What is a Clean Plant?

A clean plant, also known as ‘foundation-level’ or ‘G1’ is a plant line, variety, or cultivar that has been a) tested for, and found free of, economically important and/or harmful plant viruses and virus-like organisms, and b) been maintained under controlled conditions to prevent reinfection.

It is important to note that a clean plant is not guaranteed to be free of all viruses or virus-like organisms, nor bacteria, fungi or invertebrate pests. Clean plants are also not guaranteed to meet quarantine and phytosanitary requirements in other countries; this is a U.S.-based program, generating plants for U.S. phytosanitary and certification requirements.

In the U.S., clean plants are produced and maintained by centers, such as the Clean Plant Center Northwest (CPCNW), supported by the National Clean Plant Network (NCPN), a federally-funded program managed by the United States Department of Agriculture. The NCPN centers distribute propagative material from their foundation collections to nurseries and growers throughout the U.S., and are the basis of state-managed nursery certification programs, such as in Washington, or California.

How is a clean plant made?

A clean plant is the product of a long and methodical process, designed to ensure that harmful and economically important pathogens are identified and removed before a plant is distributed to growers and nurseries throughout the U.S. An outline of the process used at the CPCNW is detailed below:

  1. Receipt and propagation of source material – The CPCNW receives source material from the variety owner, and either propagates it onto virus-tested rootstock or via rooted cuttings and/or tissue culture.
  2. Initial diagnostics – Once propagated and actively growing, tissue samples are collected and tested for harmful or economically important pathogens using of a combination of polymerase chain reaction (PCR), high-throughput sequencing (HTS) and biological indexing assays. If a plant is found to be positive by any of these methods, it is considered infected.
  3. Virus elimination – If a plant is infected with an economically important or harmful pathogen, it enters the virus elimination process by which pathogens are removed using a combination of temperature-based therapies, targeted tissue culture and micropropagation.
  4. Follow-up diagnostics – To confirm that virus elimination was successful, plants generated during the virus elimination process are tested for pathogens identified during the initial diagnostic step.
  5. Final diagnostics – Plants that passed all the previous steps are tested again against a species-specific list of harmful or economically important pathogens by PCR. This is to ensure no pathogens were missed, for the protection of U.S. agriculture.
  6. Foundation and distribution – Once a clean plant has been produced it can either be returned to the owner, or can be placed into the foundation collection, in which plants are maintained under contained conditions to prevent reinfection, and regularly re-tested to ensure that they remain clean. 

How long does it take?

The time needed to generate a clean plant can vary greatly depending on the following factors:

  1. Plant species and/or cultivar – While hops grow rapidly when propagated, grapevines, pome and stone fruit take a lot longer. Also, the diagnostic process may be longer for some species, such as pome fruit, due to the biological indexing approaches required.
  2. What pathogens are detected – A plant that is pathogen-free based on initial diagnostics will proceed much more rapidly than an infected one. Also, should a new or quarantine-significant pathogen be detected, state and federal regulatory agencies may impose restrictions on the use or movement of the plant until a risk assessment has been performed.
  3. Amenability to virus elimination – Some cultivars are sensitive to the virus elimination or therapeutic approaches used, requiring experimentation to determine what it can tolerate whilst eliminating the viruses present. Furthermore, some pathogens are recalcitrant and difficult to remove via standard therapeutics and require multiple approaches to be used.

Based on the above, and the CPCNW’s standard operating procedures, the estimated minimum time required for a variety to pass the process is as follows:

Type Uninfected Infected
Hop (Humulus spp.) 18 Months 2-3 Years
Grapevine (Vitis spp.) 18 Months 2-3 Years
Pome (Malus & Pyrus spp.) 3 Years 4-5 Years
Stone (Prunus spp.) 2 Years 3-4 Years

How do I submit plants for cleanup?

Instructions about the CPCNW’s hop, grapevine, pome and stone fruit programs are detailed on the following pages:

Submitting new grapevine varieties

Submitting new hop varieties

Submitting new pome and stone fruit varieties

Where can I get clean plants?

The National Clean Plant Network Centers hold foundation collections of clean, virus-tested, G1-level propagative material for distribution, in limited quantities, to growers and nurseries throughout the U.S. Links to collections that hold material for specialty crops that the CPCNW processes are as follows:

Specialty Crop Available From:
Grapevine Clean Plant Center Northwest (WSU)Foundation Plant Services (UC Davis), & Missouri State University
Hops Clean Plant Center Northwest (WSU)
Pome and Stone Fruit Clean Plant Center Northwest (WSU), & Foundation Plant Services (UC Davis)