How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Plant parasitic nematodes are microscopic, unsegmented roundworms that live in soil and plant tissues and feed on plant roots. The predominant species parasitic on citrus in California is the citrus nematode. This nematode is reported to be present in most citrus orchards and in all soil types. It also parasitizes grape, lilac, olive, and persimmon. Citrus nematode attacks roots by burrowing its anterior end deep inside the root cortex while the posterior end remains outside in the soil.

The sheath nematode is less widespread than the citrus nematode; it has been found on citrus in the Coachella Valley and on some native desert plants. However, it has a broad host range and thrives well at high temperatures and at low moisture levels.

DAMAGE

Damage caused by a citrus nematode infestation depends on the age and vigor of the tree, density of the nematode population, and susceptibility of the rootstock. Mature trees can tolerate a considerable number of these nematodes before showing lack of vigor and decline symptoms.

Susceptible trees planted in lightly infested soil may grow for many years without apparent damage and then decline slowly. Resistant rootstocks generally do well even in heavily infested soils. If, however, a heavily infested orchard site is replanted with a susceptible rootstock without soil fumigation, the roots of the young trees will soon be heavily parasitized, tree growth will be stunted, and fruit production reduced. This condition is also referred to as the citrus replant problem. The damage is greater when trees are predisposed by other factors such as Phytophthora root rot and water stress.

Sheath nematodes feed on root tips and may reduce root growth and vigor of trees.

SYMPTOMS

The symptoms described below are typical of a nematode problem but are not diagnostic, because they could result from other causes as well. Aboveground symptoms of nematode damage are lack of vigor, twig dieback, decline in growth, and reduced fruit size and yield. Nematode infestations may occur without inducing any aboveground symptoms. Belowground symptoms of a citrus nematode infestation include poor growth of feeder roots and soil adhering to roots giving them a dirty appearance. Sheath nematode causes swelling (galling) of root tips.

FIELD EVALUATION

To make management decisions, it is essential to know the nematode species present and their population estimates. If a previous orchard or crop had problems caused by nematodes that are also listed as pests of citrus, population levels may be high enough to cause damage to the ensuing citrus crop. If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification.

Monitoring
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 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 an established orchard irrigated by sprinklers or furrows, collect soil and root samples at the drip line of trees that show symptoms and samples from adjacent, healthy looking trees for comparison. In drip‑irrigated orchards, take samples around emitters where feeder roots are most abundant. The soil should not be too dry or too wet.

You can sample fallow land at any time of year. The best time to sample an established orchard is March through April, so that measures can be taken, if necessary, to protect the spring growth flush of the roots. 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 60 cm, a Viehmeyer tube is recommended to reduce the soil volume taken. The tube can easily be hammered down to 48 inches; however, the amount of roots collected will be much smaller than with a soil auger.

From each sampling block, collect 10 to 20 cores or subsamples. 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. Attach labels providing name and address, location of the orchard, sample block, soil texture, cropping history, and notable symptoms and, if possible, rootstock and soil and air temperature; this information is critical for a meaningful analysis. Send or deliver the samples to the lab as soon as possible. Ship them in a cardboard box insulated with newspaper, or in a styrofoam ice chest. If any delay occurs, keep the samples in a cool place (41° to 50°F).

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

Interpreting Soil Analysis
Although it will vary greatly with soil moisture, soil type, and temperature, the number of nematodes in the soil, as determined by soil analysis, can give some indication about 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 juveniles and females at different sampling times; different soil types are not taken into account. The table gives a rough estimate of low, medium, and high populations. A preplant treatment 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 treatment is not economical, but you should continue sampling at least once a year to see if the population remains low. At medium levels, treatment may be advantageous if the site has a history of nematode damage. At high levels, a treatment can prevent substantial reduction in fruit size and yield, but healthy, vigorous trees can often tolerate high populations without apparent damage. In both cases successful treatment requires precise and repeated applications. Available postplant nematicides are expensive; you have to weigh treatment costs and age and condition of the orchard as well as projected crop loss.

Rating of Population Levels of the Citrus Nematode Juveniles and Females as Determined by Soil Analysis¹

The number of females per unit of feeder roots is more representative of the damage potential to the tree than the number of free juveniles in the soil. If the population of females exceeds the medium level, tree growth and fruit production are likely to be reduced.

MANAGEMENT

Cultural. Good sanitation practices are essential to avoid nematode infestations. Use certified nematode-free material for planting. Rotation with annual crops for 1 to 3 years before replanting citrus helps to reduce citrus nematode populations.

Rootstock selection. Using a resistant rootstock is recommended whether or not nematodes are present. Trifoliate orange is known to be tolerant to citrus nematode. Troyer citrange is also resistant to citrus nematode, but this nematode has resistance-breaking biotypes that may develop on this rootstock after a period of time, thus increasing its susceptibility. Sweet orange, Trifoliate orange, grapefruit, Thompson seedless grape, and cotton are reported to be resistant to sheath nematode, making its management relatively easy.

Chemical. If the site was previously infested with nematode pests of citrus, preplant fumigation may be necessary to reduce nematode population levels. When replanting a citrus orchard, a preplant treatment is recommended even if a resistant rootstock is used. Trees planted on fumigated orchard sites are generally known to have improved growth and yields compared to those on nonfumigated sites.

In established orchards, treat when sampling indicates more than 400 female citrus nematodes are present in 1 gram of roots in February to April or more than 700 in 1 gram of roots during May and June.

PUBLICATION

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Nematodes
J. O. Becker, Nematology, UC Riverside
B. B. Westerdahl, Nematology, UC Davis
Acknowledgments for contributions to the Nematodes:
U. C. Kodira, Plant Pathology, UC Davis
P. B. Goodell, UC IPM Program, Kearney Agricultural Center

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