Plant Health Problems
Diseases caused by Fungi:
Leaf spots, Ascochyta, Cercospora.
Circular to irregular tan to brown spots develop on leaves. These can vary in size, color, and number.
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 and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Wilt, Fusarium sp., Verticillium sp.
Vascular wilts are troublesome diseases of perennial asters. Plants may be attacked at any stage; young plants can suddenly dry up or older plants might develop a pale green color accompanied by wilting of lower leaves, often first appearing on one side of the plant. This is frequently followed by a general wilting and death of the entire plant. Plants may wilt in the middle of the day and seem to recover at night. Some plants may show no signs of infection until they come into flower, when they suddenly collapse. When the stem is cut, a brown discoloration or streaking may appear in the vascular tissues.
Control of these diseases is difficult since the pathogens are commonly found in soil. One of the key strategies for control of vascular wilts is prevention. Therefore, it is important to avoid planting asters in infested soil. It is also helpful to maximize plant vigor by good cultural care and watering. Careful handling of plants will avoid root injury which enables the fungus to enter the plant. Since repeated use of the same area greatly increases the amount of disease, rotation is essential. When available, it is also helpful to use resistant varieties. Chemical controls are not effective for these fungi.
Rusts, Coleosporium, Puccinia, Uromyces.
Symptoms of rust infection are first visible as chlorotic lesions on the upper leaf surface. Diagnostic symptoms then develop on the underside of the leaf and appear as pustules which break open to reveal the orange-rusty powdery spores for which these diseases get their name. Depending upon the particular rust fungus, alternate hosts may be involved, including 2- and 3-needle pines, some grasses, and sedges. Rusts can result in some defoliation, especially when plants are crowded.
These diseases can be minimized by cleaning up plant refuse in the fall and by adequate spacing of the plants to promote good air circulation. 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 mancozeb, maneb, and triadimefon. Consult the label for dosage rates and safety precautions.
Botrytis blight, 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 and foliage 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. Consult the label for dosage rates and safety precautions.
Powdery mildew, Erysiphe.
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.
Diseases caused by Phytoplasmas:
Symptoms consist of a yellowing or clearing of the veins in newly infected leaves, shortening of the internodes of the main stem, and production of long axillary breaks which create a yellowish witches' broom. If plants are infected young, they remain stunted, and if flowers are produced, they are small, abnormal, and 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), asters 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.
These plant-parasitic worms attack virtually all plant parts and may cause leaf lesions, yellowing, necrosis and leaf drop, and bud malformation. Lower leaves first show brown wedge-shaped areas between the veins which eventually involve the entire leaf. Discoloration then progresses from the bottom to the top of the plant. The nematodes live and move in water films.
Reducing leaf moisture and removal of infected tissues, debris, or plants is important.
Aphids, Macrosiphum artemisiae, M. asterifoliae.
These aphids are found on stems and the undersides of leaves. Large numbers of aphids may justify sprays of ultra-fine horticultural oil, insecticidal soap or malathion, which are among the compounds registered for control of this pest in Connecticut. Imidacloprid applied as a systemic to be taken up by the roots will also provide season-long control. Consult the label for dosage rates and safety precautions.
Aster leafhopper, Macrosteles quadrilineatus.
This insect feeds by sucking the juices from the plants and is responsible for the transmission of aster yellows from diseased to healthy plants. In the spring, the insects feed on diseased wild plants and then carry the virus to cultivated asters, marigolds, calendula, chrysanthemums, cosmos, dahlia and gaillardia. The adults are about one eighth of an inch long and greenish gray in color. Control of aster yellows is difficult because ornamental plants are continually reinfested by leafhoppers that have fed on diseased wild plants. Apply carbaryl, which is among the compounds registered for control of this pest in Connecticut, to manage leafhoppers. Imidacloprid applied as a systemic to be taken up by the roots will also provide season-long control. Consult the label for dosage rates and safety precautions. Whenever possible, put plants in a fine-screened cage. Discarding diseased plants throughout the season will also help.
Blister beetles, Epicauta sp.
Aster flowers are frequently devoured by the black blister beetle, occasionally by the margined blister beetle, and rarely by two or three other species, the gray blister beetle, and the ash-gray blister beetle. When needed, sprays of methoxychlor, which is among the compounds registered for control of this pest in Connecticut, will be effective against the adults. Consult the label for dosage rates and safety precautions.
Chrysanthemum lacebug, Corythucha marmorata.
The adult of this pest is about 1/8" long with sculptured, lace-like wings. One sure sign of lacebugs is the small dark spots of feces left on the undersides of leaves. This lacebug overwinters as adults in protected areas near host plants. Eggs are laid in groups on the undersides of leaves near veins. There may be two generations each year depending on climatic conditions. Both adults and nymphs suck the sap from the underside of the leaves, causing a mottling or blanching.
When needed, malathion, insecticidal soap or ultra-fine horticultural oil, which are among the compounds registered for control of this pest in Connecticut, applied the last week in May, following egg hatching is highly effective. Spray should be directed from the bottom of the plant upward to ensure thorough coverage of the lower leaf surfaces. Imidacloprid applied as a systemic to be taken up by the roots will also provide season-long control. Consult the label for dosage rates and safety precautions.
Leafminers, Agromyza platyptera, A. posticata, Liriomyza trifolii, L. huidobrensis, Phytomyza albiceps.
The leaves of aster are sometimes infested by miners that make narrow serpentine mines (Liriomyza spp., Phytomyza albiceps), or blotch mines (Agromyza platyptera and A. posticata). Eggs are laid in the leaves by tiny flies. If abundant enough to justify control, a soil drench of imidacloprid, which is among the compounds registered for use against this pest in Connecticut, should kill larvae in the mines. Organophosphate and carbamate insecticides tend to have little impact against Liriomyza spp. due to their development of resistance. Consult the label for dosage rates and safety precautions.
Root aphids, Anuraphis maidiradicis.
The roots of asters and other plants are often infested with mealy, white, wingless aphids. These aphids are usually carried in by ants. Killing the ants usually gets rid of the aphids. Imidacloprid applied as a systemic to be taken up by the roots will also provide season-long control. Consult the label for dosage rates and safety precautions.
Stalk borer, Papaipema nebris.
This insect is a borer in aster. This borer infests an occasional stalk of many kinds of herbaceous plants and it frequently causes rather severe injury to dahlia. 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.
Grubs of Japanese beetle, Asiatic garden beetle and oriental beetles, when abundant, may feed on the roots of asters, causing a weakening of the plants. See Lawns.