Black Spots on My Dahlia Leaves

Disease

• Dahlias are susceptible to several diseases that may cause black spots on the leaves. Leaf spot is the most common disease and is usually present in the bulbs before planting. Remove infected leaves and destroy severely damaged plants so the disease doesn't spread.

Insects

• Many insects attack dahlias, including aphids, slugs, thrips, earwigs and caterpillars. These insects may damage leaves by feeding on them but can also spread disease. Spray dahlias with an insecticide or miticide every seven to 10 days to control mites and other insects. Spray both the tops and undersides of the leaves and reapply after rainy weather.

Growing Conditions

• To control leaf spot, plant dahlias in an area with full sun and rich, slightly moist soil. Space plants at least 12 inches apart so air circulates freely. Stake dahlias, as they grow, to avoid damage to the stems. Fertilize the plants monthly with a water-soluble, balanced fertilizer. Do not fertilize them after August.

Tuber Storage

• Dahlias are not cold hardy and must be dug up every fall and stored over the winter. Proper storage is the best way to ensure disease-free plants the following year. Dahlia tubers are easily damaged during storage, allowing diseases to take hold. Dig the tubers up carefully after the first frost. Use a pitch fork or spade and dig in a large area around the plant to avoid nicking the tubers. Brush the soil off gently and stand the tubers upside down to allow moisture to drain from the stems. Brush off any remaining soil and dusty surfaces with sulfur. Store the tubers in a shallow box filled with sawdust or vermiculite. Leave the stems exposed and moisten the vermiculite slightly. Store the tubers in a cool, dry location with temperatures between 40 to 55 degrees Fahrenheit. Examine the tubers before planting them the next spring and discard any that show signs of decay or disease, such as gray, green or pink growth, or an unusual odor. Softness is also a sign of decay.