Delphinium (Delphinium)

Plant Health Problems
See Perennials for a detailed discussion of problems that may occur and are common to most herbaceous ornamentals.

Diseases caused by Fungi:

Crown rot, Sclerotium delphinii.
Yellowing of lower leaves followed by wilting and death of the rest of the plant are symptoms of this disease. A white cottony mass of mycelium may be growing around the crown or on the soil near the crown and distinguishes this crown rot from others. In this fungus web may be found the whitish to cream color or buff or reddish-brown seed-like sclerotia the size of a pinhead.

No control is known other than removing and destroying all infected plant parts and removal of top soil around the plant. New soil may replace sclerotia-filled soil.

Crown rots by various organisms, Diplodina delphinii, Fusarium osysporum. F. delphinii, Diaporthe arctii.
Determination of the fungus is unimportant as these diseases occur infrequently.

Removal and destruction of diseased plant parts is the only known control.

Powdery mildew, Erysiphe spp.
These fungi are obligate plant parasites which grow vegetatively on the plant leaf surface, sending haustoria, structures which absorb food from the host, into epidermal cells. The white mildew seen on the leaf is a combination of vegetative mycelium and spores borne in chains on upright conidiophores. Wind-dispersed mildew spores can germinate without free water under high humidity conditions, and disease is often severe when conditions are humid but dry. Small black over-wintering structures called perithecia are often found in powdery mildew affected areas.

Control may also be achieved with the use of fungicides applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are potassium bicarbonate, ultra fine oil, sulfur, triadimefon, or thiophanate-methyl fungicides. Consult the label for dosage rates, safety precautions, and directions for use.

Diseases caused by Bacteria:

Black leaf spot, Phytomonas delphinii.
Irregular shiny black spots appear on the upper surface of the leaves, particularly during cool wet seasons, showing first on the lower leaves.

Removal and destruction of old plant parts in the fall may help.

Bacterial blight, Erwinia carotovora.
Irregular black discoloration and softening of leaves and stem which may extend nearly to the ground. When the stem is split, masses of bacteria ooze out and black streaking is visible inside. The bacteria appear to dissolve the cell membranes and the affected tissues, and give off a foul odor. The plants may be stunted or killed back severely. The causal bacteria appear to be carried both in the seed and in the soil, but are usually lethal only under very moist conditions.

Excessive watering or irrigation intensifies the disease. Stem cracking at the ground level also provides a means of entry for the bacteria. Avoid over-watering and wounding of plants.

Diseases caused by Phytoplasmas:

"Greens" or stunt, Aster yellows phytoplasma.
Plants may be dwarfed with small leaves showing mottling or chlorosis. Flowers are characteristically green or flower parts may be leaf-like. A bunchy flowerhead is typical. This pathogen is spread by leafhoppers and has many plant hosts, including aster, ragweed, and chrysanthemums.

Control consists of removing all infected or suspicious plants and control of leafhopper vectors.

Diseases caused by Viruses:

Cucumber or Tobacco mosaic, Ringspot or Tomato spotted wilt, viruses.
A number of different viruses can attack perennials. These viruses are obligate parasites which are not themselves alive, but use plant cell metabolism to replicate and produce more virus. Symptoms of infection are commonly loss or mottling of color, yellowing, yellow rings, stunting, and/or deformed leaves, flowers, or buds. Viruses can be transmitted by mechanical means such as touching or rubbing leaves with hands or tools contaminated by contact with diseased plants, by dodder, nematodes, grafting or insects such as aphids, leafhoppers, whiteflies, or thrips.

Control is based on starting with virus-free stock or seeds, control of insect vectors, eliminating weed virus hosts, resistant varieties, and eliminating diseased plants.

Diseases caused by Nematodes:

Root-knot nematodes, Meloidogyne hapla.
The northern root-knot nematode, M. hapla, is a sedentary endoparasite, meaning that it infects host roots after hatching from eggs, stimulating the formation of a small gall containing specialized feeding cells, and feeds in the same location through several molts to produce several hundred offspring. Because most of its life cycle is inside roots, it may be spread to new locations with vegetative propagation material. This nematode is parthenogenetic, a single female can reproduce without males, resulting in a new generation every 28 days under ideal conditions. The galls produced on roots interrupt translocation and act as a nutrient sink. As a result, plants may be stunted, wilt easily, and show signs of nutrient deficiency. The nematode has a wide host range, but a number of ornamentals, including Rudbeckia, Aster, and others, have been shown to be resistant.

Growing resistant plants or rotating to small grains can greatly reduce or eliminate nematode populations in infested soil.

Insect Problems

Aphids, Brachycaudus rociadae.
This species causes delphinium leaves to cup downwards. Another species, the green peach aphid, also attacks delphinium. They may be managed by spraying with insecticidal soap or ultrafine horticultural oil, which are among the compounds registered for control of this pest in Connecticut. Aphids on delphiniums outdoors can be controlled using malathion sprays or imidacloprid can be applied as a soil drench for season-long, systemic control. Consult the labels for dosage rates and safety precautions.

Blackened leaves.
Leaves and flowers are badly deformed, leaves are brittle and thickened with black or brown spots or streaks. This trouble is caused by cyclamen mites. The cyclamen mite, Phytonemus pallidus, is a translucent, microscopic mite that often infests the new leaf and blossom buds, and can proliferate on spent blossoms. Characteristic injury includes dwarfed, thickened and wrinkled leaves. On azalea, dahlia, and other plants with stems, the distance between internodes becomes drastically shortened. Infested plants do not bloom, or blossoms may be misshapen. Cyclamen mite is not easily controlled when mature plants are badly infested: effective miticides are not available to homeowners, so a commercial applicator may be required for effective miticide applications. Effective restricted use products are abamectin, dicofol, and endosulfan.

Cutworms, Feltia subgothica.
New transplants are often cut off at ground level by this worm. Other species crawl up the stem and eat foliage. They feed at night and hide in the soil during the day. These cutworms are large and easily found if you dig in the soil around the base of the plant. They can then be destroyed.

Stalk borer, Papaipema nebris.
This borer infests an occasional stalk of many kinds of herbaceous plants. As a rule its presence escapes notice until the plant begins to wilt. Then it is too late for the plant to recover. The larva tunnels up and down inside the stem, and the top portion usually wilts and later dies. There is one annual generation. The moths emerge in September and October and lay eggs on the stalks of their food plants, in which stage the insect passes the winter. The eggs hatch in May or early June. The young larva begins to feed on the leaves of the nearest food plant, and later tunnels in the stem. The mature larva is nearly 1 ½" long, grayish brown with one white dorsal stripe and two white lateral stripes on each side. On the front half of the body the lateral stripes are interrupted, and the lower brown stripe extends forward onto the side of the head.
Burning all the old stalks, if allowed and destroying weeds at the edges of the garden helps control this insect. When needed, methoxychlor, which is among the compounds registered for use against this pest in Connecticut, applied as a dust, in June, should control this pest. Consult the label for dosage rates and safety precautions.

Larkspur leafminer, Phytomyza delphiniae.
This is a blotch miner causing large areas of the leaf blade to become blackened, distorted and shrunken. Larvae pupate in a case that hangs from the leaf. Handpicking and destroying affected leaves as well as all plant material left in the fall will reduce their populations. Imidacloprid applied as a systemic to be taken up by the roots may provide season-long control. Another option is spinosad, or abamectin, which is a restricted use product. Consult the label for dosage rates and safety precautions.