Plant Health Problems
Diseases caused by Fungi:
Powdery mildew, Erysiphe cichoracearum.
This is the most common disease of zinnia. 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.
Botrytis blight, Botrytis cinerea.
Flowers develop tan or brown spots 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. Stems may also be infected and symptoms appear as tan-brown cankers which girdle the stem and cause plants to collapse. 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, copper sulphate pentahydrate, mancozeb, and thiophanate-methyl.
Initial symptoms appear as small, reddish-brown spots with gray centers. The spots gradually enlarge and coalesce. Dark brown cankers may occasionally develop on stems and petals may be spotted.
Efforts to maximize plant vigor by fertilizing and watering are helpful. However, watering should be done early in the day to give the foliage a chance to dry before nighttime. It is also helpful to pick and remove symptomatic leaves as soon as they develop. Although not usually necessary, applications of fungicides can be made when new growth emerges in the spring. Among the compounds registered for use in Connecticut are chlorothalonil, iprodione, mancozeb, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Diseases caused by Bacteria:
Bacterial leaf spot, Xanthomonas.
Symptoms appear as angular, brown spots on the leaves. These usually start on the lower leaves first and progress up the stem.
Control strategies are aimed at prevention. However, removing and roguing of diseased plants or plant parts is critical. Any equipment or tools that come in contact with diseased plants should be disinfested with 10% household bleach, 70% alcohol, or one of the commercially available compounds. It is also important to avoid overhead irrigation since these bacteria are easily spread in splashing water. Crop rotation is also effective since planting in clean soil gives best control. Starting with clean seed or healthy transplants is helpful.
Diseases caused by Phytoplasmas:
Aster yellows, phytoplasma.Symptoms consist of extreme distortion of the leaves. If plants are infected young, they remain stunted, and if flowers are produced, they are small, abnormal, and often exhibit phyllody, a condition in which flower parts revert to green tissues. Symptoms do not always appear on the entire plant, but may show on only one stalk, depending on the time of infection. This phytoplasma is transmitted by leafhoppers. Since the pathogen infects many plants (e.g., daisies, chrysanthemums, plaintain, echinacea, and rudbeckia), zinnias may become infected from neighboring plants.
Strategies for control include promptly roguing and removing diseased plants as well as control of the leafhopper vectors.
Diseases caused by Nematodes:
Foliar nematodes, Aphelenchoides spp.
Angular spots delimited by veins appear first on bottom leaves. Eventually the whole leaf is involved and turns brown. Leaf-browning progresses up the plant. When the plant is covered with a thin film of water from rain or heavy dew, the pathogens emerge from the interior of the leaves and swim up the stem to infect the leaves above.
Reducing leaf moisture and removal of infected tissues, debris, or plants is important.
Asiatic garden beetle Maladera castanea.
Adults are 3/8" long, dull cinnamon brown, with a faint iridescent sheen. They are active night fliers, attracted to lights, and are injurious as adults when they feed on many kinds of plants. During the day, the beetles hide in the soil around the plants and are seldom seen unless one knows where to find them. If needed, azadirachtin or carbaryl, which are among the compounds registered for control of this pest in Connecticut, can be applied to foliage when adults are present. Otherwise, treating with imidacloprid as a systemic may kill adults feeding on the foliage. Consult the label for dosage rates and safety precautions. Grubs of this and other beetle species can eat Zinnia roots as well as those of turf. Treating nearby lawns for grubs will reduce the population. See Lawns, oriental beetle.
Japanese beetle, Popillia japonica.
This beetle, in the adult stage, feeds upon a great variety of trees and plants, including zinnia. The beetle is ½" long, bright, shining green, with copper colored wing covers. Two white spots on the tip of the abdomen show beyond the ends of the wing covers, and there are five white spots formed by patches of white hairs on each side of the abdomen. The beetles begin emerging in late June but their greatest abundance is usually about the middle of July. During drought periods populations of Japanese beetle are drastically reduced.
Japanese beetle traps containing floral and sex attractant lures are readily available at home and garden centers. These traps may help reduce the overall number of beetles. However, beetles often land and feed on plants close to traps, so traps should be placed well away from valued plantings. Feeding on zinnia foliage can be prevented by using sprays of acephate, azadirachtin, carbaryl or malathion, which are among the products registered for use against this pest in Connecticut. Imidacloprid, applied early in the season as a soil drench, will provide season-long systemic control. Consult the labels for dosage rates and safety precautions. Larvae develop in turf, so treatment of lawn areas should reduce the numbers of beetles.
The single zinnia, Z. angustifolia, is susceptible to lacebugs. If infestations are severe, apply insecticidal soap, ultrafine horticultural oil or malathion, which are among the compounds registered for control of this pest in Connecticut, when insects are first noticed. Spray needs to contact insects in order to be effective. Alternatively, imidacloprid can be applied as a systemic to be taken up by the roots. Consult the label for dosage rates and safety precautions.
Stalk borer, Papaipema nebris.
This borer infests an occasional stalk of many kinds of herbaceous plants, including zinnia. 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.