Dahlia Problems

Dahlia Problems
SymptomProbable Cause
Lush Foliage; Sparse BloomOver-fertilizing
Plants Don't Bloom in SummerWeather Too Hot
Buds Discolored; Tunnels in Stems; Plants WiltedBorer
Leaves Discolored; Distorted; Growth StuntedLeafhopper
Leaves Curled And DistortedAphid
Leaves Discolored And Deformed; Webbing PresentMites
Leaves And Flowers Discolored And WitheredThrips
Leaves Covered With White PowderPowdery Mildew
Leaves Turn Yellow and Wilt Stem Rot

Over fertilizing Causes Lush Foliage, Sparse Bloom
If plants receive too much nitrogen fertilizer, they often stop blooming and put all their energy into leaf growth. Leach excess nutrients from the soil by watering thoroughly several times. Use only a slow release type of nitrogen fertilizer.

Weather Too Hot; Plants Don't Bloom In Heat
Sometimes during the dog days of August, dahlias stop blooming or bloom sparsely. Be sure they are well watered and wait until cooler weather, when they will spring back.

Buds Discolored; Tunnels in Stems; Plants Wilted - Borer
European corn borer and stalk borer sometimes attack dahlias. The stalk borer is a striped worm is more likely to appear in early summer, whereas the corn borer arrives later in the summer. These nuisances feed on bud tips, flowers and leaves and then, left unchecked, burrow down inside the dahlia stem. This causes the plants to wilt. To treat borer attacks, spray BT (Bacillus thuringiensis) on the dahlia foliage when the worms are feeding. They will ingest the bacteria, sicken and die within a matter of days. Repeat applications of BT if rain washes it away.
For more information see the file Controlling Borers

Leaves Discolored, Distorted; Growth Stunted - Leafhopper
The potato leafhopper is a small, slender, pale green insect about 1/8 inch long. These pests cause leaf discoloration, which appears first along one margin and spreads toward the mid-vein. The affected area is at first pale yellowish, later turning brown and brittle. Infested dahlia plants are often stunted. Apply a preventive spray of insecticidal soap and seaweed extract to vulnerable plants during the first month of growth. Later, spray any leafhoppers with insecticidal soap laced with rubbing alcohol.
For more information see the file on Controlling Leafhoppers

Leaves And Flowers Discolored And Withered - Thrips
Adult thrips are tiny, slender insects, 1/25 inch long, variously colored pale yellow, black or brown, with four long, narrow wings. Their larvae are usually wingless. Thrips infest flowers by rasping the surface and feeding on the exuding juice, which causes petals to turn whitish and wither. Set out yellow sticky traps about 4 weeks after last frost as early warning devices. As soon as you spot thrips on the traps, spray vulnerable plants with insecticidal soap every 3 days for 2 weeks.
For more information see the file on Controlling Aphids

Leaves Covered With White Powder - Powdery Mildew
Fungi that live on the surface cells of the plant cause powdery mildews. Infested dahlia leaves are covered with a white or ash-gray powdery mold. Badly infected leaves become discolored and distorted, then drop off. Powdery mildews thrive in both very humid and very dry weather. Spray affected plants thoroughly with wettable sulfur once or twice at weekly intervals starting as soon as the whitish coating of the fungus is visible. Allow ample spacing between plants and collect and discard all aboveground refuse in the fall.
For more information see the file on Controlling Fungal Disease

Leaves Turn Yellow and Wilt- Stem Rot
Soil-dwelling fungi cause root and stem rot of dahlias. Typically, plant stems are attacked at or near the soil level. Foliage turns yellow, wilts, and dies. Usually root systems rot, causing plants to topple over. Remove and discard infected plants, or cut away affected plant parts with a clean, sharp knife or razor blade. Disinfect tools after use in a bucket of water and household bleach. Keep the yard clear of old plant debris and keep mulch away from stem bases. For long-term prevention, lighten heavy soil with a mixture of perlite, vermiculite or peat moss and provide good drainage. Avoid over watering and space plants further apart to prevent crowding.