How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Plant parasitic nematodes are microscopic, unsegmented roundworms that feed on plant roots by puncturing and sucking the cell contents with a needlelike mouthpart called a stylet. They live in soil and within or on plant tissues. Of the many genera of plant parasitic nematodes detected in soils from California vineyards, root knot, dagger, citrus, lesion, and ring nematodes are the most important ones. Other nematodes associated with grape in California include stubby root nematode, Paratrichodorus minor; spiral nematode, Helicotylencus pseudorobustus; and needle nematode, Longidorus africanus. Of these, only needle nematode has been found to be damaging to grapes in California. Pin nematode, Paratylenchus hamatus, is frequently found in vineyards but is not thought to cause damage to this crop.

Dagger, ring, and lesion nematodes are most prevalent in north and Central Coast vineyards, and in the San Joaquin Valley. Root knot and citrus nematodes occur most commonly in the San Joaquin Valley and southern California. The needle nematode is found mainly in southern California.

Species introduction, native habitat, soil texture, grape cultivar, cropping history, weed spectrum, and growing region are the determining factors as to which nematode is present in which vineyard.

DAMAGE

Plant parasitic nematodes feed on roots, reducing vigor and yield of the vine usually in irregular patterns across the vineyard. Damage patterns are frequently associated with soil textural differences. Root knot nematodes penetrate into roots and induce giant cell formation, usually resulting in root galls. Giant cells and galls disrupt uptake of nutrients and water, and interfere with plant growth. Xiphinema americanum, the most common species of dagger nematode, weakens vines by feeding near the root tip and is a specific vector of yellow vein virus (also known as tomato ringspot virus). The dagger nematode, Xiphinema index, can cause yield reduction in some varieties, but is more important for its transmission of grapevine fanleaf virus. Both ring and dagger nematodes feed from outside the roots, but can reach the vascular tissues with their long stylet. Root lesion nematode restricts the growth of roots as it feeds and migrates in and out of roots; it can be especially damaging to newly planted vines. Citrus nematodes establish feeding sites with their heads embedded in cortical tissue and their posterior ends outside the roots. Their feeding disrupts the uptake of nutrients and water, and interferes with plant growth.

SYMPTOMS

The symptoms described below are indicative of a nematode problem, but are not diagnostic as they could result from other causes as well. Generally, nematode infestations result in areas of the vineyard with vines that lack vigor and have restricted growth and reduced yields. Root knot nematodes produce small galls on the roots (about 0.125 inch in diameter), but they can be larger when multiple infections cause galls to coalesce. The dagger nematode, X. index, feeds on root tips causing them to swell in a manner similar to the nodosities caused by phylloxera. Virus transmission by dagger nematode produces symptoms on leaves such as yellowing of veins, mosaic, and malformation with symptom expression less apparent among white varieties and in warmer regions. Infestation by root lesion nematode restricts top growth of young vines. If young vines are planted in soil infested with lesion nematode, root systems may be severely restricted and sometimes exhibit brown lesions. Soil adheres to roots infested with citrus nematode giving them a dirty appearance. Ring nematodes cause general aboveground lack of vigor and reduced vine growth and yields.

FIELD EVALUATION

To make management decisions, it is important to know the nematode species present and to estimate their population. If a previous orchard or vineyard had problems caused by nematodes that are also listed as pests of grape, population levels may be high enough to cause damage to the young vines.

If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification. The best time to sample differs according to region, type of nematode, and variety of grapes. Bloom and harvest times, which influence nematode populations, also differ according to region. Research has shown that populations of citrus nematode are highest in the Coachella Valley from February through March and again in October. In the San Joaquin Valley, X. index populations are most likely to be detected in November through February. Root knot nematodes are more likely to be found at any time of the year.

To get the greatest numbers and the most reliable indication of nematode populations, irrigate three days before sampling (or wait for three consecutive days of rain). Collect soil samples from 0 to 18 inch depth between the dripper spot and the vine trunk (as long as the emitter is halfway between the two vines). Roots encountered during sampling should be included. Divide the vineyard into sampling blocks that are representative of cropping history, crop injury, or soil texture. Take samples of soil and symptomatic roots from around five randomly chosen vines per block, mix them thoroughly, and make a composite sample of about 1 quart (1 liter) for each block. Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, the previous crop/variety, and the current variety grown or that you intend to grow. See UC/ANR Publication 3343, Grape Pest Management, 2nd edition, for more details). Keep samples cool (do not freeze), and transport as soon as possible to a diagnostic laboratory. Contact your farm advisor for more details about sampling, to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results.

Look for nematode symptoms in the vineyard late in the growing season to prepare for future management.

MANAGEMENT

Cultural practices. To naturally rid an old vineyard site of the effects of X. index and grapevine fanleaf virus, it is necessary to forgo planting grapes for more than 10 years. This period of time is required to allow old roots to decompose and nematode numbers to decrease. This will increase the length of time before a new vineyard exhibits virus symptoms, but will not prevent reinfestation. Do not rotate sites with crops or plant cover crops that are hosts to nematodes. No single rootstock is resistant to all root knot nematodes and there are numerous other nematodes of concern. Broadest resistance is present in Ramsey, Freedom, and several rootstocks in the Teleki series (5C is the only one that has been specifically tested). Selection of a rootstock is a risky endeavor because of their excessive or inadequate growth in certain situations and their limited breadth of resistance. Consult your farm advisor for the appropriate site selection and cultural practices associated with each rootstock.

Manures and other soil amendments can improve vine vigor and frequently reduce the effect of nematode infestation. To reduce stress on vines, take measures to prevent soil compaction and stratification, to improve soil tilth and drainage, and to control other pests. Proper irrigation and fertilizer application also reduce stress on vines and help lessen the effect of nematodes such as root knot.

Chemical. Vineyards planted in fumigated ground are known to have generally improved growth and yields compared to those planted on nonfumigated ground. Contact your local farm advisor to discuss the most effective application method and timing when making a postplant application.

Always read and carefully follow all label information when applying soil fumigants.

PUBLICATION

UC IPM Pest Management Guidelines: Grape
UC ANR Publication 3448
Nematodes
B. B. Westerdahl, Nematology, UC Davis
Acknowledgment for contributions to Nematodes:
U. C. Kodira, Plant Pathology, UC Davis

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