Cause Verticillium dahliae and/or Fusarium oxysporum, fungi that live indefinitely in soil and may be carried in infected tubers. Verticillium grows into the xylem where it colonizes the plant through mycelial growth and conidial production. Conidia are transported passively by the fluid movement in the xylem. Once in the xylem, this fungus partially blocks water movement and produces toxins that result in wilt symptoms. Last sample of Verticillum wilt came in the OSU Plant Clinic in 1968.
Aside from mentions of occurrence in state fungal disease indexes and lists, there has been little plant pathology research on either of these two fungi on dahlia in the literature. More attention has been made of Fusarium as an endophyte with antimicrobial properties (human pathogens) or as a rot of tubers in storage.
The OSU Plant Clinic has had a few dahlia samples with Pythium root and stem rot. This pathogen rots roots when soils or media are saturated with water for extended periods of time. It can also gain a foothold when media dries out resulting in salt injury to the roots. Root rot under these conditions can also result in a wilt of the plants.
Symptoms Vascular stem tissues darken near the soil level. The top of the plant wilts and dies back. Roots have brown-black streaks.
Sampling Send soil samples to any of various private and public laboratories to assay for Verticillium propagules. Nurseries may wish to test individual core samples to determine the distribution in a particular field. The presence of any microsclerotia in the soil should be interpreted as a potential disease risk.
Cultural control
- Use only healthy tubers for propagation.
- Remove wilted plants to prevent pathogens from accumulating in soil.
- Practice flowerbed (crop) rotation.
- Soil pH should be 5.8 to 6.2.
- Destroy all affected dahlia stems. Do not add them to the compost pile unless you are hot composting.
- Sterilize soil with steam, or use a soilless potting mix.
- Use a low ammonium fertilizer and irrigate thoroughly to avoid salt accumulation. Avoid severe dry-downs of soil/media to prevent salt injury.
Reference Ardhi, A. 2015. Antimicrobial activity and molecular characterization of endophytic fungi strain isolated from dahlia (Dahlia variabilis). Journal of Chemical and Pharmaceutical Research 7(9S):201-208.