Plant Health Problems
Diseases caused by Fungi:
Botrytis blight, Botrytis cinerea.
Flowers develop whitish spots, 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 spent 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 chlorothalonil and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Diseases caused by Viruses:
Mosaic and other viruses, Tobacco mosaic virus and others.
Petunia is susceptible to several of the viruses which are also found on tomatoes and potatoes. Light and dark green mottled areas, mosaics, and crinkling of leaves indicate infection by the tobacco mosaic virus. Other viruses may result in dwarfed, cupped leaves and witches'-brooms symptoms.
Control of these diseases 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 these viruses usually have very broad host ranges. It is also helpful to place petunias at some distance from other solanaceous plants such as tomato, flowering tobacco, potatoes, eggplant, and peppers to further reduce the chances of infection. It is also critical to manage insect populations.
Impatiens necrotic spot virus, virus, (INSV).
Symptoms can appear as black ringspots or blotches on the leaves. 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.
Greenhouse orthezia. Orthezia insignis.
This greenhouse insect often infests bedding plants out of doors, including petunia. This is a dark-green or brown scale-like insect that secretes wax as it feeds, leaving behind a long plate of white wax. It damages plants by withdrawing sap. When needed, malathion or imidacloprid, which are among the compounds registered for control of this pest in Connecticut, can be applied according to label directions.
Potato flea beetle, Epitrix cucumeris.
This tiny black flea beetle feeds upon a great variety of plants, of which one is petunia. This is a small, black, jumping flea beetle that feeds upon the leaves of potato, tomato, tobacco, and many other plants, eating out small round holes from the underside, but leaving the upper epidermis. The remaining tissue soon dies, however, and falls away, leaving holes through the leaves which turn yellow and later turn brown and die. This beetle is not much more than one-sixteenth of an inch in length. It lives through the winter under rubbish and in other sheltered places, and may first be found in the spring on plantain and other weeds, sometimes also on the leaves of apple, wild cherry, and maple. As soon as the preferred food such as petunia plants appear in the garden or field, the flea beetles gather upon them. The overwintering beetles lay eggs in the soil in June. The larvae feed upon tubers and roots of the host plants. An abnormal growth sometimes takes place around each injury. The larvae transform and beetles emerge early in July.
These beetles feed on the petunia foliage as long as the plants are green and temperatures are favorable. These insects then hibernate until the following spring. If the flea beetles are controlled in the spring when they first begin feeding, fewer beetles will be present in the fall and the following spring. See Flea Beetle fact sheet.
Yellow woollybear, Diacrisia virginica.
The whitish, yellowish, or brownish hairy caterpillars feed upon petunia and many other kinds of plants in the garden in late summer. This caterpillar is about 2" long when fully grown. It makes its cocoon of its own hairy coat and silk and hides 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.