**Recovery Plan**

**for**

**Potato Wart Disease**

Caused by

*Synchytrium endobioticum* (Schilberszky) Percival

January 9, 2007

[Table of Contents Page]{.underline}

Executive Summary 2

List of Contributors 3

I. Introduction 3

II\. Biology and Symptoms 6

III\. Spread 9

IV\. Monitoring and Detection 10

V. USDA Pathogen Permits 11

VI\. Response 12

VII\. Infrastructure and Experts 13

VIII\. Economic Impact and Compensation 14

IX\. Mitigation and Disease Management 15

X. Research, Extension and Education Priorities 18

References 21

Web Resources 23

This recovery plan is one of several disease-specific documents produced
as part of the National Plant Disease Recovery System (NPDRS) called for
in Homeland Security Presidential Directive Number 9 (HSPD-9). The
purpose of the NPDRS is to insure that the tools, infrastructure,
communication networks, and capacity required to mitigate the impact of
high consequence plant disease outbreaks are such that a reasonable
level of crop production is maintained.

Each disease-specific plan is intended to provide a brief primer on the
disease, assess the status of critical recovery components, and identify
disease management research, education and extension needs. These
documents are not intended to be stand-alone documents that address all
of the many and varied aspects of plant disease outbreak and all of the
decisions that must be made and actions taken to achieve effective
response and recovery. They are, however, documents that will help USDA
guide further efforts directed toward plant disease recovery.

**Executive Summary**

*Synchytrium endobioticum*, the causal agent of potato wart disease, is
the most important world-wide quarantine plant pathogen infecting
potato. *S. endobioticum* is on the United States Select Agent list - a
list of pests capable of considerable economic damage to agricultural
crops, but not resident within the United States. Potato wart disease
was recently evaluated as a top tier threat to agriculture on the United
States Department of Agriculture, Animal and Plant Health Inspection
Service, Plant Protection and Quarantine (USDA/APHIS/PPQ) list of
regulated pests. The need for intense regulatory scrutiny is due to the
latent persistence of the resting spores, a lack of effective chemical
control measures and few resistant varieties that are horticulturally
accepted in the United States.

Potato wart is a dramatic disease characterized by development of
tumor-like galls or warts on tubers and underground portions of potato
stems. Occasionally, galls form on the upper stem, leaf or flower. The
warts act as a sink for nutrients and increase rapidly in size at the
expense of the tubers, which may be replaced entirely by warts. Because
the symptoms generally occur underground, with only a slight reduction
in vigor noticed above ground, the disease is insidious and may not be
evident until potatoes are dug at harvest.

Spread of potato wart between countries or between farms is through
infected seed potatoes and movement of contaminated soil by machinery
and workers. *Synchytrium* *endobioticum* has a limited capacity for
natural spread, making effective control possible by statutory means.

Exclusion of the pathogen from non-infested areas is the most efficient
method of disease control.

More than eighteen million metric tons of fresh and seed potatoes with a
market value of \$2.9 billion were produced in the United States in
2005. The economic impact of potato wart disease is not from direct
disease losses but from loss of international trade markets, long-term
quarantines, and regulatory restrictions placed on infested areas and
the buffer zones surrounding infested land.

There is little funding available for research of a pathogen that
USDA/APHIS/PPQ declared eradicated from the United States in 1974. While
a large body of research on potato wart exists in Europe, much of it is
not applicable to the United States because different potato varieties
are used on the two continents. Canada has some research projects
underway due to a potato wart outbreak and containment in 2000-2004.
These North American projects should be expanded by the U.S. in the
areas of developing molecular methods of pathogen identification,
surveillance, evaluating current potato varieties for resistance and
breeding for resistance.

**Potato Wart Disease**

Caused by

*Synchytrium endobioticum* (Schilberszky) Percival

**Contributors**

E. Heyward Baker, USDA RMA; Dave Bell, USDA RMA; Terry Bourgoin,
USDA-APHIS-PPQ; Larry Brown, USDA-APHIS-PPQ; Russ Bulluck,
USDA-APHIS-PPQ; Joel Floyd, USDA-APHIS-PPQ; Lynn Goldner,
USDA-APHIS-PPQ; Ray Hammerschmidt, Michigan State University; Sharon
Hesvick, USDA RMA; Lynnae Jess, Michigan State University; Steven B.
Johnson, University of Maine; Willie Kirk, Michigan State University;
Liz Lopez, USDA RMA; Sandy Perry, Michigan State University, Stephen
Poe, USDA-APHIS-PPQ; Gary Secor, North Dakota State University; Jim
Sheldon, USDA RMA; Kent Smith, USDA-ARS; Walt Stevenson, University of
Wisconsin.

