National Plant Board

Plant Quarantine, Nursery Inspection, and Certification Guidelines

Appendix L

Nursery Stock Certification: Its Role and Limitations1

Nursery stock is known to be attacked by a wide range of pests including disease causing organisms (mycoplasmids, mycoplasma like organisms, viruses, bacteria, fungi, etc.), root and foliar feeding nematodes, insects (mealybugs, whiteflies, scales, fruit flies, bugs, beetles, moths, etc.), and higher animals (slugs, snails, birds, etc.). When infected or infested nursery stock is moved from one place to another and planted, pests can easily become established in new areas. Consequently, the movement of pest infested nursery stock creates various levels of pest risk that must be managed.

Most states administer nursery inspection programs, the aim of which is to assure compliance with various legal requirements including pest cleanliness, grade and quality standards, and proper identification. When nursery stock has been inspected and found to be in compliance with established requirements, wholesale sellers ship it accompanied typically by an official nursery stock certificate authorized by the state department of agriculture of the state where the firm operates. What role does such nursery stock certification play in pest prevention? The answers to these questions can best be understood in connection with a brief review of 1) the elements of pest prevention, 2) pest risk analysis, and 3) pest management strategies.


The goal of pest prevention is to prevent the entry and permanent establishment of exotic pests which could cause serious harm to agriculture or the environment. The basic elements of pest prevention are pest exclusion, pest detection, pest eradication, pest analysis/identification and record-keeping, and public information and education. The goal of pest exclusion is to keep serious pests out of any area which has been identified for protection. Since there is no perfect pest exclusion system, pest detection activities (such as insect trapping and various kinds of surveys) must be conducted to discover incipient infestations while they are still small in population numbers and in the size of the area affected. The goal of eradication is to eliminate such incipient infestations as quickly as possible.

Pest analysis/identification and record-keeping are technical support functions for pest exclusion, pest detection, and pest eradication. Timely and accurate professional diagnostic and identification services are essential for effective decision-making and action. Public information and education also are essential functions. A well educated public is less likely to engage in smuggling activities or even to act in a way that results in the unintentional spread of serious pests. A well educated public also is more supportive of pest detection and pest eradication activities.

Pest prevention, for the purposes of this discussion is separate and apart from pest management or pest control which have as their goal the reduction of pest populations to levels which enable the economic production of a crop.

Pests which could cause serious harm to agriculture or the environment in a defined area are referred to as “quarantine pests.” Whether they are quarantine pests or not is determined by a process called “pest risk analysis.”


Pest risk analysis is a pest management prerequisite. It is absolutely essential as a means of determining whether a particular pest is a “quarantine pest” or not. The North American Plant Protection Organization (NAPPO, consisting of the U.S., Canada and Mexico) has defined “quarantine pest” as “a pest of potential economic importance to the area endangered thereby and not yet present there, or present but not widely distributed and being officially controlled.”

Of course, many factors are involved in determining whether a pest is of economic importance or not; but, even after a pest is identified as being economically important, the degree of importance is a function of how serious the economic impact would be. When introduced into areas where they do not already exist, highly destructive pests are of quarantine significance and they merit the exercise of pest risk management strategies which are strict enough to reduce the risk to an acceptable level; whereas, other pests may warrant no management, or only those management strategies which would protect or help to assure product quality or orderly marketing.

A pest risk analysis minimally consists of determining: if the pest could survive in the area of concern, what crops it attacks, whether susceptible crops are grown in the area or not, what the economic value and importance of susceptible crops in the area are, what pathways exist for the pest to move from infested to noninfested areas, whether there are effective controls and whether control is easy/difficult or inexpensive/costly, whether there are effective growing season or post-harvest commodity treatments that can be applied to assure that products imported will not carry the pests, and what the feasibility is of eradication should the pest become established.

If a pest is very destructive, there are no effective growing season or post-harvest commodity treatments that can be used to assure that shipments are acceptably free of the pest, there are uncontrollable pathways or there are no reliable ways to inspect shipments or to detect infestations and eradication would be infeasible, the appropriate management strategy might be to prohibit or severely restrict the entry of all hosts and potential carriers.

On the other hand, if there are effective treatments and inspection methods, few pathways, good detection tools and eradication is feasible, a quarantine could be promulgated to identify the origin area to which restrictions will apply, the commodities and articles regulated, what treatments are required and what official certification must accompany each shipment.


The fact is that quarantines are the principal tool for managing the pest risk associated with very destructive pests. When properly developed and strictly enforced, they are a very effective pest management strategy. However, quarantine tactics are seldom perfect and must be complemented by pest detection and eradication measures to assure that an area remains free of serious pests. They are effective when commodity treatments are available and applied to render shipments virtually free of target pests; the origin regulatory agency is competent and trustworthy in its issuance of certificates attesting to compliance with required treatments; when they are strictly enforced; and when exporters, importers, and the traveling public know, support, and comply with them.

