Cause Verticillium dahliae, a soilborne fungus that survives in soil indefinitely. It can spread in infested soil, container media, irrigation water, or in symptomless infected cuttings. The fungus grows into the xylem where it colonizes the plant through mycelial growth and conidial production. Fluid movement in the xylem passively transports the conidia. Once in the xylem, this fungus partially blocks water movement and produces toxins that result in wilt symptoms. Many other ornamentals can be hosts.
Symptoms Infected plants may not show symptoms for several weeks or months. Symptoms can develop suddenly and lead to rapid death. Wedge-shape areas on affected leaves turn yellow; then the entire leaf turns yellow, wilts, or even drops off the stem. A few branches may show foliage collapse, defoliation, a dieback, and/or a brown to black discoloration. Affected plants may be stunted or defoliated. If leaves are left, they may become pale green and show interveinal yellowing. Symptoms can be very similar to those of bacterial blight.
- Propagate only from pathogen-free plants.
- Rogue out and destroy visibly affected plants.
- Provide good fertility and cultural conditions to promote vigorous growth.
- Plant in heat-sterilized or fumigated soil or soilless container media.
- Avoid soil previously planted to susceptible hosts such as tomatoes, strawberries, potatoes, or chrysanthemums.
Reference Nelson, P.E. and Nichols, L.P. 1982. Verticillium wilt. In: Geraniums: A Manual on the Culture of Geraniums as a Greenhouse Crop, 3rd ed. J.W. Mastalerz and E.J. Holcomb (eds.) Pennsylvania Flower Growers.