How to Manage Pests
UC Pest Management Guidelines
Nematodes are tiny (usually microscopic), unsegmented roundworms. Depending on the type of nematodes, they feed on roots, bulbs, stems, leaves, or seeds. Root knot nematodes (Meloidogyne spp.) are the most prevalent nematodes attacking floricultural crops. However, there are numerous other nematodes, both ectoparasitic (feed externally on plants) and endoparasitic (enter plants to feed and reproduce) that can attack floricultural crops. Because growing media used in containers is usually pasteurized, soil-dwelling nematodes are primarily a problem in field soils.
Root Knot Nematodes. Root knot nematodes occur throughout California and most of the United States (http://plpnemweb.ucdavis.edu/nemaplex/Taxadata/G076.htm). Root knot nematodes can enter roots only as second-stage larvae. The nematodes become immobile soon after entering roots and spend most of their active life cycle in galls on roots. After several weeks, females develop inside the galls and deposit up to 400 eggs in a protective gelatinous mass. Under favorable conditions (temperature, host crop) larvae hatch from eggs to repeat the cycle, or eggs can remain infective in the soil as a source of inoculum for the next crop or growing season. Root knot nematodes are most commonly pests of plants grown in warm, sandy, irrigated soils. The particular Meloidogyne species that is active can depend on seasonal temperature and cropping history. At least some Meloidogyne species also show host preferences, so cropping history can influence the species of root knot nematodes present.
To tentatively diagnose an infestation, dig the plants up after they have grown for about 4 to 6 weeks in soil above 65°F (19°C). 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 root knot nematode infection, send roots and/or soil to a laboratory.
Foliar Nematodes. Foliar nematodes, also called bud and leaf nematodes, prefer moderate temperatures and moist or humid conditions (http://plpnemweb.ucdavis.edu/Nemaplex/Taxadata/G011.htm). Aphelenchoides fragariae and A. ritzemabosi are the leaf-infesting nematodes that attack ornamental plants in California. Strawberries, ferns, tropical foliage plants, and vegetatively propagated ornamentals are important hosts of A. fragariae. Foliar nematode damage in California occurs mostly in certain greenhouses and along coastal areas where ornamental hosts and strawberries are grown. Foliar nematodes are tiny, only 0.02 to 0.04 inch (0.5-1 mm) long, and must be sent to a diagnostic laboratory to confirm infestation.
Foliar nematodes infest new plants by swimming up stems and along the surface of moist plant tissue. After entering leaves through stomata, females lay their eggs in intracellular leaf spaces. 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 on 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 allow them to resume activity.
Growers can make an initial diagnosis by tearing symptomatic tissue into small pieces and placing 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 roots of many broadleaf plants. Severely infected roots may subsequently be attacked by a variety of decay- or disease-causing organisms, including crown gall and root rot fungi. Aboveground symptoms are usually nonspecific, reflecting a poorly functioning root system and may include yellowing, stunted growth, and early wilting. Many weeds and nonfloral crops host root knot nematodes. Some infected plants, especially annual grasses, may exhibit no galls.
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 be pressed into a bacterial gall easily, but not into a root knot gall. To provide positive identification, collect galled roots and surrounding soil and send the material to a diagnostic laboratory.
Foliar nematodes. Foliar nematode damage can be confused with damage caused by bacteria, fungi, viruses, nutrient deficiencies, or chemical injuries. Nematodes may interact with certain fungi or bacteria to cause severe foliar blight.
General damage symptoms appear as vein-limited blotches and lesions on leaves. If young leaves or shoots are infested, they may remain undersized, become bushy or distorted, and produce little or no marketable foliage or flowers. Damage usually appears beginning in spring (or winter in coastal areas) and becomes most severe by summer.
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 main veins. Because monocots have parallel veins, discoloring on them looks like streaks. Damaged foliage may become brittle or shrivel and then drop.
