Verticillium Wilt

Symptoms vary somewhat with the kind of plant and the environment, but some symptoms are common to most situations. The leaves may wilt and turn yellow, first at the margins and between the veins, then they turn tan or brown and die starting from the base to the tip of the plant or branch. Dead leaves can either fall off or remain attached to the plant. Woody plants often are affected first on one side, and affected branches usually die. The water-conducting tissues (xylem) of infected plants are often discolored with dark brown to black streaks. In some plants, including olive, ash, and roses, there is little or no discoloration.

Verticillium wilt, one of the most widespread and destructive soilborne diseases of plants, affects a large number of herbaceous and woody species throughout the world. The causal fungus, Verticillium dahliae, infects susceptible plants through the roots and plugs the water conducting tissues.

Susceptible flower crops include China aster, chrysanthemum, cineraria, dahlia, geranium, gerbera, heather, marigold, peony, pelargonium, rose, snapdragon, statice, stock, and strawflower. The V. dahliae fungus forms microscopic black resting structures (microsclerotia) capable of surviving in soil for many years in the absence of a susceptible plant. When a susceptible plant is planted in infested soil, the microsclerotia germinate and infect the plant. Long rotations with nonsusceptible plants are not effective in controlling the fungus.

The fungus also produces conidia that can be transported in irrigation water; however, they are not long-lived. The fungus can be disseminated by leaves dropping from infected plants and being blown around by the wind.

Many horticultural crop plants have been selected or bred for resistance to the fungus. Use resistant cultivars and pathogen-free plants whenever possible.

Steam (at 140°F for 30 minutes), solarize (double-tent at 160°F for 30 minutes or 140°F for 1 hour), or chemically treat growing medium. For outdoor cut flower or nursery production, avoid fields previously used for susceptible crops (e.g., tomato, cotton, potatoes, strawberries, as well as the ornamentals listed above) unless disinfected. Soil fumigation or soil solarization (in warmer climatic areas) can be useful. During the growing season, remove and destroy any plants that exhibit symptoms of Verticillium wilt.