Chrysanthemum Diseases & Insect Pests

Chrysanthemums, or hardy garden mums, like full sun and fertile, well-drained soil. They need regular watering because their roots are very shallow. These hardy garden mums were recently transferred into the genus Dendranthema, which is now a separate genus from the marginally hardy florist mums. Although the list of diseases that may attack chrysanthemums is long, mums are relatively trouble-free. However, drought will cause woody, stunted growth, and overwatering will cause yellowing leaves that blacken and drop.

Leaf Spots

Several different kinds of fungi cause leaf spot on chrysanthemum: Septoria chrysanthemi, Septoria chrysanthemella, Alternaria species, and Cercospora chrysanthemi. Symptoms consist of spots on the leaves. These spots are at first yellowish and then become dark brown and black, increasing from ⅛ to 1 inch or more in diameter. Leaf spots may be surrounded by a purple margin. As the disease progresses, leaves may wither prematurely. Typically, the lower leaves are infected first.

Prevention & Treatment: Handpick and destroy the infected leaves. Also, regularly clean up and destroy dead plant debris in the garden to reduce spore populations. These fungi overwinter as spores in such debris. A layer of mulch helps prevent spores from splashing from the soil onto plants. If the disease is severe enough to warrant chemical control, fungicides containing chlorothalonil, mancozeb, myclobutanil, propiconazole, or thiophanate methyl may be used. See Table 1 for examples of brands and products. Follow all the directions on the label for mixing rates, use, reapplication intervals, and safety.

Foliar Nematodes (Aphelenchoides ritzema-bosi)

Hardy chrysanthemums that develop yellow-brown spots starting on the lower leaves and gradually moving up the stems may be infested with foliar nematodes. Nematodes are slender, unsegmented roundworms that are barely visible to the unaided eye. Foliar nematodes overwinter in the soil, in infested plant material. They swim up the film of water on the plants created by spring rains and enter leaves through the stomata. Nematodes can become dormant and survive for over a year in fallen leaves. Yellow-brown spots on the leaves eventually run together and cover the entire leaf, which dies, turns brittle, and falls. Severe infestations can kill entire plants. Foliar nematodes are easily confused with leaf spot damage (see above), but fungal leaf spots are most often black, not brown. They additionally infest hosta and ferns.

Prevention & Treatment: Mulch plants in the spring to discourage nematodes from climbing up from the soil and avoid spraying water on the leaves when watering. Use drip irrigation to help prevent leaf wetness and to reduce the spread of foliar nematodes.

Remove, bag up, and dispose of infected plant material, along with the surrounding surface soil. Foliar sprays with insecticidal soap may help reduce nematode populations. Immediately after removing plants with obvious symptoms, thoroughly spray insecticidal soap on any nearby plants. The soap spray may kill any foliar nematodes that have spread onto the nearby plant foliage but have not yet entered the foliage of these plants. See Table 1 for examples of brands and products.

Rust

This disease is caused by the fungus Puccinia chrysanthemi. Rust infection causes pale areas to appear on upper leaf surfaces, with powdery orange pustules or spots directly beneath on the undersides of the leaves. Severely infected plants are weakened and fail to bloom properly. A white rust caused by Puccinia horiana produces yellow spots on the upper leaf surface and tan pustules that turn white with spore production on the lower leaf surface.

Prevention & Treatment: Remove infected leaves as soon as possible. Set new plants farther apart and provide better ventilation. Water the soil without wetting the plants. If the disease is severe enough to warrant chemical control, use a fungicide with myclobutanil, chlorothalonil, or mancozeb as the active ingredient. See Table 1 for examples of brands and products. Follow all directions on the label.

Some chrysanthemum varieties, which are resistant to rust, are ‘Achievement’, ‘Copper Bowl’, ‘Escapade’, ‘Helen Castle’, ‘Mandalay’, ‘Matador’, ‘Miss Atlanta’, ‘Orange Bowl’, and ‘Powder Puff’.

