Plant Health Problems
Diseases caused by Fungi:
Root rot, Pythium, Rhizoctonia, Fusarium.
Symptoms appear as wilting, stunting, and general droopiness of the foliage and are non-specific. This flaccid appearance is often accompanied by browning and rotting of the roots and the crown. Yellowing and death of the outer leaves follows until finally the entire plant collapses and dies. The fungi associated with root rots are soilborne organisms that can persist in the soil for many years.
Control can be achieved by maximizing plant vigor by careful watering and fertilizing. It is also helpful to locate the plants in different areas of the garden using crop rotation. Highly symptomatic plants can be rogued and removed since recovery is unlikely. Fungicides are not effective for control.
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 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, mancozeb, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Greenhouse orthezia, Orthezia insignis.
This insect is a common pest of ageratum in greenhouses. 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, which is among the compounds registered for control of this pest in Connecticut, can be applied according to label directions.
Mealybugs, Planococcus citri.
Ageratum plants in greenhouses are often infested with mealybugs. White cottony masses appear on leaf surfaces, in leaf axils and sheaths. These insects damage plants by sucking plant sap. Among the compounds registered for control of this pest in Connecticut are insecticidal soap, ultrafine horticultural oil or resmethrin. These products are most effective against crawlers. Because of overlapping life stages in a home environment, multiple applications will be needed to control this pest. Spray needs to contact the insect. Alternatively, use imidacloprid as a drench to be taken up as a systemic by the roots. Consult the label for dosage rates and safety precautions.
Twospotted spider mite, Tetranychus urticae.
Spider mites infest the undersides of the leaves, which become light yellow in color, and the plants have a generally unhealthy appearance. Sometimes the mites form webs, which more or less enclose the upper as well as the lower leaf surface. Among the compounds registered for control of this pest in Connecticut are insecticidal soap and ultrafine horticultural oil. Spraying with insecticidal soap will give sufficient control if applied at least twice at 7-10 day intervals. The predatory mite, Neoseiulus fallacis, is most commonly found feeding where there are mite infestations. A single application of ultrafine horticultural oil (1% dilution) can be effective if predatory mites are present. Special care should be taken with soap or oil to obtain thorough spray coverage, because they only work on contact. Abamectin is an effective restricted use material. Consult the label for dosage rates and safety precautions. Avoid applying carbaryl or pyrethroids, which tend to be much more toxic to the predators than to the pest spider mites.
Whiteflies, Trialeurodes vaporariorum.
The greenhouse whitefly, Trialeurodes vaporariorum, the sweetpotato whitefly, Bemisia tabaci, and silverleaf whitefly, Bemisia argentifolii, commonly infest many kinds of plants grown under glass and are often carried into the field where they may persist. The life cycles of these species are similar. The tiny, white moth-like adult has a mealy appearance due to the small particles of wax that it secretes. It lays groups of eggs on the underside of leaves. The eggs hatch into small oval crawlers, which then settle down and become scale-like nymphs that suck sap from stationary locations on the leaves. These then spend about 4 days in an immobile pupal stage before becoming adults. About 5 weeks are required to complete the life cycle in the greenhouse.
Yellow sticky traps are an effective way to monitor populations of whiteflies, and may even be attractive enough to reduce minor infestations. Biological controls can also be effective against whiteflies, especially in a greenhouse environment. The predatory ladybeetle, Delphastus pusillus, specializes in whiteflies and feeds on all three whitefly species. The parasitoid, Encarsia formosa, can control the greenhouse whitefly, but not the other species, in the greenhouse. Another parasitoid, Eretmocerus californicus, attacks all three species and can assist in controlling minor infestations in the greenhouse. Insecticidal soap or ultrafine horticultural oil, which are among the compounds registered for control of this pest in Connecticut, sprayed on the undersides of leaves, can be used against whiteflies in the greenhouse or the field. When applied at a low, half-percent concentration, soap is selectively toxic to whiteflies rather than parasitic wasps. Azadiractin (neem) and fenoxycarb, also directed to the undersides of the leaves, act as insect growth regulators and can also be used. Repeat applications of sprays will probably be needed because insecticides do not kill some stages in the life cycle. Imidacloprid can be applied as a systemic to be taken up by the roots. Consult the label for dosage rates and safety precautions. Chemical control using conventional insecticides is difficult because of widespread insecticide resistance.