Plant Health Problems
See Perennials for a detailed discussion of problems that may occur and are common to most herbaceous ornamentals.
Diseases caused by Fungi:
Downy mildew, Plasmopora sp.
This pathogen typically causes leaf spots with downy white or gray patches under the leaves. The downy growth results from the production of spores called sporangia which are wind-dispersed between plants. Disease is usually favored by cool wet weather.
Control may include cultural means of reducing humidity and leaf wetness. Control may also be achieved with the use of fungicides applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut is mancozeb. Consult the label for dosage rates, safety precautions, and directions for use.
Leaf spots, Alternaria, Cercospora, Colletotrichum, Phyllosticta or Septoria spp.
Leaf spots are very common, typically sharply delimited necrotic areas on plant leaves caused by a wide variety of pathogenic species. Leaf spots usually are favored by wet conditions and may become important if a large number of lesions are present or if they start to coalesce.
Under those conditions, control may also be achieved with the use of fungicides applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are thiophanate-methyl and sulfur. Consult the label for dosage rates and safety precautions.
Powdery mildew, Erysiphe.
These fungi are obligate plant parasites which grow vegetatively on the plant leaf surface, sending haustoria, structures which absorb food from the host, into epidermal cells. The white mildew seen on the leaf is a combination of vegetative mycelium and spores borne in chains on upright conidiophores. Wind-dispersed mildew spores can germinate without free water under high humidity conditions, and disease is often severe when conditions are humid but dry. Small black over-wintering structures called perithecia are often found in powdery mildew affected areas.
Control may also be achieved with the use of fungicides applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are potassium bicarbonate, ultra fine oil, sulfur, triadimefon, or thiophanate-methyl fungicides. Consult the label for dosage rates, safety precautions, and directions for use.
Rust, Puccinia, Uromyces, or Aecidium spp.
The term rust refers to both the disease and pathogen causing the disease. Rusts are specialized obligate parasites which can cause disease on one (monoecious) or two (heteroecious) host species. Symptoms of rust infection include rust-colored spores or gelatinous horns in powdery pustules on leaves or stems. Surrounding tissue is discolored and yellowed, and plants are often stunted.
Control of heteroecious rusts may be aided by removal of the alternate host, but for most perennials, control may also be achieved with the use of fungicides applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are sulfur and mancozeb. Consult the label for dosage rates, safety precautions, and directions for use.
Stem rot, Sclerotium rolfsii, or Sclerotinia sclerotiorum.
Symptoms include yellowing of lower leaves followed by wilting and death of the rest of the plant. A white cottony mass of mycelium growing around the crown or on the soil near the crown distinguishes this crown rot from others. In this fungus web may be found the whitish to cream color or buff or reddish-brown seed-like sclerotia the size of a pinhead.
Control may include removing and destroying all infected plant parts and removal of top soil around the plant. New soil may replace sclerotia-filled soil.
Wilt, Verticillium dahliae.
This pathogen infects the vascular or water-conducting tissues of plants, and cause wilt symptoms by impairing water flow. As a result, symptomatic plants may flag or wilt on one side of the plant, leaves may be twisted and yellow on one side, turn brown and hang down prior to drying up. Overall, the plant exhibits drought symptoms despite adequate soil moisture. If the stem is cut open near the base, the vascular tissues are typically brown or discolored.
Wilt control involves removal of infected plants and associated roots and soil. The pathogens survive in soil and plant debris for long times, so disposal of plants and soil without spread is important.
Diseases caused by Phytoplasmas:
Aster yellows, phytoplasma.
The pathogen is a prokaryotic organism without cell walls. It infects the phloem of susceptible plants and causes a general yellowing and dwarfing symptom. The phytoplasma is spread by a leafhopper vector.
Infected plants should be removed and destroyed. Early season control of the leafhopper vector and removal of weed hosts may help prevent re-infection.
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 Rudbeckia roots as well as those of turf. Treating nearby lawns for grubs will reduce the population. See Lawns, oriental beetle.
