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A National Standard

Fruit tree production can be severely impacted by the introduction and spread of harmful viruses and virus-like pathogens that reduce plant vigor and yield, product quality, and in some cases, lead to death of the afflicted plants. Control of these pathogens is limited, for there is no cure, only preventative measures to reduce their introduction and spread, and elimination of the source of inoculum in the environment.


The three participating National Clean Plant Network Fruit Tree centers perform the first role, helping prevent the introduction of harmful and economically important viruses into the U.S. fruit tree industry. We do this by carrying out a rigorous testing regime to detect targeted pathogens in fruit tree cultivars submitted to the centers by private, state, and federal stone and pome fruit breeding programs from across the country, and around the world. If pathogens are detected, virus elimination is performed, and the plants tested, and retested, to ensure that the pathogen is gone.


The three centers have harmonized their testing programs so that stakeholders across the country can rest assured that whichever center they obtain propagative material from, it has been tested for the same pathogens. The testing approach for producing stonefruit (Prunus sp.), is shown below:

PCR and/or HTS
Biological Indexing
Apple chlorotic leaf spot virus
Screening for viruses and virus-like pathogens of economic importance
PCR panel and/or HTS as per initial diagnostics.
Prunus avium ‘Bing’
Apple mosaic virus
P. persica ‘GF305’
Arabis mosaic virus
Chenopodium sp.
Candidatus Phytoplasma sp.
Nicotiana sp.
Cherry green ring mottle virus
Cucumis sativus
Cherry leaf roll virus
Cherry mottle leaf virus
Cherry necrotic rusty mottle virus
Cherry rasp leaf virus
Cherry rusty mottle virus
Cherry twisted leaf-associated virus
Cherry virus A
Hop stunt viroid
Little cherry virus-1
Little cherry virus-2
Nectarine stem pitting-associated virus
Nectarine virus M
Peach latent mosaic viroid
Peach mosaic virus
Plum bark necrosis stem pitting-associated virus
Plum pox virus
Prune dwarf virus
Prunus necrotic ringspot virus
Tobacco ringspot virus
Tomato ringspot virus
Xylella fastidiosa

Initial Diagnostics

Final Diagnostics

But the story doesn’t end there. Once a G1 mother plant has been produced by one of the centers it is entered into their foundation so that propagative material can be distributed to stakeholders. To ensure that each plant continues to meet state certification standards, and to maintain stakeholder confidence, the three participating centers retest each plant for a series of harmful, vector-transmitted or pollen transmitted pathogens on a three year cycle (table to right).

Each center performs regular risk assessments, taking advantage of advances in the scientific community’s understanding of pathogen biology, and may proactively test for more pathogens, or more frequently, should they see a need. The three participating centers are at the forefront of protecting the U.S. fruit tree industry from the introduction of harmful pathogens, and stakeholders should be confident in this harmonized, national standard.

Prunus Retention Testing Protocol

  • Candidatus Phytoplasma sp.

  • Cherry leaf roll virus

  • Little cherry virus-2

  • Plum pox virus

  • Prune dwarf virus

  • Prunus necrotic ringspot virus

  • Xylella fastidiosa

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