**I. Introduction**

Potatoes rank fourth in the list of world food crops, after rice, wheat
and maize. The United States ranks fifth in fresh market production
after China, the Russian Federation, India and the European Union.

Information in the following table ranks the states by production
figures, an indication of where the economic losses would be greatest if
*Synchytrium endobioticum* is identified in commercial fields. Weather
conditions approximating those known to be conducive to potato wart
development occur throughout the northeastern states and a large part of
the Great Lakes states. Those areas comprise about 25% of the potato
production dollar value in the U.S. However, there is no information
available about the possibility of wart survival in arid but irrigated
potato production areas of western states or hot but humid southern
states.

  -----------------------------------------------------------------------
  **United States Potato                          
  Production (2005)**                             
  ------------------------ ---------------------- -----------------------
  *State*                  *Weight in Billions of *Value of Production*
                           Pounds*                

  Idaho\*                  11.8                   \$674 M

  Washington\*             9.5                    \$535 M

  Wisconsin\*              2.8                    \$219 M

  Colorado\*               2.5                    \$214 M

  Oregon\*                 2.2                    \$129 M

  North Dakota\*           2.1                    \$139 M

  Minnesota\*              1.8                    \$114 M

  Maine\*                  1.5                    \$128 M

  California\*             1.5                    \$202 M

  Michigan\*               1.4                    \$110 M

  Nebraska\*               0.8                    \$70 M

  Florida                  0.8                    \$114 M

  Texas                    0.6                    \$73 M

  New York\*               0.5                    \$64 M

  Pennsylvania             0.3                    \$31 M

  Montana\*                0.3                    \$32 M

  North Carolina           0.3                    \$22 M

  Kansas                   0.2                    \$14 M

  Nevada                   0.2                    \$13 M

  Illinois                 0.2                    \$13 M

  New Mexico               0.2                    \$12 M

  Missouri                 0.2                    \$12 M

  Arizona                  0.1                    \$14 M

  Virginia                 0.1                    \$11 M

  Maryland                 0.09                   \$7 M

  Ohio                     0.09                   \$7 M

  Delaware                 0.08                   \$7 M

  Massachusetts            0.06                   \$5 M

  New Jersey               0.05                   \$4 M

  Alabama                  0.02                   \$2 M

  Rhode Island             0.01                   \$0.9 M
  -----------------------------------------------------------------------

  ------------------------- --------------------- -----------------------
  United States (2005)      42.4                  \$2.9 Billion

  ------------------------- --------------------- -----------------------

*\*States having potato seed certification agencies.*

***USDA potatoes 2005 summary,
<http://usda.mannlib.cornell.edu/usda/current/Pota/Pota-09-21-2006.pdf>***

*Synchytrium endobioticum*, the causal agent of potato wart disease, is
the most important world-wide quarantine plant pathogen infecting
potato. *S. endobioticum* is on the United States Select Agent list and
was recently evaluated as a top tier threat to agriculture on the Animal
and Plant Health Inspection Service (APHIS) list of Regulated Pests
(Rossman et al., 2006).

Presence of the pathogen does not affect the safety of potatoes for
consumption, but causes cauliflower-like deformities to grow on potatoes
making them unmarketable, and reduces yield. The pathogen affects the
growing points on the potato plant, such as eyes, buds and stolon tips.
At the end of the growing season, resting spores are produced that can
remain viable in the field soil for 40 years or more, effectively
eliminating infested fields from commercial potato production. The need
for intense regulatory scrutiny is due to the latent persistence of the
resting spores, a lack of effective chemical control measures and
scarcity of resistant varieties that are horticulturally accepted in the
United States.