In addition to commodity treatments, other pest management tactics for destructive pests include preclearance inspection, growing season crop protection methods which prevent or reduce infestation to an acceptable level of risk, plant registration and certification programs, and pest-free zones. Typically, the quarantine is the regulatory vehicle through which these strategies are employed and given the force of law. Commodity treatments, arrival inspection, preclearance inspection, growing season controls, plant registration and certification programs, and pest-free zones can be used alone or in various complementary combinations to achieve the desired goal of reducing pest risk to an acceptable level. When inspection is required as part of the overall strategy, the most valid and reliable inspection methods generally are imposed. When available, these methods include well-designed, statistically sound sampling protocols and sophisticated laboratory technology for the examination or testing of the samples that are collected.

On the other hand, the economic importance associated with pests which are not highly destructive, or which already exist in a defined area, generally are viewed in terms of product quality and other marketing considerations. That impact or risk is managed via quality standards which provide for various tolerances — either explicit or implicit (built-in).

Compliance generally is determined by visual inspection for the presence of the pest itself or for those symptoms which are highly indicative of its presence. The latter is particularly true of disease-causing agents, genetic disorders, etc. The target is the disease or disorder itself. When disease or other symptoms are identified, a sample might be collected and subjected to laboratory diagnostic procedures to confirm the presence of the causal agent; but, enforcement of any standard which has been imposed is accomplished by requiring that all diseased or affected fruits, plants or other such commodities be removed. The causal agent might still be present in or on other fruits or plants, but since they are not diseased, the standard is met. The commodity in question is eligible to be marketed, and shipments of it may be accompanied by a certificate which states that the product has met the standard.

Where does nursery stock certification fit? What is its role and what are its limitations?



Despite the fact that an official nursery stock certificate accompanying a shipment of plant materials at times has been accepted by regulatory officials as providing assurance of freedom from “quarantine pests,” the foregoing discussion helps to see why this is not likely to be valid. It is a fact that most states have established nursery inspection programs. Some have licensing and license fee requirements to identify those persons engaged in the business, the number and size of locations that will need to be inspected, and to generate the funding needed to support the program. However, nursery regulatory programs generally are established to accomplish one or another or a combination of these purposes:

1) To assure equitable and orderly marketing.

2) To establish a level of quality that enables nurserymen to compete in various markets.

3) To protect consumers.

Most states have imposed pest cleanliness standards, but the scope of pests covered varies based on the intended purpose of the nursery regulatory program that has been established. If the only purpose is to assure a certain level of quality for consumers, those surface pests which affect the appearance of the plant material are likely to be the primary target. This would include obviously diseased plants. If the consumer is a farmer, then (as in the case of the production and sale of fruit and nut trees, grapevines, and other plant materials for farm planting) nematode pests might be an important factor. In turf or sod for commercial and residential landscaping, weed pests are likely to be a main concern. However, (except in the case of nematodes in farm planting stock) the basic method of inspection is the visual examination of the plants. When nursery stock has been inspected and found to be in compliance with whatever standards the state has established, nurserymen may use officially approved and authorized nursery stock certificates to verify that compliance.

Nursery stock certificates also may be used by states to certify the results of visual inspections performed for private parties who want to ship noncommercial plant materials. These materials can include products purchased from local nurseries, products grown by the sender, and products collected from native habitats.

However, visual inspection of either commercial or noncommercial plant materials is very limiting as to the valid and reliable determination of pest freedom from disease causing agents. For this reason, when higher levels of assurance are needed for disease causing agents, nematodes, etc., states frequently establish special nursery stock registration and certification programs which target specified pests or disease causing agents. Seed potato certification is the oldest example of these kinds of programs. Plant materials which meet the requirements of these special programs often are accompanied by officially approved and authorized certification tags and/or seals as indicia of compliance.

The nursery inspection activities performed in connection with nursery inspection and plant registration and certification programs can provide a solid foundation for any special or additional inspection or testing requirements that might have to be met in connection with a quarantine governing the particular kind of nursery stock being grown and sold. Still, quarantines often present either a second or third set of requirements that must be met. When the required inspections or commodity treatments have been performed and quarantine requirements have been satisfied, a phytosanitary certificate indicating compliance is issued by an official of the state.

Thus, it is possible for a single shipment of nursery stock to bear three different forms of certification for compliance with pest requirements: 1) a nursery stock certificate to indicate compliance with the state’s basic standard of pest cleanliness, 2) a registration and certification certificate in the form of an official tag or seal indicating compliance with a requirement for specified pests or diseases, and 3) a phytosanitary certificate (certificate of quarantine compliance) attesting to inspection for and freedom from a particular “quarantine pest”; or to the application of a required commodity treatment. In this way it can be seen that each of these certificates plays a different role and serves a separate purpose.


As has already been shown, a nursery stock certificate does not have the same meaning as a phytosanitary or quarantine certificate, and that regulatory officials should not rely on a nursery stock certificate as an affirmation of compliance with quarantine requirements. The role of a state nursery stock certificate is to affirm that the nursery stock it accompanies has been inspected and that it meets established state standards; but, there are several factors which also limit the reliability of a nursery stock certificate as an indicator of general pest cleanliness.