The best time to sample nematodes for a coming crop is at or around harvest of the current crop, when plants with damage symptoms are available for testing. Nematodes are usually concentrated near or in plant roots. Unless your laboratory recommends other procedures, the following general 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 quart 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 part of the field and send them for 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 material reaches the laboratory. Laboratories report the genus of the nematodes that were found, the number of nematodes per unit of soil (usually per pint or liter), and the extraction efficiency. It is important to know the laboratory's method (and the method's efficiency) for extracting nematodes from soil. Certain methods are not adequate for detecting the presence of certain genera of nematodes; or certain tests 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 will tolerate heat without damage, cuttings can be disinfected by dipping them in hot water at 122°F (50°C) for 5 minutes or at 111°F (44°C) for 30 minutes. Foliar nematodes infesting Easter lilies may be controlled by dipping bulblets in 125°F (52°C) water for 10 minutes before planting. However, treatment at the same temperature for 20 minutes resulted in severe damage to the crop. Thus, to avoid damage to plant material, it is critical that both temperature and exposure time are accurately controlled.
Employ good sanitation practices 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. Avoid crowding plants and using overhead irrigation to reduce the risk that foliar nematodes will spread throughout the crop by traveling in a water film on plant surfaces.
MANAGEMENT OF ROOT KNOT AND
OTHER SOIL-DWELLING NEMATODES
Manage foliar nematodes according to the guidelines in the above section.
Sanitation and Cultural Practices. Avoid introducing nematode-infested plants into growing areas. Use only good-quality stock from a reliable supplier and, if available, from participants in the California Certification Nursery program to minimize the risk of introducing nematodes with the planting stock. Use growing media known to be free of nematodes or pasteurize growing media before planting. Remove infested plant 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 soil is treated first, do not plant susceptible crops in field soils where nematodes have previously been a problem. For example, do not replant the same plant genera into the old site; rotate crops by replanting with species or varieties 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 population levels of nematodes.
Heat Pasteurization. Pasteurizing media with heat, such as steam, can control nematodes and other pests 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. 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 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 the "double-tent" solarization technique can completely eradicate plant-parasitic nematodes, pathogens, and weed seeds from containerized growth media (Nursery Inspection Procedures Manual, NIPM Item 7; http://www.cdfa.ca.gov/phpps/pe/nipm.htm).
Hot Water Dips. Hot water dips can control 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 control depend on the nematode species and crop variety. Exceeding temperature or exposure time can damage plants, but insufficient temperatures or exposure time may not kill nematodes. Cool plants immediately afterward with clean, cold water, then dry thoroughly in warm air or sunshine. Store material afterwards under cool, low-humidity conditions until plants are used.
Amendments and Biological Control. Although amendments and biological control agents reduce plant parasitic nematodes in certain situations,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 controls 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: inorganics, animal-based, 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. Animal-based amendments include chitin-containing crab shells and shrimp shells that apparently stimulate populations of soil-dwelling fungi that feed on chitin. Because chitin is a component of nematodes' egg shells, these chitin-feeding fungi also feed on these egg shells. Incorporating animal manure, organic fertilizers, crop residue, and compost increases the organic matter content of soil. This improves water and nutrient availability to plants, reduces plant stress, and can encourage greater numbers of nematode predators and parasites. However, organic amendments sometimes contain contaminants such as weed seeds, and their effectiveness is largely limited to the depth of material incorporation.
Barley, marigold, perennial rye, certain legumes such as clover and vetch, and other plants with bioactive properties are grown as cover crops, trap crops, or crop rotations 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, suppress nematode growth, stimulate premature egg hatch, or disrupt the attraction between nematodes seeking to mate. However, crops suppressive to one species of nematode often host other nematode species. Rotating certain marigold cultivars with crops such as lilies grown for bulb production has been somewhat successful in controlling 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, as phytotoxicity to lilies and other crops is often observed when they are grown in rotation after incorporating marigolds into the soil.
Some new biological pesticides (mycopesticides) contain nematode-killing microorganisms. These beneficial microorganisms include certain Burkholderia and Pseudomonas species bacteria and natural by-products of Myrothecium species fungi. At least one mycopesticide, Myrothecium verrucaria (Ditera), is registered for nematode control in California.
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; improper application is not effective and may be hazardous. Postplant nematicides for use in soil around established plants may be available, but it is generally more effective to employ other controls and preventive measures before planting.
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries