Pale Cyst Nematode Program: 

Frequently Asked Questions 

Pale cyst nematode (PCN) is a microscopic, soilborne 

pest of potatoes and other plants in the nightshade 

family, including tomatoes, eggplants, and some weeds. 

Although PCN does not infect potato tubers, large 

numbers of nematodes can cause wilting, stunted 

growth, poor root development, and early plant death. 

PCN is not harmful to people or animals. 

PCN has been detected in a small area spanning parts 

of Bingham and Bonneville Counties in southeast Idaho. 

Although no one knows how PCN came to Idaho or 

from where, scientists agree that the pest was 

unintentionally introduced in the area decades ago, 

well before the activities of the current growers.

The U.S. Department of Agriculture’s (USDA) Animal 

and Plant Health Inspection Service (APHIS) and the 

Idaho State Department of Agriculture (ISDA) are 

working together to eradicate PCN, protect Idaho’s 

potato industry, and maintain markets for Idaho 

potatoes. Together, we regulate infested and 

associated fields (cultivated plots that may have been 

exposed to PCN-infested soil transferred through 

shared farming equipment and other regulated articles) 

and restrict movement of potatoes and other regulated 

items from quarantined areas to prevent this pest’s 

spread. These actions are necessary to safeguard 

potato production areas outside of PCN-regulated areas 

and maintain domestic and international markets for 

U.S. potatoes.

Where and how large is the current PCN-regulated area?

The PCN-regulated area spans portions of Bingham and 

Bonneville counties. It represents less than 1 percent of 

Idaho’s total potato-producing acreage. Maps showing 

the current regulated area are available online at 

www.aphis.usda.gov/planthealth/pcn. 

Are all regulated fields infested with PCN? 

APHIS and ISDA regulate infested fields, where PCN has 

been confirmed, and associated fields, where PCN has not 

been confirmed but that may have been exposed to PCNinfested soil in the past. Exposure typically occurs when 

farming equipment is moved between fields, resulting in 

soil transfer from one field to another.

Why don’t APHIS and ISDA publicize the locations of 

infested fields? 

We do not share the exact locations of infested fields to 

protect the privacy and safety of individuals who own or 

farm those fields. Maps of all regulated fields (both 

infested and associated) are publicly available, but they 

don’t distinguish infested fields from associated fields. 

Are entire farming operations regulated? 

APHIS and ISDA regulate known infested and associated 

(potentially PCN-exposed) fields on a field-by-field basis. 

We do not regulate entire farming operations.

What does the PCN Final Rule, published in 

December 2020, do?

The 2020 rule requires APHIS to publish a notice in the 

Federal Register when considering any changes to PCN 

program protocols to inform the public of the proposed 

changes and solicit comments. The rule also requires APHIS 

to publish a final notice and inform the public of changes 

made and the reasons behind them. APHIS published this 

rule in response to a court order requiring us to provide a 

means for public input on the protocols used to regulate and 

deregulate fields for PCN. Current program protocols are 

available at www.aphis.usda.gov/planthealth/pcn. 

Does the 2020 PCN rule make any operational 

changes to the program?

The 2020 rule does not change any of the protocols for 

regulating and deregulating PCN-infested and associated

fields, nor does it change the interstate movement 
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requirements for regulated articles or field sampling 

protocols. Also, the final rule does not impact ISDA’s PCN rule 

(https://adminrules.idaho.gov/rules/current/02/020610.pdf), 

which mirrors APHIS’ intrastate movement requirements for 

regulated articles. 

Is PCN spreading? 

There is no evidence that PCN is spreading from infested 

fields since APHIS and ISDA put movement restrictions and 

strict sanitation requirements in place. We also conduct 

extensive outreach to the public, farm service providers, 

utility companies, and others to ensure that anyone who 

may need to enter an infested or associated field follows 

appropriate sanitation procedures to prevent soil movement.

Why are infested fields still being found if PCN is 

not spreading? 

PCN cysts can contain hundreds of viable nematode eggs 

and remain dormantin soil for up to 30 years without a 

susceptible host. When a host crop like potato is planted, 

nematode eggs hatch, the nematodes enter the root, and 

then they reproduce on the growing potato plant. With 

every planting of a potato crop, the PCN population can 

increase, becoming large enough to be detected with soil 

analysis. Depending on a field’s crop rotation, PCN population 

levels may take several years to reach detectable levels. 

Can PCN be spread by the wind or migrating geese? 

Although this cannot be ruled out, APHIS and ISDA field 

survey data do not indicate that wind or migrating geese 

contribute to PCN spread. PCN is spread primarily through 

the movement of infested soil by farming, construction, 

and other equipment and on seed potatoes and other 

regulated crops. 

What has the program achieved since PCN was first 

detected in Idaho in 2006?

When PCN was first detected, several trading partners closed 

their markets to Idaho and U.S. potatoes. Through the PCN 

program’s work to delimit and control the infestation in Idaho, 

APHIS was able to restore all lost markets by 2017. Since then, 

the program has helped maintain market access for Idaho and 

914,000 total acres of U.S. potatoes valued at $3.9 billion in 

2020. Additionally, the program has successfully contained 

PCN to an area with an 8.5-mile radius in southeast Idaho, 

preventing its spread to other potato-producing areas in the 

United States.

Have any PCN-infested fields been released from 

regulation?

No infested fields have been released from regulation yet. 

All infested fields are undergoing a three-step testing 

process to ensure the absence of viable PCN cysts and 

ultimately qualify for deregulation. Twenty of the 32 

known infested fields have progressed to the third testing 

step, which includes an in-field bioassay that requires 

growing three crops of PCN-susceptible potato varieties 

and testing negative for viable PCN after each crop. 

Achieving eradication and deregulating infested fields is 

an ongoing challenge.

Where can I find more information about the 

PCN program?

To learn more about the PCN program, go to

www.aphis.usda.gov/planthealth/pcn. There, you’ll find 

information on the pest’s biology, PCN-regulated areas, 

program protocols, quarterly program updates, and more. 

You may also call the Idaho PCN Program office at (208) 

522-2431, Monday through Friday, 8:00 a.m. to 4:30 p.m., 

excluding Federal holidays.