How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Pistachio

Verticillium Wilt

Pathogen: Verticillium dahliae

(Reviewed 2/07, updated 2/07)

In this Guideline:

Symptoms
Comments on the disease
Management

SYMPTOMS

Generally Verticillium wilt causes a rapid desiccation and death of one or more scaffolds or the entire tree, usually in late spring or early summer. The first symptoms are interveinal patches of yellowing or scorching of the leaves on affected branches. In some instances, however, it may also cause a condition known as thin leaf decline in which the disease develops slowly over several years before the tree becomes economically unproductive or dies. Thin leaf decline is characterized by slow loss of vigor, reduction in growth and yield, and gradual thinning of the canopy until most of the remaining leaves are clustered in tufts at the ends of branches and shoots.

COMMENTS ON THE DISEASE

Verticillium wilt affects a large number of herbaceous and woody species. The causal fungus, Verticillium dahliae, infects susceptible plants through the roots and plugs the water-conducting tissues. Pistachio trees of any age are subject to attack. The disease is more common in the southern half of the San Joaquin Valley than in other areas of the state and has been most destructive where pistachio trees were grown in fields previously planted to other susceptible crops such as tomato, cotton, melons, or peppers.

The fungus survives as microscopic, black resting structures (microsclerotia) capable of surviving in soil for many years. When a susceptible plant is grown in soil infested with the fungus, the microsclerotia germinate and infect the plant. It invades and colonizes the plant's vascular system, plugging the xylem and preventing or reducing the transport of water from the roots to the above ground portion of the tree. If dead or dying branches are cut in cross section, a darkened ring of plugged xylem tissue can be seen.

Verticillium wilt is favored by cool temperatures. Extended spring weather and mild summers often are accompanied by severe losses to this disease. The fungus apparently is eliminated from aboveground portions of trees in hot summer weather. Repeated attacks of wilt apparently represent new infections each year.

MANAGEMENT

The best defense against Verticillium wilt is the use of the resistant rootstock Pioneer Gold, Pistacia integerrima, or UCB I (a P. atlantica x P. integerrima hybrid). Pistacia atlantica and P. terebinthus rootstocks are very susceptible and should be avoided where Verticillium is present.

Preplant fumigation. When choosing an orchard site, have the inoculum level in the soil determined by a private laboratory. Preplant or planting site fumigation can reduce soil inoculum levels but will not eradicate the fungus or prevent its re-establishment.

Soil solarization. Placing plastic mulches on fallow soil for several weeks in summer also lowers inoculum levels through solarization. Beginning in late spring, cover the surface of an entire block with transparent plastic that has a UV-inhibitor additive. Leave the plastic on throughout the summer and as long as practical. Inferior plastic will break down and render the treatment ineffective. Application of plastic mulches to established pistachio orchards is limited in effectiveness and does not work well in shade.

Common name

Amount/Acre

R.E.I.+

(trade name)

(hours)

When choosing a pesticide, consider information relating to environmental impact.

PREPLANT

METHYL BROMIDE*

Label rates

COMMENTS: Use allowed under a Critical Use Exemption permit only. To prepare the soil for fumigation, dry it by withholding water during summer and using cover crops such as sudangrass. The drier the soil, the better for deep penetration. Deep-till the area after drying. If the soil is dusty, wait for an early rain before ripping and fumigation. Ripping a dry soil that is silty can result in large clods on the surface. Inject methyl bromide 18 to 30 inches deep with chisels and cover with gas-proof cover. Increasing the dose tends to increase the depth of penetration, but it cannot be relied upon to penetrate wet soil, especially if soils are high in clay. Do not remove the cover for at least 2 weeks and aerate 1 month before planting. Treatments may be made from late summer to early fall. In tree crops, methyl bromide often gives inconsistent control. Fumigants such as methyl bromide are a prime source of volatile organic compounds (VOCs), which are a major air quality issue. 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 entered without protective clothing.

PUBLICATION

UC IPM Pest Management Guidelines: Pistachio
UC ANR Publication 3461
Diseases
T. J. Michailides, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to the diseases section:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

Top of page

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See How to manage pests, or in the U.S., contact your local Cooperative Extension office for assistance. /PMG/r605100811.html revised: February 7, 2007. Contact webmaster.