Connecticut State The Connecticut Agricultural Experiment Station
Cosmos (Cosmos)
Plant Health Problems
Diseases caused by Fungi:
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 usuallymost pronounced during hot, humid weather. Heavily infected leaves turn brownand shrivel.
Disease can be minimized by avoiding overcrowded spacing of plants and bycarefully picking off affected leaves as soon as symptoms are evident.Symptomatic leaves can be placed into a plastic bag in order to avoid spreadingthe spores of the fungus to other plants. Use of fungicides is usually notnecessary. 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 fordosage 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 atarget-like appearance can also develop on the leaves. These patches are oftenassociated with flowers which have dropped onto the leaf surface. This diseaseis particularly troublesome during periods of extended cloudy, humid, wetweather.
Good sanitation practices including grooming the plants and removing spent orsenescing flowers can minimize the potential for infection. These affectedtissues should be carefully removed and discarded when they are dry. It is alsoimportant to avoid wetting the flowers when watering and crowding plants.Adequate spacing between the plants can promote good air circulation. Controlcan also be achieved with the use of fungicide sprays applied as soon assymptoms are visible. Among the compounds registered for use in Connecticut aresulfur and thiophanate-methyl. Consult the label for dosage rates and safetyprecautions.
Smut, Entyloma.
Symptoms first appear as small chlorotic flecks or spots on the leaves andstems. These eventually develop into whitish pustules that break open to revealthe powdery spores of the fungus. Once the pustules rupture, the leaves dry,turn brown, and shrivel.
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 chanceto dry before nighttime. It is also helpful to pick and remove symptomaticleaves as soon as they develop.
Diseases caused by Phytoplasmas:
Yellows, phytoplasma.
Symptoms consist of a yellowing or clearing of the veins in newly infectedleaves, shortening of the internodes of the main stem, and production of longaxillary breaks which create a yellowish witches' broom. If plants are infectedyoung, they remain stunted, and if flowers are produced, they are small,abnormal, and exhibit phyllody, a condition in which flower parts revert togreen tissues. Symptoms do not always appear on the entire plant, but may showon only one stalk, depending on the time of infection. This phytoplasma istransmitted by leafhoppers. Since the pathogen infects many plants (e.g., daisies,chrysanthemums, plaintain, echinacea, and rudbeckia), cosmos may becomeinfected from neighboring plants.
Strategies for control include promptly roguing and removing diseased plants aswell as control of the leafhopper vectors.
Insect Problems:Aphids, Myzus persicae and Macrosiphoniella sanborni.
The green peach aphid and the chrysanthemum aphid, Macrosiphoniella sanborni,sometimes infest cosmos. They may be managed by spraying with insecticidal soapor ultra-fine horticultural oil, which are among the compounds registered forcontrol of this pest in Connecticut. Aphids on chrysanthemums outdoors can becontrolled using malathion sprays or imidacloprid can be applied as a soildrench for season-long, systemic control. Consult the label for dosage ratesand safety precautions.
Fourlined plant bug, Poecilocapsus lineatus.
This bug lays eggs in the soft stems. They hatch about the middle of May andthe young bugs suck the sap from the tender leaves. They molt five times andwhen mature, about the middle of June, they have wings and are nearly 1/3"long. The insect body is yellow, marked lengthwise on the wings with four blackstripes alternating with three green stripes. The injury to the leaves consistsof sunken areas around the punctures. These areas later appear as circulartransparent spots and finally as circular holes. This insect injures the newleaves of many different kinds of annual and perennial plants and shrubs. Thereis one generation each year. The nymphs can be managed by spraying withazadirachtin, ultra-fine horticultural oil, insecticidal soap or malathion,which are among the compounds registered for control of this pest inConnecticut. Consult the label for dosage rates, safety precautions, andpreharvest intervals.
Twospotted spider mite, Tetranychus urticae.
This is sometimes a pest of cosmos. It infests the undersides of the leaves,which become light yellow in color, and the plants have a generally unhealthyappearance. Sometimes the mites form webs, which more or less enclose the upperas well as the lower leaf surface. Among the compounds registered for controlof this pest in Connecticut are insecticidal soap and ultrafine horticulturaloil. Spraying with insecticidal soap will give sufficient control if applied atleast twice at 7-10 day intervals. The predatory mite, Neoseiulus fallacis,is most commonly found feeding where there are mite infestations. A singleapplication of ultrafine horticultural oil (1/2 to 1% dilution) can beeffective if predatory mites are present. Special care should be taken withsoap or oil to obtain thorough spray coverage, because they only work oncontact. Abamectin is an effective, restricted use product. Consult the labelsfor dosage rates and safety precautions. Avoid applying carbaryl or pyrethroids,which tend to be much more toxic to the predators than to the pest spidermites.
Stalk borer, Papaipema nebris.
This borer infests an occasional stalk of many kinds of herbaceous plants. As arule its presence escapes notice until the plant begins to wilt. Then it is toolate for the plant to recover. The larva tunnels up and down inside the stemand the top portion usually wilts and later dies. There is one annualgeneration. The moths emerge in September and October and lay eggs on thestalks of their food plants, in which stage the insect passes the winter. Theeggs hatch in May or early June. The young larva begins to feed on the leavesof the nearest food plant, and later tunnels in the stem. The mature larva isnearly 1 ½" long, grayish brown with one white dorsal stripe and two whitelateral stripes on each side. On the front half of the body the lateral stripesare interrupted, and the lower brown stripe extends forward onto the side ofthe head.
Burning all the old stalks, if allowed, and destroying weeds at the edges ofthe garden helps control this insect. When needed, methoxychlor, which is amongthe compounds registered for use against this pest in Connecticut, applied as adust, in June, should control this pest. Consult the label for dosage rates andsafety precautions.
