Dahlias, perennial plants that blossom in the summer, brighten your garden with large flowers coloured orange, yellow and pink. Dahlias are relatively easy to care for unless attacked by insects or viral disease. If your dahlia looks weak and wilted, check its leaves, flowers and stem for symptoms of disease or insect infestation.
Ringspot - If large, yellow circles appear on the dahlia's leaves, the plant may have viral ringspot disease. Transmitted by insects called thrips, ringspot infects the dahlia's roots and spreads throughout the plant. Over time, the rings on the leaves grow larger and brown spots sometimes develop in the middle of each ring. Because the disease starts in the roots, it is not usually possible to treat the sick plant.
Dahlia Mosaic - Another root-based viral disease that commonly affects dahlias is dahlia mosaic. The virus gets its name from the alternating light and dark green patches that appear on leaves. This symptom appears because the virus causes a chlorophyll imbalance. Yellow leaf spots and veins are another symptom. Dahlia mosaic is generally spread by aphids. Once a plant gets the disease, it is very difficult to treat. It is advisable to remove and destroy the plant.
Powdery Mildew - A dahlia with gray, fuzzy leaves that fall off is most likely infected with powdery mildew. This fungal disease tends to infect dahlias when it is very humid outside, but it also appears in dry weather. Powdery mildew is also more common in plants that are not spaced far enough apart. If a dahlia exhibits signs of powdery mildew, treat it with wettable sulfur once per week.
Insects - Thrips, the pests that spread ringspot, are small, thin flying insects. They also destroy dahlia flowers by extracting and eating juice from them. Control thrips by placing sticky traps around the plant and then spraying the plant with insecticidal soap. Insecticidal soap also works to control aphids, the tiny green or red insects that spread dahlia mosaic. Leaf borers also commonly affect dahlias. These little worms burrow into the stem to feed, and this kills the plant. A bacteria called Bacillus thuringiensis kills the borers, but is safe for the plant.