First among the limiting factors is that each state has its own standard and the scope of the pests covered can vary considerably. Therefore, pests not covered by the standard could be present and the stock would still meet the requirements and be eligible for certification. To illustrate this point, consider California’s pest cleanliness requirements. Pests and diseases covered include all surface pests and diseases obviously affecting nursery stock. Soil borne pests generally are not covered; or those that are, are covered only for certain kinds of nursery stock. For example, field 40′ x 40′ grid sampling, mist extraction, and microscopic examination for and professional identification of plant parasitic nematodes is required for all farm planting stocks; but not for landscape ornamentals and other kinds of nursery stock. When sold bare-root, the requirement for the latter is visual freedom from any evidence of nematode infestation such as root-knot nematode galls and lesions caused by migratory endoparasitic and other plant parasitic nematodes.

The second most critical limitation is the fact that determining compliance with the pest cleanliness is based on visual inspection methods. Low levels of pests may be present and not detected. Also, soil inhabiting, nocturnal or microscopic pests and disease causing agents are not likely to be detected. Inspector training, experience and motivation (collectively) are a third factor that is different from, but closely related to, visual inspection. Untrained, inadequately trained, or inexperienced inspectors might not know pest/host relationships or where to look. Unmotivated inspectors might not put forth the effort that is required to do an adequate job of inspection.

A fourth factor is related to the degree to which nursery stock from outside sources is controlled. One nursery might buy stock from another nursery which has not met the same standard and then resell and ship it under its nursery stock certificate without ever bringing it into compliance with the state standard. Or, a nursery stock dealer might act as a sales agent, commission merchant or broker for plant materials without ever determining the quality or pest status of the stock being sold.

A fifth limitation arises from the fact that most state nursery stock regulatory programs rely upon annual or twice annual inspections as the basis for certification. While inspection personnel might be on the premises more frequently, total nursery inspection at a frequency greater than twice per year would be economically infeasible. Nevertheless, pest populations can fluctuate considerably over a two to three month timeframe and a pest outbreak between inspections could result in affected stock being shipped that would not meet the established standard.

The foregoing limitations clearly affect the validity and reliability of the nursery stock certificate but they also raise nonuniformity issues of concern to plant pest and disease regulatory officials. An Ad Hoc committee of the Central Plant Board examined the nonuniformity problem and identified the application and inspection process, nursery stock brokerage, transportation, reporting, communication, and state laws as areas of variability/nonuniformity. It identified several needs for improvement. Similar variability exists among the 13 states that make up the Western Plant Board. However, it should be noted that considerable uniformity does exist among those states that are major nursery stock producers and exporters.


The big question is how to achieve a greater degree of uniformity among the nursery inspection programs within the United States. Nursery inspection is not regulated by the federal government; and most states would agree that it should not be. There is no regulatory mechanism for requiring states to adopt the same set of requirements. Since there are major costs associated with administering such programs and since states are significantly concerned with satisfying intrastate needs, there is a considerable basis for arguing that no set system ought to be imposed.

However, the National Plant Board could work with its regional plant boards to develop a model nursery inspection program for adoption by the Board itself or for promulgation by the USDA as a federal, voluntary program with a special certificate that would be used by participating (complying) states. The federal model approach would appear to be the most effective approach, but National Plant Board (NPB) adoption also could be a preliminary or intermediate step to developing state government and industry consensus; and subsequent support for a federal, voluntary program.

If a federal, voluntary program were to be developed it might even be used without adoption by an individual state for its entire industry. Individual nursery owners could conceivably apply to their state nursery regulatory agency for inspection and certification based on the federal regulation in addition to or in place of an existing state program. The state agency would then perform its inspections and make its compliance determinations based on the federal requirements. The nursery would then be eligible to ship nursery stock using the official federal nursery stock certificate. Buyers of nursery stock also could include the federal standards in their purchase orders and require that shipments be accompanied by the federal nursery stock certificate.

Assuming that a large number of states adopted it, the national model approach would help to achieve a far greater degree of program uniformity and therefore improve the validity and reliability of nursery stock certification. Consequently, it seems entirely appropriate that the NPB appoint a committee to develop a model nursery inspection program. However, the goal should not be to make nursery stock certification equal to phytosanitary certification. Inspections to determine compliance with nursery stock pest cleanliness standards and for quarantine pests involve two different levels of pest risk management. Nursery inspection programs should facilitate the orderly marketing of nursery stock by assuring that it is of consistent quality and meets an acceptable standard for freedom of pests that are not of quarantine significance. Nursery inspection programs can and should serve as a platform or foundation for the additional inspections and treatments that might be required for phytosanitary certification, but they are not stand alone vehicles for quarantine compliance.

1Western Plant Board Nursery Stock Certificate Evaluation Subcommittee Report, July 15, 1992. Subcommittee members: Bill Brookreson (Washington), Bill Callison (Chairman, California), Kingsley Chong (British Columbia, Canada), and Bill Wright (Oregon).