The dahlia is a perennial flower with beautiful blossoms that are popular with gardeners. Originally native to Central America and Mexico, dahlias are grown from tubers; they are considered easy to grow (although some insects love them). They are well known, however, for having problems with fungi in the form of mold and root rot.
On warm days with low humidity and cool nights with high humidity, Dahlia plants can be infected by powdery mildew, a fine white or light gray fungus on leaves and stems. Infected leaves eventually become discolored and fall off.
Dahlias are susceptible to brown patch, a soil fungus that causes stems and roots to rot. If the humidity is high, brown patch spreads quickly to leaves, causing oval or peanut-shaped spots. The spots turn brown, and the plant wilts. The stems, marked by brown lesions, rot at the soil line. If the soil is compacted or subjected to excessive drying and rewetting, the roots crack and rot.
Pythium, a white growth that looks like cobwebs, is often seen on the dew of grass in the morning; it attacks the roots and works up the stem, causing the plant to turn yellow and die. It is water-borne and is a major cause of root rot.
Phytophthora, a soil-borne fungus, is related to Pythium. The symptoms are similar. This disease caused the Irish potato famine of 1843-47. It can also be borne in the water, air or through seeds. It travels up from the roots and first appears as white mold on the underside of leaves.
Two species of fungi called Sclerotinia can infect dahlias. Sclerotina minor is carried in the soil. A cottony fungal growth and small, black beads form on the inside and outside of the stems. The roots become soaked with water and turn brown; the plant eventually collapses. White mold, Sclerotinia sclerotiorum, infects lower stems and leaves, causing similar symptoms as Sclerotina minor, and has an aerial spore that can infect upper flowers and leaves.
You can take many actions to help avoid the above problems: Do not over-water. Do not crowd plants. Do not apply too much fertilizer. Use perlite, peat, or vermiculite to provide better draining if the soil is heavy. Keep mulch away from the bottoms of your plants. Use a razor blade or clean, sharp knife to remove infected parts or remove the plants and throw them away. Finally, disinfect your tools with water and bleach.
There are two broad types of fungicides. Protectant fungicides inhibit the germination of fungi spores. They can prevent infection of applied to healthy tissue before it is infected. Systemic fungicides are absorbed by the leaves or roots and are moved throughout the plant. There are numerous brands of fungicides on the market. Check the labels to see if the listed symptoms match the problem you have with your dahlia. If root rot is involved, you should use a systemic.