Chrysanthemum (Chrysanthemum, Dendranthema, Leucanthemum)
Diseases caused by Fungi:
Leaf spots, Septoria sp.
Symptoms first appear as yellowish areas which turn brown to black. Spots often appear on lower leaves first and can coalesce until the entire leaf turns brown-black and dies. The leaves appear to die from the bottom of the plant up the stem. Symptoms are easily confused with those associated with the foliar nematode.
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.
Wilts, Verticillium, Fusarium.
Vascular wilts are troublesome diseases of garden chrysanthemums. 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 mums 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.
Brown rust, Puccinia.
Rust pustules start as swellings on the underside of the leaves which soon break open discharging chocolate-brown powdery spores. These spores infect chrysanthemums and will spread the disease through the rest of the planting. The alternate host is not known.
This disease 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.
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.
Gray mold, 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, copper sulphate pentahydrate, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Diseases caused by Viruses:
Symptoms appear as a distinct stunting of individual branches or the entire plant. Leaves on infected branches may be smaller than normal and flowers are small and have a washed-out color. Infected plants often produce flowers earlier than their healthy counterparts.
Control is very difficult so symptomatic plants should be promptly removed.
Diseases caused by Nematodes:
Foliar nematodes, Aphelenchoides.
Black to brown V-shaped areas appear in the lower leaves, showing first at the bottom of the plant and working their way up the plant. These spots are bounded by veins at first but eventually may involve the entire leaf. The trouble is caused by microscopic eelworms which live in and destroy the leaf tissue. They come out of the leaf when the leaf surface and stems are wet with rain, dew, or syringing, and swim up the stem to infect the leaves above. The nematodes overwinter in plant debris in the soil.
This disease can be minimized by reducing leaf moisture and removal of infected tissues, debris, or plants.
Aphids, Myzus persicae and Macrosiphoniella sanborni.
The green peach aphid and the chrysanthemum aphid, Macrosiphoniella sanborni, often infest chrysanthemum plants in the greenhouse. They may be managed by spraying with insecticidal soap or ultra-fine horticultural oil, which are among the compounds registered for control of this pest in Connecticut. Aphids on chrysanthemums outdoors can be controlled using malathion sprays or imidacloprid can be applied as a soil drench for season-long, systemic control. Consult the label for dosage rates and safety precautions.
Chrysanthemum gall midge, Rhopalomyia chrysanthemi.
A small slender fly lays eggs in the leaves and tender shoots in greenhouses and each larva forms a cone-shaped gall. Plants in the open are not usually injured. Probably there is more than one generation each year, but it is difficult to estimate the number because of overlapping of generation. The midge is more abundant in spring and fall than in other seasons. Bringing only clean cuttings or plants into the greenhouse can reduce damage from this pest.
Existing infestations can be managed with sprays of malathion, which is among the compounds registered for control of this pest in Connecticut, applied according to label directions.
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, acephate 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 to the soil as a systemic, provides season-long control. Consult the labels for dosage rates and safety precautions.
Chrysanthemum leafminer, Chromatomyia syngenesiae,
Liriomyza trifolii, L. huidobrensis.
These insects overwinter as partially grown larvae inside leaves (C. syngenesiae), or are transported in commercially greenhouse-grown plants. The adult C. syngenesiae are gray flies about 1/12" long, emerge in spring and begin egg laying. Liriomyza spp. are the same size, but are striking black and yellow flies. Eggs are laid in the leaves. The leaves develop narrow serpentine mines. 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. Abamectin, a restricted use product, is also effective. Organophosphate and carbamate insecticides tend to have little value against Liriomyza spp. due to their development of resistance. Consult the label for dosage rates and safety precautions.
Chrysanthemum thrips, Thrips nigropilosus.
The insect feeds in the developing flowers, causing deformation and mottling. The adult of this and another damaging species, the western flower thrips (Frankliniella occidentalis), are brownish-yellow with feathery wings. The young are pale lemon yellow and also cause damage with their sucking mouthparts. Insecticidal soap, malathion, or spinosad, which are among the compounds registered for use against this pest in Connecticut, may control thrips. Some populations of western flower thrips are resistant to all insecticides registered for use by homeowners. Consult the label for dosage rates and safety precautions.
Cyclamen mite, Phytonemus pallidus.
This translucent, microscopic mite often infests the new leaf and blossom buds, and can proliferate on spent blossoms. Characteristic injury includes dwarfed, thickened and wrinkled leaves. On azalea, dahlia, and other plants with stems, the distance between internodes becomes drastically shortened. Infested plants do not bloom, or blossoms may be misshapen. Cyclamen mite is not easily controlled when mature plants are badly infested: effective miticides are not available to homeowners, so a commercial applicator may be required for effective miticide applications. Effective restricted use materials are abamectin, dicofol, and endosulfan.
European corn borer, Ostrinia nubilalis.
The second generation of this pest sometimes lays its eggs on chrysanthemum.
The larvae tunnel in stems, weakening them so they break. The larva is pale white or gray with black tubercles and is not more than 1" long when fully grown. Adults have a wingspread of 1" or so and are buff to brown in color. There are usually two generations annually. Eggs are laid on the underside of leaves and the larvae tunnel in the stalks and pupate in the burrows. Second-generation larvae and those of the single generation corn borer overwinter in stems and pupate in the spring.
The parasitic wasp Trichogramma has been used as alternative method of control. This tiny wasp attacks the egg masses of the corn borer, and the eggs of other caterpillars, too. Be sure to purchase the insects from a reputable supplier and make sure the strain you purchase is known to be well adapted to attacking corn borer. Bt will not harm the Trichogramma wasps, but other insecticides may.
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 one generation each year. The nymphs can be managed by spraying with azadirachtin, 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 taken up by the roots, will provide season-long control. Consult the labels for dosage rates, safety precautions, and preharvest intervals.
Mexican mealybug, Phenacoccus
Although there are other species of mealybug that attack mums, the Mexican mealybug is the most injurious. It attacks all stages of growth, stunting plants by causing distortion of the leaves. Malathion, insecticidal soap or ultra-fine horticultural oil, which are among the compounds registered for use against this pest in Connecticut, should give good control when applied according to label directions. Ants that tend mealybugs in order to harvest their honeydew, can be controlled by lightly dusting the soil surface with chlorpyrifos. 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.
Spittlebugs, Philaenus spumarius.
These insects cause a stunting, curling, twisting of the terminal growth, side branches and infested leaves of hardy mums. The insects cover themselves with a frothy material, resulting in the common names "snake-spit" and "frog-spit." When needed, malathion, which is among the compounds registered for use against this pest in Connecticut, applied to the terminals during the spring and early summer should give adequate control. 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.