Nematodes

Nematodes are typically tiny (usually microscopic), unsegmented roundworms. At maturity, they commonly are 1/100 to 1/25 inch (0.25 to 1 mm) long. Depending on the type of nematode, they feed on or inside bulbs, leaves, seeds, stems, or roots. Root-knot nematodes (Meloidogyne spp.) are the most prevalent nematodes attacking floricultural and nursery crops. Foliar nematodes (Aphelenchoides spp.) and lesion nematodes (Pratylenchus spp.) can also cause problems. However, there are numerous other nematodes, both ectoparasitic (feed externally on plants) and endoparasitic (enter plant tissues to feed and reproduce) that can attack floricultural and nursery crops. Because growing media used in containers is commonly pasteurized, soil-dwelling nematodes primarily cause problems in field crops.

Plant-parasitic nematodes generally hatch from an egg and develop through four juvenile stages before maturing into adults. Most species are active at temperatures from about 60° to 85°F. Feeding stages pierce plant cells and withdraw the contents. They also inject enzymes and other chemicals that breakdown cells and can change the appearance and physiology of plants. Tissues injured by nematode feeding become increasingly susceptible to infection by pathogenic bacteria, fungi, and oomycetes. Certain species of plant-parasitic nematodes vector some plant viruses.

Root-knot Nematodes

Root-knot nematodes occur throughout California and most of the United States. The second-stage larvae occur in soil, are mobile, and can enter roots. Once inside the roots, they become immobile near vascular cells where they induce feeding sites (giant cells). While the root swells to form galls, within those structures the nematodes further develop through three molts into mature females. Each of those pear-shaped females can produce up to 400 eggs in a protective gelatinous mass that protrudes from the surface of roots. Second-stage larvae hatch from eggs to repeat the cycle, or eggs can remain viable in the soil and hatch during the next crop or growing season. The time from root invasion to egg development is determined mostly by temperature, but also depends on the nematode species and host crop.

Root-knot nematodes are the most common pests of plants grown in warm, sandy, irrigated soils. The particular Meloidogyne species that are active can depend on cropping history, geographical location, and seasonal temperature. Although Meloidogyne species generally have a broad host range, there are differences between species, so cropping history can influence the species of root-knot nematodes present. See the University of California (UC), Davis Nemaplex website for more information.

To tentatively diagnose an infestation, dig the plants up after they have grown for about 4 to 6 weeks in soil above 65°F. Wash or gently tap the soil from their roots and examine the roots for swellings and gnarled, restricted root growth. Cut open any galls and use a hand lens or binocular microscope to examine galls for the presence of pinhead-sized, shiny white females that look like tiny pearls. For confirmation of infection by root-knot nematodes, send roots or soil or both to a nematology laboratory.

Foliar Nematodes

Foliar nematodes, also called bud and leaf nematodes, prefer moderate temperatures and moist or humid conditions. Aphelenchoides fragariae and A. ritzemabosi are the leaf-infesting nematodes that attack ornamental plants in California. Ferns, strawberries, tropical foliage plants, and vegetatively propagated ornamentals are important hosts of A. fragariae. Foliar nematode damage in California occurs mostly in certain greenhouses and in fields along coastal areas where humidity is high and ornamental hosts and strawberries are grown. Foliar nematodes are tiny, about 1/50 to 1/25 inch (0.5–1 mm) long, and samples must be sent to a nematode diagnostic laboratory to confirm an infestation.

Foliar nematodes infest new plants by swimming in a water film up stems and along the surface of moist plant tissue. After entering leaves through stomata, females lay their eggs in intracellular spaces in leaves. Foliar nematodes can mature from egg to adult in about 2 weeks, allowing many generations to develop during one growing season. Foliar nematodes can also live for a few months in soil or decomposing organic material by feeding on fungi. They typically overwinter in dormant buds, plant terminals, and soil in dead leaves that drop from infested plants. In slowly drying leaf tissue, adults of A. ritzemabosi can enter a desiccated, resting stage that allows them to survive for several years until moist conditions induce them to resume activity. See the UC Davis Nemaplex website for more information.

To make an initial diagnosis, tear symptomatic tissue into small pieces and place it in a glass dish. Add just enough water to immerse the plant tissue, then cover the dish to reduce evaporation. After 24 hours, carefully examine the water under strong light using a 10X hand lens or, preferably, a binocular microscope providing higher magnification. Nematodes will appear as tiny strands moving in the water.

Root-knot nematodes

Root-knot nematodes cause galls or swellings on the roots of many broadleaf plants. Many weeds host root-knot nematodes. Some grasses and cereals (monocots) can be infested and are suitable hosts, but root symptoms (galls) on these crops are generally not obvious. Severely infected roots may subsequently be attacked by a variety of decay- or disease-causing microorganisms, including bacteria, fungi, and oomycetes. Aboveground symptoms are usually nonspecific, characteristic of a poorly functioning root system, and may include stunted growth, wilting, and yellowing.

