Dahlia diseases and pests
Diseases
Dahlias are susceptible to bacterial, viral and fungal diseases. Most diseases appear when a pathogen is present and a plant is stressed due to environmental factors or care.
If the dahlias you are growing have a bacterial or viral disease, discard the plant and tubers. To avoid spreading these diseases to other dahlias, do not compost infected plants or tubers. Sterilize all tools by dipping them in bleach.
One of the most common bacterial diseases is crown gall, which is identified by an extra lump-like growth or multiple growths on the crown of the tuber. You can find images of this disease in the Pacific Northwest Management Handbook. Crown gall is not treatable and spreads easily. If you suspect a tuber has crown gall, discard it. Sterilize all tools used on the infected plant before using them on other dahlia tubers.
Stunted growth, yellow streaks or spots on the leaves often suggest viral diseases. Both the plant and the tuber can host viruses. There is no treatment for dahlia viruses. If you notice the symptoms of a virus on your plant, remove the plant, as viruses can also spread. Dahlia mosaic virus is one of the most common viral diseases of dahlias. Learn more about how to diagnose and treat this virus in Dahlia Mosaic Virus: A Pictorial Guide to Symptoms and Diagnosis.
Powdery mildew is a common fungal disease of dahlia plants identifiable by a gray, powdery substance on the leaves. You can often prevent fungal diseases like powdery mildew through proper plant maintenance. To prevent fungal diseases, water at the base of the plant and not overhead. Air circulation is also important for controlling fungal diseases. If a portion of a plant is infected by a fungus, remove any infected leaves and reduce overcrowding to increase airflow to the plant.
Insects and predators
Aphids, cucumber beetles, earwigs and slugs are some common insects that may affect your dahlias. Several comprehensive resources to identify and manage these insects are listed below.
- Aphids are common on plants. You can treat them with insecticidal soap. Spray both the buds and the leaves of the dahlia during the cooler parts of the day.
- Cucumber beetles are light green with black spots. The cucumber beetle chews on the leaves, flowers and stems of dahlia plants. Cucumber beetles can be challenging to control, but some gardeners have success using trap crops as well as some insecticides.
- Earwigs come out at night and are damaging to the flower blooms. Organic and conventional traps help to eliminate earwigs.
- Slugs and European garden snails attack the young dahlia plants at night. Put down bait two weeks after planting or when first sprouts appear.
- Field mice, voles and gophers will eat dahlias. Voles can use mole runways to get to the tubers. Gophers create their own tunnels. Trapping is the best control.
