How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Plant parasitic nematodes are microscopic roundworms that live in soil and plant tissue and feed on plants by puncturing and sucking the cell contents with a needlelike mouthpart called a stylet. The stem and bulb nematode lives within the plant, feeding in stems, leaves, and bulbs. It is capable of living without water and tolerates desiccation for several years. Root knot nematodes live within the roots; the second stage juveniles are motile and the other stages are sedentary. The stubby root nematode lives in the soil and feeds on the roots. Lesion nematode, Pratylenchus penetrans, has been reported on onions and garlic in other states where it suppresses the growth and yield of these crops. While this nematode is found in California, it has not been found to cause problems on onions or garlic.

DAMAGE

Stem and bulb nematode and root knot nematodes cause substantial damage and are of major concern in California. The stem and bulb nematode penetrates the germinating clove and destroys tissue as it moves through seeking food. Nematodes sucking the cell contents and salivary secretions cause the cells to collapse. Root knot nematodes can cause stunting and reduce a stand. Stubby root nematode causes stunting of plants.

SYMPTOMS

The symptoms described below may be an indication of a nematode problem, but are not diagnostic as they could result from other causes as well. Plants infested with the stem and bulb nematode have distorted and bloated tissue with a spongy appearance; the plants are stunted with shortened and thickened leaves, often with brown or yellowish spots. The bulb tissue begins softening at the neck and gradually proceeds downward; the scales appear pale gray, and the bulbs desiccate and split at the base under dry conditions. Under moist conditions, secondary invaders such as bacteria, fungi, and onion maggots induce soft rot and decay of the bulbs. Root knot infestation can cause stunting, uneven stands of plants, and produces characteristic galls on roots. Galls induced by M. hapla are generally small and difficult to see, whereas galls produced by the other root knot species are larger. Roots fed on by the stubby root nematode are extremely short with a yellow to brownish cast.

FIELD EVALUATION

It is critical to know the nematode species present and their population densities to make management decisions. If a previous field or crop had problems caused by nematodes that are also listed as pests of onions or garlic, populations of these nematodes may be high enough to cause damage to seedlings.

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 between 5 and 20 acres each that are representative of cropping history, crop injury, or soil texture. Be sure to sample the soil from within the root zone (6–18 inches deep) and include samples of suspect plants. Take several subsamples randomly from a block, mix them thoroughly and make a composite sample of about 1 quart (1 liter) for each block. Place samples from each block 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. Determine the cropping history of fields to be planted with seed garlic, onion transplants, or onion sets. If the field is not known to be infested with nematode pests of onion and garlic, make sure clean, uninfested cloves are used when planting garlic. Garlic cloves can be tested by private laboratories or the California Department of Food and Agriculture to determine if they are infested. Growing nonhost crops such as carrots and lettuce for several years is helpful in reducing populations of stem and bulb nematodes, but is not usually feasible in fields with root knot and stubby root nematodes because of their wide host range. Avoid infesting new fields by cleaning machinery and equipment with water, and preventing movement of infested soil.

Resistant cultivars. There are currently no resistant cultivars available.

Treatment decision. Treating bulbs with hot water has been shown to eradicate nematodes from garlic cloves. Contact your local farm advisor for the most recent information.

Preplant fumigation can be effective in onion fields.

PUBLICATION

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453
Nematodes
B. B. Westerdahl, Nematology, UC Davis
J. O. Becker, Nematology, UC Riverside
Acknowledgment for contributions to Nematodes:
U. C. Kodira, Plant Pathology, UC Davis

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