Potato wart disease is believed to have coevolved with the potato in the
Andes Mountains and was subsequently distributed around the world by
trade in potatoes. The disease was first described in England in 1896,
then reported on the European continent in Czechoslovakia (1898),
subsequently spreading over the entire continent except Portugal. Potato
wart has also been reported in Africa (Algeria, South Africa, Tunisia),
Asia (Bhutan, India, Nepal), New Zealand, North America (Canada and the
United States) and South America (Bolivia, Chile, the Falkland Islands
and Peru) (De Boer, 2005).

The disease is commonly known as potato wart, but is also know by
various names, including black scab, black wart, cauliflower disease,
potato tumor, wart and warty disease. Potato is the principal host, but
*S. endobioticum* has been experimentally transferred to tomato, and to
black nightshade and climbing nightshade (*Solanum nigrum* L. and *S*.
*dulcamara* L.). Both nightshades developed wart infections in pot
experiments, though neither had been observed to be attacked in nature.
Extensive testing of a number of species belonging to the Solanaceae did
not produce any infections (Weiss, 1925; Hampson, 1993).

The first report of potato wart in North America was from Newfoundland
in 1909, where it has been under quarantine since 1912. In 2000 and
2001, new infestations were reported in commercial potato fields on
Prince Edward Island (P.E.I.), Canada.

Wart was identified in the United States in the early 1900s. Poor potato
harvests in the U.S. during 1911 and 1912 resulted in the importation of
millions of pounds of tubers from Europe where potato wart was
spreading. In 1912, the United States adopted a strict quarantine
against countries where wart disease occurred. Shipments of tubers
imported prior to 1912, however, were believed to have reached diverse
sections of the U.S. There is no evidence of the spread of wart from
Newfoundland to continental North America.

In 1918 potato wart was discovered in Pennsylvania, soon after in West
Virginia and, in 1920, the disease was found in Maryland. The potato
wart discoveries were confined to home gardens in coal mining areas of
the states. Despite a 1919 national survey for potato wart by the Plant
Disease-Survey in cooperation with the Federal Horticultural Board, no
more infestations, other than those in West Virginia and Maryland, were
discovered (Lyman et al., 1920).

Infestations in Pennsylvania and West Virginia were eradicated in the
1950s and 1960s, respectively. Infestations in Maryland were thought to
be eradicated but non-viable cysts of the pathogen were found in one
home garden during a survey in 1974. Subsequent bio-assays conducted
with susceptible potato cultivars, and soil collected from the infected
site did not produce any evidence of the pathogen. Potato wart was
considered eradicated from Maryland in 1994 (Putnam and Hampson, 1989).

**II. Biology and Symptoms**

*S. endobioticum* is an obligate parasite -- a pathogen that can only
feed on the living tissues of the host plant and cannot be grown in
artificial culture medium. The pathogen does not produce hyphae. When
not actively growing, it can survive as resting spores in the soil for
as long as 40 years.

Conditions favorable for disease development include cool summers with
average temperatures of 18°C (64°F) or less, winters of approximately
160 days at or below 5°C (41°F) and annual precipitation of 70 cm (28
in.). Temperatures between 12-24°C (54-75° F) favor infection.

![](media/image1.png){width="5.992361111111111in" height="4.525in"}

***Life cycle diagram of* Synchytrium endobioticum**. *A, infected
tubers, stem and stolons with wart symptoms; B, resting sporangium; C,
resting sporangium with maturing zoospores; D, discharged motile
zoospores; E, two zoospores form a (diploid) sporangium to later form a
zygote; F, zoospore entering a host cell by direct penetration; G, young
prosorus in host cell; H, contents of prosorus passing into host cell;
I, cross section of sorus with two (haploid) sprorangia and remains of
empty prosorus; and J, three mature (haploid) sporangia in sorus with
zoospores beginning to be released. The haploid sporangia are summer
sporangia and the diploid sporangia are resting winter sporangia*
(Stevenson et al., 2001; illustrations after Walker, 1957).