Wilt

This disease is caused by the fungi Verticillium albo-atrum and/or Fusarium oxysporum f.sp. chrysanthemi). The first symptoms of wilt are yellowing and browning of the leaves, which die from the base of the plant upwards. Infected plants are stunted and often fail to produce flowers. The entire plant may wilt and die. The fungus is soil-borne and enters the plant through the roots, later invading the vessels of the stem and cutting off the water supply.

Prevention & Treatment: Control of this wilt on plants grown in infested soil is difficult. Remove and destroy all infected plant material. Ultimate control lies with purchasing certified disease-free plants and strict sanitation locally. It is best to use resistant varieties. If Fusarium has been a problem, increase the pH of the soil to between 6.5 and 7.0.

Powdery Mildew

This disease is caused by the fungus Erysiphe cichoracearum. The leaves are covered with a whitish, ash-gray powdery growth. The spores require a very moist atmosphere in which to germinate and spread the infection.

Prevention & Treatment: Remove diseased plant material. Spray with myclobutanil, propiconazole, chlorothalonil, or thiophanate methyl according to instructions on the product label. See Table 1 for examples of brands and products.

Ray Blight

This disease (also called Ascochyta ray blight) is caused by the fungus Mycosphaerella ligulicola. The ray flowers (marginal flowers of an inflorescence) are infected so that the blooms are deformed and one-sided. Early infection may cause blasting of the buds. Additionally, leaf spots and stem cankers may occur.

Prevention & Treatment: Chlorothalonil, mancozeb, myclobutanil, thiophanate methyl, or propiconazole are effective foliar sprays when applied at label rates. See Table 1 for examples of brands and products.

Ray Speck

The fungi Stemphylium and Alternaria cause ray speck disease. Infection by these fungi produces brown or white necrotic specks surrounded by colored halos on the fully expanded ray florets when humidity and temperatures are high.

Prevention & Treatment: See ray blight control.

Gray Mold

Gray mold is a disease caused by the fungus Botrytis cinerea. Leaves show brown water-soaked spots. Infected parts become covered with a grayish-brown, powdery mass of spores. This disease may be confused with Ray Blight disease.

Prevention & Treatment: Space plants for increased circulation of air. Do not wet the foliage during irrigation. Apply foliar sprays of chlorothalonil or thiophanate methyl. See Table 1 for examples of brands and products. For more information on gray mold control, please see the fact sheet HGIC 2100 Gray Mold.

Bacterial Blight

This bacterial disease is caused by Erwinia chrysanthemi. The most pronounced symptom is a rot of the upper part of the stem, resulting in wilt and collapse of the distal portion. Infected cuttings may show a brown to black decay at their bases. Occasionally, the only symptom is marginal leaf scorch.

Prevention & Treatment: For containerized plants, always use new soil-less mix and clean containers. Soil in which diseased plants grew should be pasteurized with heat before reuse. Use cuttings that are disease-free or dip cuttings for 4 hours in solutions of antibiotics such as streptomycin. See Table 1 for examples of brands and products.

Crown Gall

This disease is caused by the bacterium Agrobacterium tumefaciens. Infection with this bacterium causes large swellings on the crown and nearby roots.

Prevention & Treatment: Discard infected plants. Do not wound the stems, as this increases the chance of infection if the bacterium is in the soil. Buy containerized plants. If buying rooted cuttings, be sure they are certified to be disease-free. Growers use Agrobacterium radiobacter (strain K84) as a preventive treatment.

Virus Diseases

Chrysanthemums are subject to a large number of virus diseases, including mosaic, chrysanthemum smut virus, and tomato spotted wilt virus. Virus-infected plants generally have spindly, stunted shoots and yellowed foliage. Leaves may be marked with ring spots, lines, pale areas, or mottling. Infected plants are stunted, form dense “rosettes”, and have small flowers. Aster yellows of chrysanthemum is caused by a phytoplasma, and infection results in deformed, green, leaf-like flowers. Virus diseases are spread by sucking insects such as aphids and leafhoppers. Aster yellows is spread by leafhoppers.