Crescentmarked lily aphid, Neomyzus circumflexus.
This aphid is yellow and black. See Perennials for information on the biology of aphids.
Fleahopper, Halticus bractatus.
Both adults and nymphs of this plant bug damage plants by sucking plant sap. They overwinter as adults in protected areas. Females puncture leaves and stems with their mouthparts and then lays eggs in the damaged tissue. Nymphs are green but get darker as they mature. There are multiple generations each year. Foliar symptoms include bleached or dark spots. Some plants show red spotting. Leaves may be distorted along with flowers. If control is needed, malathion or methoxychlor, both of which are among the products registered for use against this pest in Connecticut, can be applied as foliar sprays. Imidacloprid can be used as a systemic treatment. Consult the label for dosage rates and safety precautions.
Fourlined plant bug, Poecilocapsus lineatus.
This bug lays eggs in the soft stems. They hatch about the middle of May, and the young bugs suck the sap from the tender leaves. They molt five times and when 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 black stripes alternating with three green stripes. The injury to the leaves consists of sunken areas around the punctures. These areas later appear as circular transparent spots and finally as circular holes. This insect injures the new leaves of many different kinds of annual and perennial plants and shrubs. There is only one generation each year. Among the products registered for use against this pest in Connecticut are azadirachtin, ultrafine horticultural oil, insecticidal soap, or malathion applied as sprays, or imidacloprid applied as a systemic to be taken up by the roots. Consult the label for dosage rates and safety precautions.
Fuller rose beetle, Asynonychus godmani.
Many species of plants are sometimes injured by Fuller rose beetle, which feeds on the leaves at night and rests in the leaves or in some protected place during the day. This is a brown weevil marked with patches of gray scales on wing covers that are fused. Only females exist. They vary in length from 1/4" to 3/8" (7-9 mm). The eggs are laid under the edges of bark near the ground or directly on the ground and can be found throughout the season. The white legless grubs live in the soil and chew upon feeder roots, as well as the larger roots of various plants. At maturity, they measure about 1/2" (10-12 mm). This weevil overwinters as a pupa near the soil surface or as an adult in a protected spot. Adults do the most damage. Control treatments are rarely needed. When the infestation is light, handpicking the insects is a possibility.
Sawflies, Macrophya intermedia.
The adult of this damaging caterpillar is wasp-like, but has a fat waist. The caterpillar is gray with dark stripes and is a nocturnal feeder doing most of its damage in May and June. Spraying young caterpillars with spinosad or malathion, which are among the compounds registered for control of this pest in Connecticut, will control them. 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. 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 generation each year. 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 upon the leaves of the nearest food plant, and later tunnels in the stem. The mature larva is nearly 1 1/2" long, grayish brown with one white stripe on top and two white 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.
Disposing of all the old stalks and destroying weeds at the edges of the garden helps reduce the population. When needed, methoxychlor dust, which is among the compounds registered for control of this pest in Connecticut, applied in June, should control this pest. Consult the label for dosage rates and safety precautions.
Twospotted spider mite, Tetranychus urticae.
This pest infests 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 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 (1/2 - 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. Additional materials appropriate for commercial growers include hexythiazox and abamectin (a restricted use product). 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.
The greenhouse whitefy, Trialeurodes vaporariorum, the sweetpotato whitefly, Bemisia tabaci, and silverleaf whitefly, Bemisia argentifolii, commonly infest 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 on the undersides of leaves and become scale-like. These insects 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 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 in the greenhouse. Another parasitoid, Eretmocerus californicus, attacks all three species and can assist in controlling minor infestations in the greenhouse. Insecticidal soap or ultra-fine 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. Azadiractin (neem) or fenoxycarb, directed to the undersides of the leaves, can also be used. Repeat applications of sprays will probably be needed because some stages in the life cycle may not be affected by insecticides. Chemical control using conventional insecticides is difficult because of widespread insecticide resistance. Season-long control can be achieved with imidacloprid applied as a systemic taken up by the roots. Consult the label for dosage rates and safety precautions.