from Viticulture Farm Advisor Ed Weber

1710 Soscol Avenue #4, Napa CA 94559 Tel: (707) 253-4221 / 944-2006 Fax: (707) 253-4434

Number 8. June 1997

Pierce's Disease

Information for Homeowners and Landscape Professionals

Pierce's Disease (PD) is a serious threat to vineyards planted adjacent to landscaped areas. It is one of the few diseases that kills grapevines. With sensitive grape varieties, vines often die in just one or two seasons.

PD is a complex disorder involving bacteria, insects, and other host plants. It is caused by the bacterium Xylella fastidiosa, which moves and multiplies in the water-conducting tissues of the vine. Over time, bacteria plug these tissues causing the vines to die. For more detailed information about PD, refer to the publication "Pierce's Disease in the North Coast" available from our office.

Why are landscapes important?

The causal bacterium can reside in a number of ornamental plants where it causes no apparent disorder. These plants are symptomless hosts of the bacterium.

Several species of leafhopper insects can acquire bacteria from these landscape plants and spread them to nearby grapevines. Not all leafhoppers are capable of spreading PD bacteria. Most common in the North Coast is the blue-green sharpshooter.

In accordance with applicable State and Federal laws and University policy, the University of California does not discriminate in any of its policies, procedures, or practices on the basis of race, religion, color, national origin, sex, marital status, sexual orientation, age, veteran status, medical condition, or handicap. Inquiries regarding this policy may be addressed to the Affirmative Action Director, University of California, Agriculture and Natural Resources, 300 Lakeside Drive, 6th Floor, Oakland, CA 94612-3560. (510) 987-0097.The blue-green sharpshooter feeds on a wide range of woody shrubs and trees, preferring plants that are actively growing. Its choice of plants changes during the year depending on their stage of growth. With normal pruning, fertilization and irrigation practices, ornamental landscapes are excellent habitats for these insects.

Landscapes thus provide both a source of the bacterium that causes PD, and favorable habitat for the insect vectors.

What can be done?

Two approaches homeowners or landscapers may take to try to minimize the threat of Pierce's Disease are plant selection and treatment with insecticides, either alone or in combination.

Most owners are probably unwilling to radically change the appearance of their landscape to combat PD, but may agree to treat with insecticides to reduce its severity.

Plant selection

In theory, landscape plants could be selected based on their importance relative to Pierce's Disease. For several reasons, however, this approach is not likely to succeed.

Plant selection would need to be based both on the ability of the plant to harbor the causal bacterium, and its suitability as a host for the blue-green sharpshooter.

Only a small number of ornamental plants have been tested for their ability to host the bacterium. When tests have been done, usually 50% or more of the plants are potential hosts. Eliminating host plants for the bacterium is therefore unlikely to occur.

Similarly, the blue-green sharpshooter has a wide host range. It prefers many of the woody shrubs and trees common to most landscapes.

For these reasons, a listing of plants suitable for use adjacent to vineyards where PD is a problem is not available and is not forthcoming.

Certain plants have been identified as preferred breeding hosts for the blue-green sharpshooter (places they will lay eggs). These plants should be avoided if possible.

Blue-green sharpshooter breeding plants

Virginia creeper

Many other plants not on this list are also likely to be breeding host plants.

Currently, the only other recommendations with respect to plant selection are to plant conifers, or to establish a non-irrigated xeriphytic landscape. These plants should not be attractive to blue-green sharpshooters.

Using insecticides to control the blue-green sharpshooter

Since most landscapes are likely to include host plants for both the bacterium and the blue-green sharpshooter, the use of insecticides will usually be the best choice for trying to deal with PD.

A number of products are available to homeowners and landscape professionals which are active against leafhoppers. Systemic products, such as acephate (Orthene®) and imidacloprid (Merit®) are good choices because they are taken up by the plant. Be certain to follow all label instructions.

When to treat

Grapevines are most susceptible to acquiring PD in late winter and early spring, once the vines have leafed out and the days become warmer. In general, March, April and May are the most critical months.

During this time, one or more insecticide applications may be needed. The frequency of applications will depend on the material being applied and observations of the leafhoppers.

Blue-green sharpshooter activity can be monitored through the use of yellow sticky-card traps. Place cards in the landscaped area and check them regularly. Insecticide treatments can be timed according to these trap readings.

Warning On The Use Of Chemicals

Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original labeled containers in a locked cabinet or shed, away from food or feeds, and out of the reach of children, unauthorized persons, pests and livestock.

Confine chemicals to the property being treated. Avoid drift onto neighboring properties especially gardens containing fruits and/or vegetables ready to be picked.

Dispose of empty containers carefully. Follow label instructions for disposal. Never reuse the containers. Make sure empty containers are not accessible to children or animals. Never dispose of containers where they may contaminate water supplies or natural waterways. Do not pour down sink or toilet. Consult your county agricultural commissioner for correct ways of disposing of excess pesticides. Never burn pesticide containers.

To simplify information, trade names of products have been used. No endorsement of named products is intended, nor is criticism implied of similar products which are not mentioned.

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