Although beneficial nitrogen-fixing bacteria often form nodules on the roots of legumes such as cassia, sweet pea, and vinca, these nodules rub off roots easily, whereas galls caused by root-knot nematodes are truly swellings of the roots. Also, a thumbnail can easily be pressed into a bacterial gall, but not into a gall of root-knot nematodes. To provide identification, collect galled roots and surrounding soil and send them to a nematode diagnostic laboratory.

Foliar nematodes

Foliar nematode damage can be confused with symptoms caused by certain bacteria, fungi, viruses, nutrient deficiencies, or chemical injuries. Nematodes may interact with certain bacteria or fungi to cause severe foliar blight.

Foliar nematode damage usually begins as yellowish leaf spots that eventually turn dark green to blackish brown. Discoloring typically starts near the leaf base and spreads outward. The lesions are often angular because nematodes in leaves are initially contained between the veins. Because monocots have parallel veins, discoloring on them occurs in streaks.

If young leaves or shoots are infested, they may remain undersized, become bushy or distorted, and produce little or no marketable foliage or flowers. Damaged foliage may become brittle or shrivel and drop prematurely. Damage usually appears beginning in spring (or winter in coastal areas) and becomes most severe by summer.

The best time to sample nematodes for the next crop is at or around the harvest of the current crop when plants with damage symptoms are available for testing. Nematodes of most pest species are usually concentrated near or in plant roots. Unless your nematology laboratory recommends other procedures, the following method can be used. Divide the field into areas of uniform plant growth and similar soil characteristics and cropping history. Take several soil subsamples from locations scattered throughout each uniform field area. Each subsample can be about 1 pint of soil. Collect moist (not soggy) soil from the plant root zone or the upper 6 to 18 inches of soil if no crop is present. Thoroughly mix the subsamples to make a composite sample and send about 1 pint of soil for testing. Repeat this sampling procedure for each field area. If plants have symptoms, dig them up along with their roots and surrounding soil and place them in a bag for testing. Also, bag separately at least one or two plants and soil sampled from a healthy-looking part of the field and send them for comparison testing.

Label each sample with field location, current crop, cropping history, crop injury observed, and your name, address, and phone number. Seal samples in plastic to prevent them from drying out and keep them cool at about 50° to 60°F until the material reaches the laboratory. Laboratories should report the genus of the nematodes that were found, the number of nematodes per unit of soil, and the extraction efficiency. It is important to know the laboratory's method (and the method's efficiency) for extracting nematodes from soil. Certain techniques are not adequate for detecting the presence of specific genera of nematodes, or they provide only qualitative results, which tell you that nematodes are present but not whether they are abundant enough to cause damage.

Grow plants in soilless media or pasteurize media before use. Propagate only nematode-free stock. Foliar nematodes are typically introduced into growing areas in cuttings, seedlings, and other vegetative propagation material that may be asymptomatic. Take cuttings only from the tops of long, vigorous growth to reduce the likelihood that it is infested.

If the plants tolerate heat without damage, cuttings can be disinfected by dipping them in hot water at 122°F for 5 minutes or at 111°F for 30 minutes. Foliar nematodes infesting Easter lilies may be controlled by dipping bulblets in 125°F water for 10 minutes before planting. However, treatment at the same temperature for 20 minutes results in severe damage to the crop. Thus, to avoid damage to plants, it is critical to control both temperature and exposure time accurately.

Employ proper sanitation by removing plant debris, promptly disposing of all infested plants, and eliminating weeds that can host foliar nematodes (e.g., goldenrod, groundsel, and sneezeweed) from around growing areas. To reduce the risk that foliar nematodes will spread throughout the crop by traveling in a water film on plant surfaces, avoid crowding plants and using overhead irrigation .

Rotating crops, employing good cultural practices and excellent sanitation, pasteurizing growing media, and fumigating field soil before planting are the most important strategies for preventing and managing most soil-dwelling nematodes. Soil amendments and biological control products may sometimes suppress nematode populations. Post-plant nematicides for use in soil around established plants may be permissible (check the label). But it is generally more effective to employ preventive measures before planting.

Sanitation and Cultural Practices

Avoid introducing nematode-infested plants into growing areas. To minimize the risk of introducing nematodes with the planting stock, use only good-quality stock from a reliable supplier and, if available, from participants in the California Department of Food and Agriculture (CDFA) Nursery Services Program. Use growing media known to be free of nematodes or pasteurize growing media before use. Dispose of infested plants when found and avoid moving soil from around infested plants to healthy plants. Do not allow irrigation water from around infested plants to run off onto healthy plants, as this spreads nematodes.

Unless the soil is treated first, do not plant susceptible crops in field soils where nematodes have previously been a problem. In particular, do not replant the same plant genera into the old site; rotate crops by replanting with different genera more tolerant of, or resistant to, the specific nematodes present.

Provide crops with proper cultural care so that they are vigorous and better able to tolerate feeding by nematodes and other pests. More frequent irrigation of drought-stressed plants can reduce damage caused by root-knot nematodes, but it does not reduce the abundance of nematodes.

