How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Nematodes are microscopic roundworms that live in diverse habitats. Plant parasitic nematodes live in soil and plant tissues and feed on plants by puncturing and sucking the cell contents with a spearlike mouthpart called a stylet or spear. Of the several genera of plant parasitic nematodes detected in California orchard soils, root knot, ring, root lesion, and dagger nematode are considered to be the most important.

DAMAGE

Damage caused by nematodes may first become evident during the first year after planting. Feeding by root knot nematodes can impair root functions such as uptake of nutrients and water. Root knot nematodes have been implicated in nectarine disease complexes with fungi and bacteria; for example, Meloidogyne javanica has been reported to increase the incidence of crown gall on nectarine roots. Ring nematode infestation impairs development and function of nectarine roots and predisposes trees to bacterial canker. Root lesion nematodes penetrate roots and cause damage by feeding and migrating through the root tissues. Dagger nematodes feed from outside the roots, but can reach the vascular tissues with their long stylet and are capable of reducing vigor and yield of trees. However, the main damage caused by the dagger nematode, Xiphinema americanum, is that it vectors a strain of tomato ringspot virus that causes peach yellow bud mosaic, which debilitates and can kill trees.

SYMPTOMS

The symptoms described below are indicative of a nematode problem, but are not diagnostic as they could result from other causes as well. Symptoms of root knot nematode infestation are reduced vigor and yield, patches of unevenly sized trees, and characteristic galls on roots. Heavily infested trees are more susceptible to moisture stress. Ring nematode infestation reduces tree vigor, and predisposes trees to bacterial canker complex, which can cause sudden collapse of trees in spring. Root lesion nematodes reduce overall root presence. Symptoms of dagger nematode infestation include reduced growth and vigor; if the tomato ringspot virus has been transmitted, yellow bud mosaic disease symptoms may also be seen. Newly infected trees have irregular chlorotic areas and vein clearing (mosaic) on leaf blades. In the second year of infection, the yellow bud phase is expressed in spring as extremely stunted tufts of pale yellow leaves.

FIELD EVALUATION

It is critical to know the nematode species present and to estimate their population to make management decisions. If a previous orchard or crop had problems caused by nematodes that are also listed as pests of your specific nectarine rootstock, expect population levels to be high enough to cause damage to the young trees.

If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification. Divide the field into sampling blocks of not more than 5 acres each that are representative of cropping history, crop injury, or soil texture. Within each block, take several subsamples randomly from the frequently wetted zones at the edge of the tree canopy. Take samples from within the root zone (6- to 36-inch depth) and include some feeder roots when possible. Mix the subsamples 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, and the current/previous crop and the crop you intend to grow. 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.

MANAGEMENT

Cultural practices. Before fumigating, remove old trunks and large roots brought to the surface by ripping and fallow or plant green manure cover crops for 1 to 2 years. Do not use cover crops that are known hosts of nematodes that damage the rootstock you plan to plant; contact your farm advisor for further information. Use certified nematode-free rootstocks or seedlings to establish new orchards. When the orchard is developed, use procedures that improve soil tilth and drainage to help reduce nematode damage. Where ring nematode is a problem, apply irrigations more frequently.

Rootstock selection. Use certified nematode-free rootstocks. Among nectarine rootstocks, Nemaguard is known to be resistant to root knot nematodes, but it is susceptible to ring and root lesion nematode. Lovell is a slightly poorer host to ring nematode and somewhat more tolerant of bacterial canker complex but susceptible to root knot and root lesion nematodes.

When to treat. Trees planted on fumigated orchard sites are generally known to have improved growth and yields compared to those on nonfumigated sites. Threshold levels for a postplant treatment have not been established. Yield and vigor increases of 15% can be expected following proper application of a postplant nematicide. To help determine the effectiveness of postplant treatments, leave several trees or rows of trees untreated for comparison.

PUBLICATION

UC IPM Pest Management Guidelines: Nectarine
UC ANR Publication 3451
Nematodes
B. B. Westerdahl, Nematology, UC Davis
M. V. McKenry, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to the nematode section:
U. C. Kodira, Plant Pathology, UC Davis

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