What is it? What to do about it? ♣ Download fact-sheet pdf
by Elizabeth H. Roche, Nancy J. Taylor & Francesca Peduto Hand
Verticillium Wilt is a vascular disease caused by two closely related species of the soilborne fungus Verticillium (e.g. Verticillium albo-atrum and Verticillium dahliae). The pathogen attacks a wide variety of herbaceous and woody ornamentals infecting the plant through the roots and then clogging the water conducting tissues. Flower crops that are particularly susceptible to the disease include chrysanthemum, dahlia, geranium, gerbera, impatiens, marigold, peony, phlox, snapdragon and stock, among others.
Verticillium enters and colonizes the roots of host plants invading the xylem tissue. Once in the xylem, it grows and reproduces, and moves systemically throughout the plant. As a result of the physical presence of the fungus and the production of enzymes and toxins, as well as the plant’s attempt to contain the invasion, the vascular system clogs preventing water and nutrients from moving throughout the plant. As a consequence, symptoms of the disease are characteristic of a non-properly functioning vascular system, including:
- Leaf chlorosis
- Leaf scorch
- Brown-black discoloration (streaking) of xylem tissue may be present (visible in longitudinal or cross-sections). Since this is almost identical to that caused by Fusarium wilt, a confirmatory laboratory diagnosis is recommended.
Microscopic observation of infected tissues may reveal the presence of very small black structures (microsclerotia).
For assistance in identification, contact the C. Wayne Ellett Plant and Pest Diagnostic Clinic.
Verticillium is a soilborne pathogen that can survive in soil for many years in absence of a host in the form of microscopic black resting structures, called microsclerotia or as dormant mycelium. Germination of microsclerotia is stimulated by root exudates from a host plant. The fungal hyphae penetrate the roots directly or through wounds and the pathogen then spread within the plant as mycelium or spores (microconidia).
Verticillium species can spread on contaminated seeds, vegetative cuttings and tubers, and by wind, splashing water and soil particles. The disease represents a major threat in cut-flower field production. Even though its importance as a greenhouse pathogen has diminished with the adoption of soilless potting media, it can occasionally be introduced in greenhouse production through plants that were previously grown outdoors or through contaminated cuttings.
Greenhouse pests such as fungus gnats have been shown to be vectors of V. albo-atrum in greenhouses. The insect may introduce the pathogen on healthy plants by depositing the fungus into feeding wounds on stems, roots or foliage.
Favorable Environmental Conditions
- V. dahliae optimal temperature for growth 77-82°F
- V. albo-atrum optimal temperature for growth 68-77°F
- Use resistant cultivars whenever possible.
- Use culture-indexed pathogen-free plants.
- Check incoming plant material for symptoms before introducing them into production.
- Remove and destroy any symptomatic plant.
- Control insect pest populations.
- Adopt proper soil sanitation practices. Always use clean soil. Soil can be disinfested by steam (140°F for 30 min), solarization (double tent at 160°F for 30 min or 140°F for 1 hour), or chemical treatment (fumigation).
- For field production of cut flowers, avoid fields that have previously hosted susceptible crops.
- Fungicides are generally not effective for disease control.