University of FloridaSolutions for Your Life

Download PDF
Publication #ENY-606

2016 Florida Citrus Pest Management Guide: Ch. 14 Nematodes1

L.W. Duncan, J.W. Noling, and R.N. Inserra2

Integrated pest management (IPM) for nematodes requires: 1) determining whether pathogenic nematodes are present within the grove; 2) determining whether population densities of some nematodes are high enough to cause economic loss; and 3) selecting a profitable management option. Attempting to manage nematodes may be unprofitable unless all of the above procedures are carefully followed. Similarly, some management methods pose risk to people and the environment. Therefore it is important to know that their use is justified by the actual conditions in a grove.

Nematode Pests

Although many different species of nematode have been found in association with citrus roots, relatively few have been documented to be economically important. The nematode species of major economic importance in Florida include the citrus nematode (Tylenchulus semipenetrans), causal agent of "slow decline" of citrus, and the burrowing nematode (Radopholus similis), causal agent of "spreading decline" of citrus. Other species of limited economic importance because they are more localized include the sting nematode (Belonolaimus longicaudatus) and two species of lesion nematode (Pratylenchus coffeae and P. brachyurus). The incidence and abundance of dagger nematodes (Xiphinema vulgare and Xiphinema americanum group) in citrus groves appears to be increasing. The ecology and economic importance of these dagger nematodes in citrus are the subjects of ongoing research.

Typical Symptoms. Most nematode species that are known pathogens of citrus do not actually kill the tree but can significantly reduce tree vigor, growth, and grove productivity. Nematode-infested trees generally grow more slowly and may ultimately be of smaller size and quality. Aboveground symptoms which develop as a result of damage to roots include thinner canopies with less new foliar growth, and twig dieback within the upper tree canopy. Symptoms of decline frequently increase with time, and are more apparent during periods of environmental stress (i.e., drought or freezing temperature) or when combined with other damaging soil pests (i.e., root weevils, Phytophthora).

Monitoring Nematodes. The distribution and abundance of nematodes in soil prior to or after planting will affect the severity of the problem and define the need for nematode management. The only effective way of determining the presence or distribution of nematodes within a grove is by soil and root tissue sampling of undercanopy areas of individual trees. A representative grove sample for most nematode species consists of soil and roots (using a shovel or soil sampling tube) collected from the undercanopy areas of 20-30 trees within a 5-acre block. When sampling for burrowing nematodes, it is necessary to collect a large quantity of fibrous roots (loosely fill a 1-gallon freezer bag) from the surface 0-12 inches beneath 10-12 declining trees. Immediately after collecting the sample, thoroughly rinse soil from the roots and from inside the bag and replace the roots in the freezer bag. Once soil and root samples have been collected, they should never be subjected to overheating, freezing, drying, or to prolonged periods of exposure to direct sunlight. Samples should be submitted immediately to a commercial laboratory or to the University of Florida Nematode Assay Laboratory for analysis and recommendations.

Managing Nematodes

Nematode management should be considered only after the results of soil and root sampling are available. The agency or company that processed the samples should be able to indicate whether potential nematode problems exist within a grove. In most cases, nematode management should not be considered until all other potential causes of tree decline are evaluated and corrected. For more detailed information on treatment decisions and methods of nematode management in citrus, consult the Florida Citrus Rootstock Selection Guide SP 248, Best management Practices for Soil-Applied Agricultural Chemicals in this Guide, or counte cooperative Extension personnel.

Sanitation. Once established, nematodes cannot be eradicated from groves, so the best method to manage plant-parasitic nematodes in new plantings is to exclude them from a grove by using only trees from nurseries certified by FDACS Division of Plant Industry to be nematode-free. Use of certified trees will virtually eliminate the possibility of nematode problems in new groves planted in virgin soils or in old citrus soils never infested by nematodes, provided that care is taken to always use clean equipment in those groves. Use of certified trees also reduces damage during the early years of growth in old, previously infested groves if soil nematode populations are low. High soil nematode densities hinder the beneficial effects of the use of certified trees. Sanitation of equipment to remove soil and root debris before moving between groves is an effective means of preventing the spread of nematodes.

Cultural Practices. Proper grove management is critical to mitigate damage caused by plant parasitic nematodes. There is no value to managing nematodes if other problems (poor soil drainage, insufficient irrigation, foot rot and fibrous root rot, root weevils, improper fertilization, poor disease control) limit root function and/or reduce tree quality. In the case of burrowing nematodes, specific cultural practices (avoidance of disking, frequent irrigation, and fertigation) are critical to maintain a vigorous root system in the shallow soil horizons where the nematode is much less active.

