Nematodes

Plant parasitic nematodes, which are only a small fraction of all nematode species, are microscopic, unsegmented roundworms that live in soil and plant tissues. They are distinguished from other nematodes by a mouth stylet, typically a syringe-like structure adapted for feeding on living cells. Most other soil dwelling nematodes play an important role in the cycling of soil nutrients by feeding on microorganisms. Some nematode species even parasitize arthropods, thus functioning as biological control agents against certain insects or snails.

Plant-parasitic nematodes are distinguished by their feeding and dwelling habit. Ectoparasitic nematodes feed, molt, and reproduce on the outside of their hosts, while endoparasites spend most of their life inside the plant tissues. The plants provide them with nutrition and protect against biotic or abiotic threats such as pathogens, predators, drought, frost, and even pesticides.

Citrus Nematode

The citrus nematode is the only major nematode pathogen in California citrus. Originally from Asia, it spread worldwide with infested planting stock. This invasive species is present in most California citrus orchards and in all soil types. The host range of the predominant biotype includes also grape, lilac, persimmon, olive, and trifoliate orange.

Its life cycle includes an egg, four juvenile stages, and an adult stage. The first-stage and second-stage juvenile (J2) develop within the egg from which the latter one hatches. Approximately a quarter of the J2 develop into males that do not feed. They remain long and slender throughout their development and do not penetrate the roots. The other J2 first feed on the outside of young feeder roots for 2 to 3 weeks. They then burrow deep inside the root cortex while the back end remains outside of the root. Relatively few J2 succeed at entering the root considering their abundance in the rhizosphere (root zone). After another two molts, the young females penetrate further into the cortex. They initiate a feeding site with several nurse cells. At maturity, the back end of the now sedentary (stationary) female swells and extends from the root surface. Meanwhile, its head remains at the nurse cells inside the root.

Reproduction occurs both sexually and asexually. Each female can produce about 100 eggs that are embedded in a protective gelatinous matrix. The female life cycle from egg to egg ranges from four to eight weeks. The highest numbers of nematodes are typically found in late spring and late autumn following the citrus root flushes. Hatch, feeding, growth, and reproduction is limited between 68 °F (20 °C) and 86 °F (30 °C). Second-stage juveniles are the persistent stage that can survive for a year or more in field soil.

Sheath Nematode

The sheath nematode is far less widespread than the citrus nematode; H. arenaria has been found only on citrus in the Coachella Valley and on some native desert plants. However, it has the potential to have a broad host range that includes many vegetables. The name sheath nematode refers to the presence of an extra covering over the body of all juvenile and adult life stages. They are restricted to sandy soils and complete their life cycle (egg to egg) in 15 to 18 days at an optimal soil temperature of 86°F (30°C).

Damage caused by citrus nematodes is called citrus slow decline, which refers to the gradual starvation and consequent deterioration of the tree. Mature trees can tolerate many of these nematodes before vigor declines and symptoms become apparent.

The symptoms described below are typical of a citrus nematode problem but are not diagnostic because they could result from other causes. Aboveground symptoms include reductions in leaf and fruit size, as well as leaf yellowing, curling, and twig dieback that are caused by poor root development and feeder root decay. Feeder roots that are heavily infested with mature citrus nematode females typically appear darker and thicker than those without the parasites. This impression is caused by soil particles encrusted with the egg sack gel.

Severity and speed of the tree decline depend on tree age and vigor, citrus nematode virulence (the degree of disease caused), population density, and susceptibility of the rootstock. In addition, parasitized trees are more susceptible to other biotic or abiotic stresses such as microbial pathogens, insect pests, salinity, waterlogging, drought, and freezing. Susceptible trees planted in lightly infested soil may grow for many years without apparent problems and decline slowly. Resistant rootstocks generally do well even in heavily infested soils. If, however, a highly infested orchard site is replanted with a susceptible rootstock, the roots of the young trees will soon be heavily parasitized, tree growth will be stunted, and fruit production will be reduced.

Sheath nematodes feed near the terminal and lateral root tips with a long, flexible stylet. They reach the meristematic zone (area of active plant growth) and cause galling of the affected area. This causes a stubby root system with distinct galling of root tips. Several nematodes are often firmly attached to the root tips by adhesive plugs. It anchors the nematode in place during feeding over extended periods of time. Parasitism may reduce root growth and vigor of trees, particularly young trees.

Before planting or if nematode-caused problems are suspected in an established orchard, it is essential to know the nematode species present and their population density (nematode numbers per gram of feeder root or soil volume) to make rational management decisions. If a previous orchard or crop had problems caused by nematodes that are also listed as pests of citrus, nematode numbers may be high enough to cause damage to the ensuing citrus crop.

