Researchers are calling for the exclusion of over 120 "phantom agents" from regulatory lists to optimize phytosanitary control.

Phantom agents are assumed pathogens that were described in scientific literature as early as the early 1900s without any real evidence of their existence, according to Ioannis Tzanetakis, a professor of plant virology at the Arkansas Agricultural Experiment Station and director of the Arkansas Clean Plant Center. The experimental station is a research unit of the System Division of Agriculture at the University of Arkansas.

The Arkansas Clean Plant Center leads the efforts of a group of 185 agronomist scientists from over 40 countries who conduct pathogen testing in plants. They call for elimination of over 120 phantom agents from regulatory lists as they are outdated and hinder access to pathogen-free plant materials. Clean plants are crucial for sustainable production of agricultural crops.

According to Tzanetakis, India is the second largest producer of fruits and vegetables globally, but a lack of disease-free planting material limits potential yield.

Most of these phantom agents were described before the advent of modern molecular methods, and there are no available samples or genome sequences for their study. Despite lacking evidence of their existence, presumed pathogens have been included in international rules governing the transportation of plant materials.

Tzanetakis explains that this results in a convoluted mix of real and fictional agents in regulatory lists which the shipper must exclude before sending plants from one country to another.

“We tried to clean up the list of regulated pathogens to optimize phytosanitary controls,” Tzanetakis says. “What we call phantom agents are names where there is absolutely no knowledge, and there are no places on the planet where you can go, collect that plant, and say it is infected with agent X.”

In a recent article published in the journal Plant Disease, Tzanetakis and numerous co-authors identify phantom agents in eight crops that still appear on regulated pathogen lists, despite there being no method for precise testing for them.

The work is titled “Optimizing the Global Exchange of Plant Germplasm: Integrating Scientific Rigor and Common Sense to Remove Phantom Agents from Regulatory Oversight.”

“With today’s technologies, if an indicator plant shows symptoms, it will be subjected to testing using high-throughput sequencing, also known as HTS,” explains Tzanetakis. “If this process identifies a new agent, it is unlikely to be labeled a phantom. Instead, it will be recognized as a new host pathogen. As a result, phantom agents typically persist indefinitely.”

High-throughput sequencing is a scientific method that allows researchers to quickly sequence DNA from a large number of samples and/or organisms simultaneously.

The Arkansas Clean Plant Center (ACPC) is a state-of-the-art berry center in the National Clean Plant Network. The network, also known as NCPN, was established to safeguard specific U.S. crops from the spread of economically harmful pests and plant diseases. The U.S. Department of Agriculture funds the NCPN network, which includes scientists, educators, state and federal regulators, nurseries, and producers working together to ensure the cleanliness and availability of plant propagation material.

Facilities such as the Arkansas Clean Plant Center conduct testing to detect and verify the presence of phytopathogens similar to those listed in regulatory schedules. ACPC also provides cleaning services to ensure plant materials have the best possible quality before being delivered to nurseries, breeding companies, and farmers.

Tzanetakis added that cleaning plant materials may be responsible for removing some pathogens from the list of phantom agents. Presumed pathogens may also be caused by one or more viruses now known by a different name, or possibly even eliminated due to resistance in modern varieties.

For example, the list of phantom agents includes the “Strawberry Vein Banding Virus,” which was described as a disease in Hungary in the 1960s on an old variety based on symptom appearance, with symptoms based on a single photograph provided in the publication.

“Given the limited information presented in the single report, further study of the agent is impossible,” Tzanetakis and his co-authors noted.

The ACPC laboratory is one of two in the National Clean Plant Network with its own HTS capabilities, optimizing testing and cleaning processes for breeding lines and enhancing quality control in pathogen testing.

Tzanetakis states that the goal is to improve agricultural crop production and provide farmers with access to high-quality, disease-free plants without unnecessary barriers.

“These rules are in place even though we have many more advanced disease-testing tools,” Tzanetakis said. He added that NCPN network laboratories, such as the Arkansas Clean Plant Center, are designed to test and eliminate viruses from plants.

Once plants are clean, meaning free of systemic pathogens like viruses, ACPC preserves Generation 1 (G1) materials to provide “the highest level of protection against reinfection with systemic pathogens,” Tzanetakis explained. “This ensures the long-term preservation of G1 materials, giving breeders and stakeholders confidence in the quality and integrity of their advanced breeding efforts,” Tzanetakis stated.

Tzanetakis noted that alongside virus diagnostic tools based on HTS and robotics for nucleic acid extraction, the center also operates a team of experts whose collective experience contributes to efficient work in virus elimination, providing solutions for plant strains that are difficult to propagate in vitro, while allowing for adaptation of new virus elimination protocols.

Source: University of Arkansas. Author: John Lovett.

Photo caption: Blackberry leaf being examined at the Arkansas Clean Plant Center. Photo: U of A System Division of Agriculture.