Dahlias are prized for their array of colors, flower size and long-lasting blooms. So naturally when gardeners discover wilting dahlia plants in their gardens, concern is warranted. If watering doesn’t rejuvenate the affected plants, gardeners should consider dahlia verticillium wilt as the causative agent.
What is Dahlia Verticillium Wilt?
Verticillium wilt is an opportunistic, soil-borne fungal disease that infects many types of plants, including species of flowers, garden vegetables and trees. It exists in the soil as mycelia, which is the vegetative part of the fungus comprised of branching, thread-like white filaments.
The fungus, Verticillium dahliae, infects the roots of plants where it plugs the water conducting tissue. Even though the soil is moist, water can’t reach the leaves and the plant exhibits signs of wilting. Dahlia plants affected by this fungus often begin wilting on a single branch rather than the entire plant. The leaves on that branch may turn yellow.
Eventually, the entire plant will be impacted. Upon closer inspection, the vascular tissue in infected plant stems will appear dark brown or black. Outbreaks are more common when warm temperatures follow cooler weather
Dahlia Verticillium Control
Unfortunately, there is no cure once gardeners find verticillium on dahlias. It’s best to remove and destroy plants infected with dahlia verticillium wilt to prevent it from spreading.
A laboratory culture can confirm the diagnosis of verticillium and there are methods for treating dahlia verticillium infected soil. These include soil fumigation with chloropicrin-methyl bromide or metham-sodium. In sunny locations, soil solarization has also been shown to be effective for treating dahlia verticillium living in the soil.
Home gardeners may find agricultural management techniques a suitable alternative for dahlia verticillium control. These include:
- Avoid planting dahlias where dahlia verticillium has been a problem in previous years. There are over 300 species of plants that are vulnerable to this fungus. Common gardening plants include vegetables from the nightshade family (tomatoes, peppers, potatoes) as well as raspberry canes and strawberries.
- Whenever possible, select flower species that are resistant to the fungus. Plant dahlias in mixed beds with English daisies, cleome or ageratum.
- Plant dahlias under trees that are naturally resistant to verticillium. These include oak, dogwood, and willows.
- Cut down infected shrubs or trees or prune branches exhibiting signs of dahlia verticillium wilt. Ash, maple and elm are just a few species of trees that are susceptible to the fungus.