Many deciduous trees and shrubs are susceptible to infection by Verticillium dahliae. Hosts include ash, box elder, camphor, catalpa, Chinese pistache, coral tree, dracaena, dodonaea, elm, fuchsia, hebe, Indian hawthorn, maple, mayten, olive, pepper tree, redbud, rose, syringa, and tulip tree. Various flowering herbaceous plants and garden vegetables are also susceptible to Verticillium wilt. Flower hosts include chrysanthemum, dahlia, geranium, gerbera daisy, impatiens, Marguerite daisy, marigold, peony, petunia, snapdragon, sunflower, and vinca. Vegetable hosts include brussels sprouts, cabbage, cauliflower, cantaloupe, cucumber, pumpkin, squash, watermelon, eggplant, pepper, potato, and tomato.
Identification
Verticillium wilt is a fungal disease that damages a plant's vascular system. It causes foliage to turn faded green, yellow, or brown and wilt in scattered portions of the canopy or on scattered branches. Shoots and branches die, often beginning on one side of the plant, and entire plants may die. Small plants may die from Verticillium wilt in a single season, but larger plants usually decline more slowly. Mature trees may take many years to die and may suddenly recover if conditions become favorable for plant growth and poor for disease development.
Symptoms of Fusarium wilt and Verticillium wilt can be indistinguishable. Determining which is the cause of damage may require submitting samples to a plant diagnostic laboratory that cultures the fungus for positive identification.
Life cycle
Verticillium persists as microsclerotia in soil. When soil is cool, microsclerotia produce hyphae (vegetative growth structures) that infect roots of susceptible hosts. The pathogen then spreads upward in the current year’s growth and inhibits the plant’s nutrient and water transport.
In some plants, but not all (e.g., olive), peeling off the bark on newly infected branches may reveal dark streaks following the wood grain. Depending on the plant species, the stains are dark black, brownish, gray, or greenish. Infection can occur during the spring but not become apparent until warm weather when plants are more water stressed.
Solutions
Keep plants vigorous by providing them a good growing environment and proper cultural care, especially appropriate irrigation. Applying modest amounts of slow-release fertilizer to promote new growth can increase infected plants’ likelihood of survival. If chronic branch dieback develops, prune off any dead wood and have an arborist certified by the International Society of Arboriculture or registered with the American Society of Consulting Arborists inspect trees for possible hazards (e.g., risk of limb drop). Trees affected by Verticillium wilt may need to be removed.
Where Verticillium wilt has been a problem, plant only resistant species because the pathogen persists in soil. For example "FNV" on a plant label means the plant is resistant to Fusarium (F), nematodes (N), and Verticillium (V). Before replanting, soil solarization during the summer then keeping the site plant free for at least 1 year can help reduce pathogen inoculum in soil and improve the establishment and growth of subsequent plantings.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
