Septoria leaf spot disease is caused by a fungus which attacks leaves and produces brown to black spots. The disease will begin on the lower leaves and move its way upward until perhaps half of the leaves become brown and wilted.

The fungus overwinters in debris on the soil surface. Spores produced from infested debris splash onto new foliage in the spring and initiate new infections.

The best way to control the disease is to avoid the initial infections by cleaning up infested debris from around the base of the plant. If the disease shows up on the leaves, a fungicide can be used to avoid continued infections. Fungicides labelled for leaf spot control include benomyl (Benlate), chlorothalonil (Daconil), zineb, maneb or mancozeb. No cultivars are resistant to this disease.

The same fungus which causes powdery mildew on roses, phlox and zinnias can become established on chrysanthemums.

The best conditions for powdery mildew development are moderate to cool temperatures and high relative humidity, but not free moisture on leaves. Typically this disease is more prevalent during the early to late fall when air temperatures at night are relatively cool.

The only way to control powdery mildew is to use a fungicide sprayed every 7 to 10 days. Sulfur, benomyl (Benlate), dinitrophenyl crotonate (Karathane), triademefon (Bayleton) or triflorine (Funginex) are labelled for control of powdery mildew of chrysanthemum.

Virus diseases like mosaic and stunt or virus-like diseases such as aster yellows are occasionally a problem on chrysanthemums. These diseases are typically transmitted by insects and vary in their occurrence each year according to the insect survival rate after winter.

They may cause plants to be severely stunted with distorted leaves or have mottled and yellowed leaves with no apparent reduction in growth. Aster yellows is caused by an organism called a mycoplasma which resembles bacteria, but behaves much like a virus in symptom development.

There is little you can do to prevent these diseases and other virus disease in the garden. The best approach to this is to rogue out infected plants as soon as they are discovered and control insects which transmit the viruses; particularly aphids and leafhoppers.

Aphids are soft-bodied insects that range in color from green to black and are about one-eighth inch long at the largest. They feed by sucking out plant juices from cells, and most feeding activity will take place from the underside of the leaves and buds.

Under severe infestations, aphid feeding will cause tissues to become yellowed.

The only way to effectively control aphids is with insecticides. Suitable materials include malathion, diazinon and insecticidal soap which when use properly and sprayed to provide uniform coverage can be quite effective.

A couple different types of caterpillars may chew holes in the leaves or feed on the buds. Generally, these pests will cause little lasting damage and therefore should be tolerated unless the plant shows more than 50% loss of tissue.

Caterpillars can otherwise be controlled by using an insecticide like those mentioned for aphid control. In addition, carbaryl (Sevin) should be added to the list as well as one biological insecticide called Bacillus thuringinesis (Dipel, Thuricide, Bactur). Insecticidal soaps are not effective against caterpillars.

These small, wedge-shaped insects are always found on the underside of leaves where they feed by sucking plant juices out of cells. Typically, they are green in color and fly or run sideways when disturbed. Heavy feeding will cause leaves to become mottled, curled and withered.

The same insecticides as those recommended for aphids plus carbaryl (Sevin) will control leafhoppers.

Leafminers spend most of their pest life inside leaves burrowing between the upper and lower cell layers. Light-colored, serpentine mines can be easily distinguished. Heavily infested leaves may curl and wither.

Insecticides sprayed over the surface are not likely to control the feeding of leafminers in the leaves, however, malathion and diazinon can be used to control egg-laying adults.

Plant bugs do not typically cause real problem on chrysanthemums, but can be found feeding on sap by
puncturing tissues with their mouth parts. These insects are about one-quarter of an inch long, brown or green in color with some black marking on the back.

If necessary, plant bugs can be controlled by use of malathion, diazinon or carbaryl.

These are related, but not true insects which feed on the underside of leaves by rasping and rupturing cells with their mouth parts. The plant sap which leaks out is then lapped up. Heavy feeding will cause the leaves to become bronzed and dry looking.

Insecticidal soaps as well as those pesticides which control aphids can be effective against spider mites. Carbaryl (Sevin) will not control mites.