A typical disease cycle of potato wart begins when cool spring
temperatures, and rainy weather or irrigation cause the soil-borne
thick-walled overwintering sporangia to germinate. Instead of hyphae,
each germinating sporangium produces as many as two- to three-hundred
motile zoospores (2-4 µm). The zoospores have a tail-like flagellum
enabling them to move through soil water. Zoospores can survive for 1-2
hours, infecting susceptible potato tissue with which they come into
contact. The plant cells around the penetration point swell and
proliferate, producing the characteristic warty, cauliflower appearance.
The internal cells entered by the zoospores also become greatly
enlarged. More spores are discharged from the infected tissue, re-infect
surrounding cells which, in turn, produce more spores and subsequent
infections as long as conditions remain favorable (cool, wet summer
growing conditions). The persistent resting spores form at the end of
the growing season and are capable of surviving in the soil for decades
at depths of up to 50 cm (20 inches).

![](media/image2.jpeg){width="3.004861111111111in"
height="2.0034722222222223in"}

Potato tuber with gall

![](media/image3.jpeg){width="2.9034722222222222in"
height="1.9527777777777777in"}

Stem galling on potato

[]{.mark}

**Photos: Michael Hampson**

The warts are tumor-like galls affecting tubers and underground portions
of potato stems. Occasionally, warts form on the upper stem, leaf or
flower. The warts range from the size of a pea to large masses that
cover the entire tuber. Warts at first appear green or white, turn brown
and then black at maturity as they decay. Roots are not known to be
attacked. The warts act as a sink for nutrients and increase rapidly in
size at the expense of the tubers, which may entirely transform into
warts. Because the symptoms generally occur underground with only a
slight reduction in vigor above ground, the disease is insidious and may
not be evident until potatoes are dug at harvest (Stevenson et al.,
2001; EPPO Bulletin 34, 2004).

***Potato wart on tomato***

There is one report from 1920 that several varieties of tomato became
infected with potato wart when grown in infested soil.

[]{.mark}

In 1918, L.O. Kunkle and C.R. Orton completed a survey of vegetables
other than potatoes, and weeds growing in infested Pennsylvania gardens.
They found no evidence of potato wart infection. However, they were
aware of the range of potato varieties susceptible to potato wart, and
set out to see if tomatoes, close relatives of potatoes, also showed
varietal susceptibility. In 1919, they planted two plants each of fifty
tomato varieties in an *S. endobioticum* infested garden in Lehigh,
Pennsylvania. About half the plants died from damping-off fungus, but of
the remaining plants that survived until harvest, seven varieties showed
signs of potato wart on the roots and stems. The warts on the roots were
the size of a pea or smaller. The warts on the stems were considerably
larger than on the roots. Warts were confined to the underground
portions of the stem or to portions just above the surface of the soil.
Microscopic examination revealed that both root and stem warts contained
large numbers of resting sporangia. The warts on the tomato were harder
and less succulent than those on the potato. Kunkle and Orton felt that
the large numbers of warts on the roots of a tomato plant might somewhat
lessen the yield. It was not believed that the disease would cause
serious damage to susceptible tomato varieties, but growing these
varieties on infested soil would keep the disease alive from year to
year and transplanting seedlings from infested soil might spread it to
new fields. The warted stem of the Maulc's New Imperial tomato variety
is show below (Lyman et al., 1920).

![](media/image4.jpeg){width="2.7243055555555555in"
height="4.604166666666667in"}

**III. Spread**

*Synchytrium endobioticum* has a very limited capacity for natural
spread, making it possible to control it effectively by statutory means.
Spread of potato wart disease between countries or between farms is
primarily through infected seed potatoes. The potato industry in the
United States has excellent programs in place to safeguard the
disease-free quality of seed and fresh potatoes on a state by state
basis, and is in the process of establishing a nationally verifiable
system. It may still be possible for infected potatoes with incipient
warts to pass undetected through visual inspection. However, many
programs start with tissue cultured stock that eliminates all pathogens.
Wart in a seed field would be detected early because in the early seed
generations the potatoes are checked individually.

Local spread may occur by the movement of fungal spores in soil, water,
or in contaminated soil adhering to tubers, equipment, implements or
other carriers. Manure from animals that have fed on infected tubers may
also spread the disease. USDA-APHIS-PPQ's prohibition on the movement of
foreign soil into the United States makes it unlikely that potato wart
could enter the U.S. on imported plants or plant parts (EPPO bulletin
34, 2004; Stevenson et al., 2001).

**IV. Monitoring and Detection**

*Synchytrium endobioticum* has been subject to quarantine and domestic
legislation to prevent its spread worldwide for more that 65 years. Crop
protection chemicals have not proven effective in controlling potato
wart. Exclusion of the pathogen from non-infested areas is the most
efficient method of disease control.

