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:
Powdery mildew, Erysiphe cichoracearum.
White powdery spots or patches develop on leaves and occasionally on stems. Symptoms often first appear on the upper surfaces of the leaves and are usually most pronounced during hot, humid weather. Heavily infected leaves turn brown and shrivel.
Disease can be minimized by avoiding overcrowded spacing of plants and by carefully picking off affected leaves as soon as symptoms are evident. Symptomatic leaves can be placed into a plastic bag in order to avoid spreading the spores of the fungus to other plants. Use of fungicides is usually not necessary. However, applications can be made as soon as symptoms are visible. Among the compounds registered for use in Connecticut are horticultural oil, sulfur, potassium bicarbonate, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Gray mold, Botrytis cinerea.
Flowers turn a papery brown and become covered with gray, fuzzy masses. Senescing flowers are particularly susceptible. Tan to brown spots with a target-like appearance can also develop on the leaves. These patches are often associated with flowers which have dropped onto the leaf surface. This disease is particularly troublesome during periods of extended cloudy, humid, wet weather.
Good sanitation practices including grooming the plants and removing spent or senescing flowers can minimize the potential for infection. These affected tissues should be carefully removed and discarded when they are dry. It is also important to avoid wetting the flowers when watering and crowding plants. Adequate spacing between the plants can promote good air circulation. Control can also be achieved with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are mancozeb and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Diseases caused by Viruses:
Spotted wilt, virus, Impatiens Necrotic Spot Virus (INSV).
Initial symptoms of infection appear as a mottling of the leaves. Purple spots then develop on these leaves and stems. This virus is transmitted by the western flower thrips.
Control of this disease is focused on prevention since once plants are infected, they cannot be cured. It is important to eliminate and remove infected plants as soon as they are recognized and to eliminate other symptomatic plants since this virus has a very broad host range. It is also critical to manage the thrips population.
The green peach aphid, Myzus persicae, sometimes infests verbena. Among the compounds registered for control of this pest in Connecticut are insecticidal soap and ultrafine horticultural oil. Apply them as foliar sprays when insects are present. Alternatively, imidacloprid can be applied as a systemic to be taken up by the roots. Consult the label for dosage rates and safety precautions.
Verbena bud moth, Endothenia hebesana.
The larva of this insect is a borer in the new shoots of verbena and Physostegia, causing them to wither. The larva is greenish yellow with a black head and is a little less than 1/2" long when mature. The adult is a purplish brown moth with wingspread of 1/2". Ordinary infestations can be controlled by handpicking, or by or clipping and disposing of the infested tips.
Garden fleahopper, Halticus bractatus.
The adult plant bug resembles a black aphid and is about 1/16" long. The nymphs are greenish. Both suck the juices from the leaves and stems causing pale spots to appear. Badly infested plants may die. The insects survive the winter in the adult stage. Disposing of all weeds and trash in the garden before spring controls the overwintering adults. Spraying with malathion, which is among the compounds registered for control of this pest in Connecticut, should control both adults and nymphs. Imidacloprid applied as a systemic to be taken up by the roots also controls this pest. Consult the label for dosage rates and safety precautions.
Verbena leaf miner, Agromyza artemisiae.
The tiny adult midge or fly lays its eggs in the tissue of verbena leaves, and the maggots form blister-like mines usually conspicuous from the upper surface of the foliage. Handpicking and destruction of infested leaves when mines have just started should be sufficient to control the usual infestation. Serious outbreaks can be controlled by spraying with malathion or spinosad, which are among the compounds registered for control of this pest in Connecticut. Imidacloprid applied as a systemic taken up by the roots also controls this pest. Commercial growers may also consider abamectin, a restricted use product. Consult the label for dosage rates and safety precautions.
Yellow woollybear, Diacrisia virginica.
Whitish, yellowish, or brownish hairy caterpillars feed upon the leaves of verbena and many other kinds of plants in the garden in late summer. This caterpillar is about 2" long when fully grown and is covered with long hairs that vary in color from pale yellow to reddish brown. It makes its cocoon of its own hairy coat and silk, hiding in sheltered places, sometimes 20 to 30 being clustered together. The insect overwinters in this stage and the moths emerge in June and July. The moth has a wing spread of between 11/2 and 2", and is pure white with a few black dots, blackish antennae, and orange abdomen with a row of black spots on the back and along each side. Handpicking is the usual means of control.