Heat Pasteurization

Pasteurizing media with heat, such as aerated steam, can control nematodes and other pests or pathogens in container mix and greenhouse beds. Special tractor-drawn steam rakes are available, but except for raised beds, steam is difficult to use in field soils. The heat generated by decomposer microorganisms during composting of container media can control certain nematodes, but preparing pathogen-free compost requires careful management and monitoring.

Solarization

In sunny, warm climates, field solarization before planting can temporarily reduce nematode populations in the upper 12 inches of soil. Solarization involves covering moist, bare soil or container mix with single or double layers of clear plastic for several weeks during hot weather. In some cases, incorporating amendments (such as compost or green manure) or applying lower than normal rates of fumigant pesticides in combination with solarization can provide better control than using any single method. Correct use of "double-tent" solarization can completely eradicate plant-parasitic nematodes, most other pathogens, and weed seeds from containerized growth media. See the table of Some Flower and Nursery Crop Nematodes Controlled by Solarization of Container Mix for which nematodes are controlled in this situation. See CDFA's Nursery Inspection Procedures Manual (NIPM Item 7) and UC's Using Solarization to Disinfect Soil for Containerized Production (PDF) for details.

Hot Water Dips

Hot water dips can reduce the number of nematodes and certain other pests infesting bulbs, corms, and rhizomes of crops such as amaryllis, daffodil, gladiolus, lily, and tulip. The temperature and time needed to provide sufficient control depend on the nematode species and crop variety. Exceeding the proper temperature or exposure time can damage plants, but insufficient temperatures or exposure time may not kill the nematodes. Cool plants immediately afterward with clean, cold water, then dry thoroughly in warm air or sunshine. Consider making a fungicide application after the heat treatment. After heat treatment, store the plant material under cool, low-humidity conditions until plants are used.

Amendments and Biological Control

Although amendments and biological control microorganisms reduce plant-parasitic nematodes in certain situations, sufficient control has been unreliable. The reasons for this variable effectiveness are not well known. To provide a basis for comparison, growers using amendments and biological control products should consider leaving several randomly selected areas of their fields untreated or treated with more conventional methods or both.

Soil amendments used for nematode control can be placed into four categories: animal-based, inorganics, plant-based, and microbial. Except for inorganics (such as ammonium sulfate fertilizer and powdered rock), nematode suppression from most amendments is at least partly the result of biological control. Incorporating animal manure, compost, crop residue, and organic fertilizers increases the organic matter content of the soil. It improves water and nutrient availability to plants, reduces plant stress, and can encourage higher numbers of nematode predators and parasites. Some residues can produce by-products with nematicidal properties upon degradation. However, organic amendments sometimes contain contaminants such as weed seeds, especially in non-composted or incompletely composted materials, and their effectiveness is largely limited to the depth of material incorporation.

Barley, certain legumes such as clover and vetch, French marigold, perennial rye, and other plants with bioactive properties are grown as crop rotations, cover crops, or trap crops in some row crops. These plants may sometimes reduce populations of certain soil-dwelling plant-parasitic nematodes by producing chemicals that kill or repel nematodes, stimulate premature egg hatch, suppress nematode growth, or disrupt the attraction between nematodes seeking to mate. However, crops suppressive to one species of nematode often host other nematode species. Rotating specific marigold cultivars with crops such as lilies grown for bulb production has been somewhat successful in controlling certain nematodes. The marigolds must be left in the soil, either through cultivation or by mowing the tops and leaving the roots underground. However, this practice is generally not recommended because phytotoxicity to lilies and other crops is commonly observed when they are grown in rotation after incorporating marigolds into the soil.

Several biologically-based (microbial) nematicides are registered in California. Their efficacy for controlling root-knot nematodes has been studied in vegetable crops in Southern California. These field experiments found their effectiveness to be inconsistent at best.

Organically Acceptable Methods

Certain amendments and biological control products, anaerobic soil disinfestation, cultural practices, heat pasteurization, hot water dips, sanitation, and solarization are organically acceptable management methods.

Fumigants

A soil fumigant can be used in certain situations to reduce nematode populations before planting. Before using a fumigant, be sure that nematodes or other soil pests are the cause of your problem by having a laboratory test performed or by having an expert examine your plants and soil. Consider alternatives before using a nematicide. Be sure the nematicide is registered for that crop or growing situation. Follow label directions strictly; the improper application is not only illegal but often ineffective and may be hazardous. Fumigants such as 1,3-dichloropropene* and metam sodium* are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. (*Permit required from county agricultural commissioner for purchase or use.)

Fumigate only as a last resort when other management strategies have not been successful or are not available. Soil fumigants may only be applied by a regulated, commercial applicator. Consult UC's Field Fumigation: Pesticide Application Compendium, Vol. 9, the Department of Pesticide Regulation's Addendum to the Field Fumigation Study Guide (PDF), and the office of the local county agricultural commissioner for more information.