Garden chrysanthemums suffer from fewer pests and diseases than those grown in glasshouses, but they are not totally trouble free.
Pests
Aphids
Chrysanthemums are attacked by several aphids. The most damaging is melon-cotton aphid (Aphis gossypii), which can form dense colonies on foliage and flowers, causing leaves to become sticky with honeydew and soiled with cast aphid skins and sooty mould.
Melon-cotton aphid is often resistant to older pesticides. More recently-introduced systemic insecticides, with imidacloprid or thiacloprid as active ingredients, are likely to give the best results. These are available as concentrate and ready-to-use forms of Provado Ultimate Bug Killer. Caterpillars, earwigs and chrysanthemum leaf miner can also cause problems outdoors.
Leaf and bud eelworm
A microscopic worm-like pest that lives within chrysanthemum foliage, Aphelenchoides ritzemabosi causes areas between larger leaf veins to turn brown or black.
Symptoms mainly develop in late summer to autumn and gradually spread up the plant. Infestations develop more rapidly
in periods of wet weather. Read more about leaf and bud eelworm
Diseases
Rusts
There are two types of rusts affecting chrysanthemums: white and brown rusts caused by the fungi Puccinia horiana (white rust) and P. chrysanthemi (brown rust); treatment is the same.
White rust is the more serious disease. Infected leaves exhibit yellow to tan spots on the upper surface, which correspond to dirty-buff/white pustules on the undersides. This disease is rapidly spread and difficult to control; destroy all affected plants. Read more about white rust.
Verticillium wilt
This disease is caused by Verticillium fungus. Symptoms are yellowing and browning of the leaves, which die from the base of the plant upwards. 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. There is no effective chemical control: remove and destroy infected plant material.
Virus diseases
Chrysanthemums are susceptible to many viruses. Infected plants often 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 produce small flowers. There is no cure for viruses: destroy infected plants and disinfect any tools used.
More information
For more information on chrysanthemums see the RHS Chrysanthemum page