Chrysanthemum Diseases & Insect Pests

Prepared by Marjan Kluepfel and J. McLeod Scott, HGIC Information Specialists, James H. Blake, Extension Plant Pathologist, and Clyde S. Gorsuch, Extension Entomologist, Clemson University. Revised by Joey Williamson, HGIC Horticulture Information Specialist, Clemson University. (New 07/99. Revised 12/06.)

HGIC 2101

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Chrysanthemums, or garden mums, like full sun and fertile, well-drained soil. They need regular watering because their roots are very shallow. Drought will cause woody, stunted growth. Overwatering, on the other hand, causes yellowing leaves that blacken and drop. Although the list of diseases that may attack chrysanthemums is long, mums are relatively trouble-free.

Leaf Spot

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. Leaves may wither prematurely. The lower leaves are infected first. With a hand lens white masses of spores may be seen on the leaf spots.

Prevention & Treatment: Hand pick 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 helps prevent spores from splashing from the soil onto plants. If disease is severe enough to warrant chemical control, fungicides containing chlorothalonil (Bonide Fung-onil Multi-Purpose Fungicide, Ortho Garden Disease Control, Ferti-Lome Liquid Fungicide, or Daconil 2787), mancozeb (Bonide Mancozeb Flowable, Southern Ag Dithane M-45), myclobutanil (Spectracide Immunox), propiconazole (Ferti-lome Liquid Sytemic Fungicide or Bonide Infuse Fungicide), or thiophanate methyl (Ferti-lome Halt Fungicide, Green Light Systemic Fungicide, Cleary's 3336) may be used. Follow all the directions on the label.

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 (see above), but fungal leaf spots are most often black, not brown. They additionally infest hosta and ferns.

Prevention & Treatment: Remove infested plant material, along with the surrounding soil. Mulch plants in the spring to discourage nematodes from climbing up from the soil, and avoid spraying water on the leaves when watering. Foliar sprays with insecticidal soap may help reduce nematode populations.

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 much weakened and fail to bloom properly.

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 disease is severe enough to warrant chemical control, use a fungicide with mancozeb (Bonide Mancozeb Flowable, Southern Ag Dithane M-45) or triadimefon (Green Light Fung-Away) as active ingredient. 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). 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 6.5 - 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 (Spectracide Immunox), triadimefon (Green Light Fung-Away), propiconazole (Ferti-Lome Liquid Sytemic Fungicide or Bonide Infuse Fungicide) or thiophanate methyl (Ferti-lome Halt Fungicide, Green Light Systemic Fungicide, Cleary's 3336) according to instructions on the product label.

Ray Blight

This disease is caused by the fungus Mycosphaerella ligulicola. The ray flowers (marginal flowers of an inflorescence) are attacked, so that the blooms are deformed and one-sided. Early infection may cause blasting of the buds.

Prevention & Treatment: Chlorothalonil, mancozeb, myclobutanil, thiophanate methyl or propiconazole are effective as a foliar spray when applied at label rates. See leaf spot control for brands.

Ray Speck

This disease is caused by the fungi Stemphylium and Alternaria. These fungi cause 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

This disease is 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 free circulation of air. Apply a foliar spray of chlorothalonil or thiophanate methyl (see brands under leaf spots).

Bacterial Blight

This 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 a marginal leaf scorch.

Prevention & Treatment: 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 (Agri-mycin).

Crown Gall

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

Prevention & Treatment: Discard infected plants. Do not wound the stems. Buy rooted cuttings, which are certified to be disease-free. Use Agrobacterium radiobacter (strain K84) as preventive treatment.

Virus Diseases

Chrysanthemums are subject to a large number of virus diseases, including mosaic, chrysanthemum smut virus, tomato spotted wilt virus, and aster yellows. 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. Virus diseases are spread by sucking insects such as aphids and 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 infected plant.

Insects & Related Pests

Chrysanthemum aphids (Macrosiphoniella sanborni) and other aphid species are pests on chrysanthemums. The chrysanthemum aphids are brown to black, which other species 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 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 (Safer Insecticidal Soap or Concern Insect killing Soap), horticultural oil (Bonide All Season Spray Oil, Ferti-lome Scalecide, Green Light Horticultural Oil Spray, or Ortho Volck Oil Spray), acephate (Ortho Japanese Beetle Killer), malathion (Ferti-lome Mal-A-Cide or Hi-Yield Malathion Insect Spray), cyfluthrin (Bayer Advanced Garden Power Force Multi-Insect Killer), permethrin (Spectracide Bug Stop Gardens & Lawns Insect Control) and esfenvalerate (Ortho Bug-B-Gon Garden & Landscape Insect Killer). 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 damage is more severe.

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. Ortho Systemic Insect Killer contains acephate and fenbutin oxide, the latter of which is a miticide. 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 (Phytomyza syngenesiae) is the larva (immature form) of small (about 1/8-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 (Ortho Japanese Beetle Killer).

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