Before planting or replanting a citrus orchard, obtain a professional soil analysis; the analysis will help you determine the potential for nematode damage and plan a management strategy. In an established orchard, a soil analysis will confirm visible symptoms that may be present. Some laboratories collect soil samples, or you may have to do it yourself.

To collect samples

• Before planting: visually divide the orchard into sampling blocks representing differences in soil texture, drainage pattern, or cropping history.
• In established orchards irrigated by sprinklers or furrows: collect soil and root samples at the drip line of trees that show symptoms.
• In drip-irrigated orchards: take samples around emitters where feeder roots are most abundant. The soil should not be too dry or too wet.
• For all irrigation types in established orchards: take another set of samples from adjacent, healthy-looking trees for comparison.

In loamy soils, sampling down to 24 inches is sufficient; in sandy soils, take samples to a depth of 36 inches. Use a soil auger, Viehmeyer tube, or shovel. A soil auger (3 inches in diameter) is convenient for depths to 24 inches in sandy soils. To sample deeper than 24 inches (60 cm), a Viehmeyer tube is recommended to reduce the soil volume taken.

From each sampling block

1. Collect 10 to 20 cores or subsamples.
2. Combine the subsamples, mix thoroughly, and pour the soil and roots into durable plastic bags or other moisture-proof containers. Seal tightly and place bags in the shade until you have taken the last sample.
3. Attach labels providing name and address, location of the orchard, sample block, soil texture, rootstock, soil and air temperature as well as notable symptoms; this information might be critical for a meaningful analysis.
4. Send or deliver the samples to a lab as soon as possible. Ship them in a cardboard box insulated in a styrofoam ice chest.
5. If any delay occurs, keep the samples in a cool place (41° to 50°F).

Most labs extract nematode juveniles from soil samples using the Baermann funnel, the elutriation, or the flotation method. The method used and often the extraction efficiency is reported together with the results. Juvenile counts are generally sufficient for estimating relative infestation levels. Extracting females from the citrus roots, however, is more accurate, especially when checking the success of a pesticide application at the end of the season when juvenile counts are usually low because of low temperatures.

The number of nematode juveniles in the soil or mature females in the roots, as determined by laboratory analysis, can give some indication of the damage potential of an infestation. Samples cannot provide an accurate prediction of yield at the end of the season because many other factors, including alternate-bearing habit of citrus and other pest problems, may influence yield.

The table below shows the average number of citrus nematode juveniles and females at different sampling times; different soil types or rootstock susceptibility are not taken into account. The table gives a rough estimate of low, medium, and high nematode numbers for every 500 grams of soil.

• A preplant pesticide application is recommended at all levels when replanting an orchard with either a tolerant or a susceptible rootstock.
• At low levels in an established orchard, a pesticide application is not economical, but sampling at least once a year is recommended to see if the nematode numbers remain low.
• At medium levels, nematicide application may be advantageous if the site has a history of nematode damage. If the number of females exceeds the medium level, tree growth and fruit production are likely to be reduced.
•  At high levels, a nematicide application can prevent substantial reduction in fruit size and yield, but healthy, vigorous trees can often tolerate high nematode numbers without apparent damage. In both cases, successful treatment requires precise and repeated pesticide applications.

Sanitation is the most important and economical means to avoid plant-parasitic nematode problems and for ensuring long-term citrus productivity.

• Use certified nematode-free planting stock, citrus nematode-resistant rootstocks, and nematode-free planting sites.
• Prevent citrus nematode infestation by contaminated run-off or irrigation water.
• Rotate with annual crops for 1 to 3 years before replanting citrus to reduce the number of citrus nematodes.

Use a citrus nematode-resistant or citrus nematode-tolerant rootstock. Some rootstock hybrids are not only resistant or tolerant against the citrus nematode but also against Phytophthora spp. and Tristeza. In general, trifoliate orange and its hybrids have performed well against the citrus nematode and Phytophthora spp. in California.

If the site was previously infested with citrus nematodes, consider applying a preplant fumigation to reduce their numbers, even if a tolerant rootstock is used. Trees planted on fumigated orchard sites are generally known to have improved growth and yields compared to those on nonfumigated sites.

Postplant nematicides are expensive. Growers should weigh application costs, age, and condition of the orchard, as well as projected crop loss. In established orchards, a pesticide might be justified when sampling indicates medium to high population levels, as shown in the table above.