Prevention & Treatment: There is no cure for virus-infected plants. Remove and destroy them. To control the insects that transmit these viruses, see the insect portion of this fact sheet. Remove weeds that may harbor the viruses—Wash tools used around the infected plant.

Insects & Related Pests

Chrysanthemum aphids (Macrosiphoniella sanborni) and other aphid species are pests on chrysanthemums. The chrysanthemum aphids are brown to black; however, other aphid species that may feed on chrysanthemums range in color from green to pink. Aphids feed by piercing plant tissue and sucking plant sap. They prefer feeding on new growth in such areas as shoots, the undersides of leaves, buds, and flowers. Their feeding can result in distorted growth, stunting, and sometimes death of the entire plant. As they feed on plant sap, they excrete honeydew (a sugary material). The sooty mold fungus feeds on the honeydew, resulting in unsightly, dark fungal growth. In addition to damage caused by their feeding, chrysanthemum aphids can transmit various plant viruses.

Control: Aphids can be removed from plants by applying a forceful spray of water to the plants every 2 days, especially to the undersides of leaves. Continue as needed, but at least 3 times. Several naturally occurring enemies feed on aphids. As much as possible, these predators should be allowed to reduce aphid populations. As a result of their phenomenal ability to reproduce, aphids are very difficult to control with insecticides. Leaving one aphid alive can result in the production of a new colony very quickly. In addition, the use of contact insecticides kills the beneficial insects that normally keep aphid populations under control.

If natural predators do not keep aphids under control and serious damage is occurring, spray with one of the following materials: insecticidal soap, horticultural oil, acephate, malathion, cyfluthrin, permethrin, or bifenthrin. See Table 1 for examples of brands and products. As with all pesticides, read and follow all label directions and precautions.

Two-Spotted Spider Mites (Tetranychus urtica) and other mite species are pests of chrysanthemums. Mites are not insects but are more closely related to spiders. They tend to be more of a problem during hot, dry periods. Mites are extremely small and can barely be seen without a magnifying lens. They have piercing mouthparts with which they puncture plant tissue and suck plant sap. With a light infestation, leaves develop stipples (tiny yellow spots) and appear dusty. Early damage is often overlooked until the damage is more severe. Spider mites will be initially on the lower surfaces of leaves.

With heavier infestations, symptoms include distorted leaves and withered and discolored blooms. In addition, fine webbing can be seen on flower buds, between stems, and on the undersides of leaves.

Control: Consider destroying severely infested plants or portions of plants, as spider mites are difficult to control under these circumstances. Spider mites can be removed by spraying plants forcefully with water. Repeat as needed, but at least 3 times. Insecticidal soap, if started early in the infestation, is effective at controlling spider mites. Stronger miticides are tau-fluvalinate and bifenthrin. See Table 1 for examples of brands and products. Usually, two or more applications at 5- to 7-day intervals are required. Be sure to apply the spray to all leaf surfaces. As with all pesticides, read and follow all label directions and precautions.

Chrysanthemum Leafminer (Chromatomyia syngenesiae) is the larva (immature form) of small (about ⅛-inch) dark-colored flies. The adult female lays eggs on the undersurfaces of leaves. The larvae hatch and penetrate the surface to enter the leaf and live between the upper and lower surfaces of the leaves. As they move through the leaf feeding, they create winding trails that are pale green to brown in color. Dots of black waste products are visible in some of the trails. Severely infested leaves may dry up and droop downward along the stems.

Control: Prune off and destroy infested leaves. Any leaves that fall to the ground should be picked up and destroyed. Remove and destroy any plant remains in the fall. If damage is severe, spray with a foliar systemic insecticide, such as acephate or spinosad. See Table 1 for examples of brands and products.

Table 1. Disease, Insect Pest, & Mite Control on Chrysanthemums

Pesticides are updated annually. Last updates were done on 10/24 by N. Jordan Franklin.

Originally published 07/99