Rootstock Resistance. Resistant rootstocks are also available to manage citrus and burrowing nematodes. Swingle citrumelo is a widely planted rootstock with resistance to citrus nematode. Milam lemon, Ridge Pineapple, and Kuharski Carrizo citrange are all resistant to burrowing nematode. The existence of races of these nematodes capable of breaking resistance compromises their value somewhat; nevertheless, large numbers of groves are currently growing well on resistant rootstocks in the presence of these nematodes.

Chemical Control. Environmental concerns and deregistration of numerous pesticides have dramatically reduced the availability of chemical products for nematode management. Currently, there are no soil fumigants recommended for preplant nematode control. Postplant nematicides can provide temporary suppression of nematodes in the shallow part of the root zone. Because of Florida's uniquely porous soils, soil-applied pesticides have the potential to contaminate groundwater. Consequently, their use should be restricted to the late autumn and early spring when rainfall is least in Florida and these materials should not be applied near irrigation or drinking water wells, or where the water table is close to the soil surface. Irrigation systems should always be inspected prior to pesticide application to soil to prevent over-application of pesticide or water due to line breaks, faulty line-end pressure valves, or missing emitters. Additional considerations for the application of fumigants and nematicides to soil are outlined below.

Tree response to postplant chemical treatment often requires a period of one to two years of repeated treatment for growth improvement and significant yield returns. Response to preplant fumigants in newly planted young trees may be particularly slow, since nematode population increase may be delayed until canopy closure of adjacent trees occurs. Note that to protect groundwater, preplant fumigants can only be used in areas with an underlying impermeable layer within 6 feet of the soil surface capable of supporting seep irrigation. Since nematicides are not eradicants, repeated treatments are required to periodically suppress nematode repopulation of soil and roots to maintain high grove productivity. Preplant nematode management programs (sampling, selection of appropriate rootstocks, use of certified trees) are therefore important considerations for maximizing young tree growth and eventual long-term productivity, since it may not be possible to assure satisfactory tree growth with postplant chemical management programs alone. However, if nematode problems do arise on young trees, early management of the populations can have a prolonged beneficial effect on subsequent growth and productivity of the trees. Nematode control with postplant, nonfumigant nematicides occurs primarily within the zone of application and to a much lesser degree, due to the systemic activity of these pesticides, within and around roots outside of the zone of application. Since a large majority of fibrous roots grow within the surface 24-30 inches of soil and decrease in abundance from the tree trunk to the row middle, proper nematicide placement to maximize undercanopy coverage is of critical importance. Nematicide placement under the tree canopy can significantly improve overall nematode control by targeting applications to areas of highest fibrous root and nematode density. Treatments will be most effective if made when soil temperatures are warm enough for nematode development and uptake by the tree. Natural degradation of nematicides moving downward in soil also increases with increasing soil temperature, thereby reducing the likelihood of groundwater contamination. To confirm the value of treatment programs, it is wise to designate areas of grove that will remain untreated in order to evaluate product performance and tree growth response.

A lack or loss of nematicidal efficacy and citrus yield response can be associated with factors other than improper pesticide application rate, placement, and application timing. The repeated use of nematicides often results in diminished efficacy in successive years due to accelerated microbial degradation. This process is caused when populations of microorganisms capable of metabolizing these products increase in soil following use of the compound. The degradation process can be initiated after a single treatment. Most postplant nematicides do not necessarily kill nematodes upon direct contact: efficacy usually requires long, continuous exposure to sublethal, yet toxic, concentrations in soil. Nematode population reduction results from a disruption of normal nematode behavior necessary to complete the life cycle. Disappearance rates of nematicides in soil (due to leaching and/or microbial degradation) are therefore critical determinants of treatment efficacy.

Pesticide leaching to depths below the primary root zone can occur as a direct result of excessive irrigation or rainfall. Given the sandy, permeable nature of citrus soils and generally low soil organic matter content, irrigation schedules based on soil moisture deficits are likely to improve nematode control and grove response to treatment by maximizing retention of toxic concentrations within the citrus tree root zone and prevent problems of environmental contamination. Undercanopy weed growth may reduce nematicide effectiveness by interception or absorption of pesticide residues targeted for citrus roots or nematodes in soil. Undercanopy weeds also interfere with microsprinkler operation and can prevent uniform coverage of chemigated nematicides.

Footnotes

1.

This document is ENY-606, one of a series of the Entomology and Nematology Department, UF/IFAS Extension. Original publication date December 1995. Revised September 2013 and April 2016. This publication is included in SP-43, 2016 Florida Citrus Pest Management Guide. Visit the EDIS website at http://edis.ifas.ufl.edu. For a copy of this handbook, request information on its purchase at your county Extension office.

2.

L.W. Duncan, professor, and J.W. Noling, professor, Entomology and Nematology Department, Citrus REC, Lake Alfred, FL; R.N. Inserra, regulatory nematologist, FDACS, Division of Plant Industry, Gainesville, FL; UF/IFAS Extension, Gainesville, FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.