file on Dealing with Fungal Disease.
Pale Patches On Leaves - Downy Mildew
Downy mildews are caused by fungi that grow and spread on moist leaf and stem surfaces. These fungi form pale areas on upper leaf surfaces and gray or white or purplish "downy" patches on undersides. They are especially active in periods of cool wet nights and warm humid days, causing leaves to wilt and die. Plant delphiniums on well-drained, fertile soil with enough space between them to encourage good air circulation. Water them early on sunny days. Try to keep their foliage dry by watering just the soil. Promptly, dig out all heavily infected plants together with adjacent soil and discard them in the trash. Spray lightly infected plants with flowable sulfur or dust them with lime sulfur or Bordeaux mixture when downy mold first appears on leaf undersides. Several applications may be needed to achieve control. For more information see file on Dealing with Fungal Disease.