How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

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 usually fall; sometimes they remain attached. Woody plants often are affected first on one side, and affected branches usually die. The water-conducting tissues (sapwood) of infected plants are often discolored with dark streaks occurring in the xylem tissue; discoloration varies with the species but frequently is an olive green, dark brown, or black. In some plants there is little or no discoloration (including olive, ash, and roses).

COMMENTS ON THE DISEASE

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.

MANAGEMENT

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 open field 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 disinfested. Soil fumigation or soil solarization (in warmer climatic areas) can be useful. During the season, remove and destroy any plants that exhibit symptoms of Verticillium wilt.

Common name		Amount to Use	R.E.I.+
(trade name)			(hours)
When choosing a fungicide, consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
SOIL FUMIGATION
Note: The 2007 Critical Use Exemption List allows fumigation with methyl bromide for moderate-to-severe disease infestation.
A.	METHYL BROMIDE*/CHLOROPICRIN*	Label rates	48
	Allowable for use under a Critical Use Exemption only. Methyl bromide is a gas at temperatures over 40°F. It escapes rapidly from soil if not applied under a gas‑proof cover. Polyethylene sheeting is commonly used to confine methyl bromide, although the gas does slowly escape through polyethylene. Diffuses rapidly through the soil and kills many kinds of organisms, weeds, and many seeds. Soil generally can be planted a few days after removal of plastic covers, although there are exceptions. A few plants such as Allium spp., carnations, and snapdragons are sensitive to and may be damaged by inorganic bromide that remains in the soil following fumigation. Leaching the soil with water before planting is helpful in reducing the amount of bromide in the rooting area. Fumigants such as methyl bromide are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone: methyl bromide depletes ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
*	Permit required from county agricultural commissioner for purchase or use.
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.

PUBLICATION

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

Top of page