With only 42 species but more than 57,000 cultivars, there’s a dahlia (DAL-ee-a) out there for just about every taste.
These amazing plants can be quite easy to care for and produce heavy blooms ranging in size and shape from tiny pompons to sunflower-sized dishes.
But even the best of the plants can sometimes fall ill, and dahlias are no exception.
What Diseases Affect Dahlias?
There are a wide array of potential diseases that can affect the many types of Dahlia plants.
Here are the ones to especially watch out for.
- Aster Yellows: This peculiar disease is carried by leafhoppers and is incurable.
- Botrytis Blight: This fungus prefers cloudy, moist weather and can become deadly if not treated.
- Crown and Leaf Gall: These incurable diseases have similar effects but on different parts of the plant.
- Impatiens Necrotic Spot Virus (INSV): Transmitted by thrips, this disease is incurable and highly aggressive.
- Mosaic Virus: There are many kinds of mosaic virus, but they are all incurable and will be inherited through all propagation methods.
- Powdery Mildew: This annoying fungus often crops up on honeydew, the waste product of piercing pests such as aphids and mealybugs. More on Dahlia mildew here.
- Root Rot and Stem Rot: These two deadly diseases may be caused by the same fungi, as well as independent fungal or bacterial causes. The biggest cause of these two infections beyond contaminated soil is excess water in the soil or getting on the stem.
- Verticillium Wilt: This incurable disease usually only affects already weakened plants and can have symptoms similar to root rot.
What Damage Do Dahlia Diseases Cause?
Aster yellows is an incurable disease where the plant will grow leaves instead of buds.
While it won’t kill the plant, you probably didn’t buy your dahlia strictly for the foliage.
Meanwhile, the mosaic virus can sometimes be considered attractive and even led to the “tulipomania” craze in 17th century Holland.
It can cause leaves and flowers to become discolored and distorted and will usually result in the plant becoming dwarfed.
Galls are abnormal growths that develop on the plant and can lead to additional infections.
INSV can have a wide range of symptoms, often making it hard to diagnose.
Some fungal infections such as powdery mildew are largely cosmetic but may weaken the plant by hindering photosynthesis.
This can lead to a higher susceptibility to other infections or infestations.
Meanwhile, botrytis blight (sometimes called grey mold) will infect the stems and slowly work its way down the plant.
If not treated early enough, it can kill the plant.
Rots and verticillium wilt have similar symptoms.
More about Verticillium Wilt symptoms.
By clogging up the roots, the plant is unable to get water and nutrients, causing the leaves to die.
Stem rot affects the plant in reverse, killing the stem and working its way down to the tubers.
Both fungal stem rot and root rot are treatable if caught early enough, but verticillium wilt and bacterial stem rot are a death sentence for your plant and will continue to infect the soil long after the plant is uprooted.
Related: Why Are My Dahlias Not Blooming?
How To Control Dahlia Diseases?
Unfortunately, any plant contracting an incurable disease must be completely destroyed and the soil sterilized or discarded.
Fungicides will work on most fungal infections, especially topical ones.
- For some fungal infections, such as fungal stem rot, the infected parts of the plant must be pruned away before it spreads to healthy tissue.
- Neem foliar sprays work well against many surface infections, while neem soil soaks can help fend off infections that break through the plant’s surface.
- Apply one or both every 2 weeks as a preventative.
Of course, nothing beats proper care when it comes to preventing these diseases.
Check your plants regularly for signs of infestation and treat infested plants quickly.
Be sure to use the soak-and-dry method of watering for both indoor and outdoor plants, being careful not to get the leaves or stems wet when you water.
Finally, when buying potting soil or similar products, never get a cheap brand unless it has a reputation for sterilizing the soil.
It’s this important step that shady companies skip to keep costs down.
When buying cheap, make sure it’s a reputable company such as Miracle-Gro, or you could unwittingly infect your dahlias.