Dahlias can experience problems with insect pests and diseases, but these can usually be resisted if plants are kept vigorous. When problems do emerge, results can be simply cosmetic or can lead to severe injury. Treat your dahlia plants as soon as you notice signs of illness or insect damage. Most importantly, always keep dahlias in full sunlight and well-drained soil for vigorous, healthy plants that can defend themselves against problems.
Injury from improper care is preventable if you observe the maintenance requirements of dahlias. Significant problems can result from environmental hazards based on improper siting. Always plant dahlias in their ideal conditions: full sunlight and well-drained soil. Poorly drained, excessively wet soil can cause your dahlia plant to rot. Also, beware of frost, which may injure or kill your dahlias. Be sure to plant them outdoors only after temperatures are warm enough during the spring.
Diseases of dahlias include fungal infection and viruses. Powdery mildew is a fungal infection that causes white/grey velvety mildew to form on leaf surfaces, leading to wilted leaves that drop early. To prevent powdery mildew, apply a fungicidal spray up to once every ten days from the month of June through September; fungicides are not recommended as a cure, so treat dahlias before fungal infection occurs. Viruses can cause a number of different symptoms such as mottled patterns on leaves, wilting, or streaked stems; however, your dahlias may be infected with viruses without any symptoms. There is no pesticide control for viruses; if you suspect viral infection, remove and destroy infected plant parts, recommends the Iowa State University Extension.
Insect infestations pose problems for dahlias by causing cosmetic and health damage. Some insects that affect dahlias include aphids, grasshoppers and spider mites. Aphids are small insects that cause leaves to curl; the presence of a sticky, glossy film on the surface of leaves is an indicator of an aphid infestation. Aphids cause overall diminished health; control them by spraying plants with water or appropriate insecticidal treatments. Young and mature grasshoppers both attack dahlias by chewing the plant and leaving holes; for control, simply catch grasshoppers and release them away from your dahlia plants. The Iowa State University Extension explains that grasshoppers are easier to catch during the morning, when their activity level is lowest. Spider mites also pose a threat to dahlias; these insects create a delicate web on plant foliage and small dots on leaf surfaces; leaves eventually turn a bronze color. To combat such infestation, which will diminish dahlia health, give your plants a "forceful spray" with water or soapy water up to 4 times a week or locate an appropriate miticide. For soap solution, simply mix 1 tbsp. liquid dish soap with 1 quart of water.