Potato wart is not a disease that is regularly surveyed by the APHIS
Cooperative Agricultural Pest Survey (CAPS) program. Some limited survey
work was conducted in parts of Idaho, Washington, Massachusetts, Oregon,
Vermont and Nevada between 2000 and 2005. All results were negative. The
state of Montana will complete a CAPS survey for potato wart in 2006 but
results are not yet available (NAPIS website; Personal communication
with Montana Department of Agriculture).

Potato wart falls under both state and federal regulations within the
United States. Monitoring for potato diseases is built into the U.S.
potato industry at several levels. There is a formal program of
inspection and certification of seed potatoes to ensure that the tubers
sold for seed meet the quality standards mandated by state law. The
standards ensure that only high quality seed is marketed to growers of
fresh market and processing potatoes. Authority for certifying seed
potatoes at the state level resides with the agency granted authority by
state law. The agency is usually a land grant university, state
department of agriculture, crop improvement association or a combination
of all three.

There are formal and informal inspections of tablestock potatoes during
and after harvest, when potatoes travel by conveyor belt into cold
storage and again, on the conveyer moving potatoes from cold storage to
be washed and sold. Seed potatoes are subject to inspection by state
regulatory officials when crossing state lines.

Each state land grant university diagnostic laboratory, as part of the
National Plant Diagnostic Network (NPDN), has a protocol for preliminary
identification of *S. endobioticum* and instructions for sending
suspicious samples to the USDA national lab for confirmation and species
identification.

Because *S. endobioticum* is on the federal Select Agent list (42 C.F.R.
Part 73, 7 C.F.R. Part 331, and 9 C.F.R. Part 121), any suspected
incidences of the pathogen in the United States must be sent for
confirmation to the Plant Protection and Quarantine (PPQ) division of
APHIS. Morphological identification of *S. endobioticum* will be
performed by the National Mycologist at the USDA-APHIS-PPQ Plant
Safeguarding and Pest Identification National Identification Services in
Beltsville, MD. Confirmatory molecular testing and validation of primers
for PCR or serological tests for the pathogens is performed at the
USDA-APHIS-PPQ National Plant Germplasm and Biotechnology Laboratory
(NPGBL) in Beltsville, Maryland. This laboratory also has all the
necessary permits, registrations, and containment approvals to handle
select agents.

In the future, other laboratories may obtain certification and
registrations for making presumptive determinations but must abide by
guidelines set under various permits and authorizations maintained by
APHIS Plant Protection and Quarantine.

**V. USDA Pathogen Permits**

The possession, use and transfer of select agents as well as plant pests
or infected plant material, regardless of quarantine status, is
specifically regulated.

PPQ permit and registration requirements for plant diseases and
laboratories fall under two authorities, the Plant Protection Act (7 CFR
Part 330) and the Agricultural Bioterrorism Protection Act of 2002 (7
CFR Part 331). Laboratories receiving suspect infected plant material or
cultures are required to have PPQ permits. Laboratories possessing,
using, or transferring select agents are required to be registered as a
select agent laboratory. However, diagnostic laboratories that identify
select agents are exempt from this requirement as long as they complete
an APHIS/CDC Form 4 and destroy or transfer infected material to a
laboratory registered with the APHIS Select Agent Program within seven
calendar days of learning of a positive confirmation.

-   The Plant Protection Act permit requirements apply to all plant
    pests and infected plant material, including diagnostic samples,
    regardless of their quarantine status. If any material is shipped
    interstate, it is a requirement that the receiving laboratory has a
    permit. For further guidance on permitting of plant pest material,
    consult the PPQ permit website at:
    <http://www.aphis.usda.gov/ppq/permits/> or contact PPQ Permit
    Services at (301) 734-8758.

-   Federal regulation by the Agricultural Bioterrorism Protection Act
    of 2002 (7 CFR Part 331) specifies requirements for possession, use,
    and transfer of organisms listed as select agents and toxins. Once a
    diagnostic laboratory identifies a select agent, it must immediately
    notify the APHIS Agriculture Select Agent Program, complete an
    APHIS/CDC Form 4 and submit within 24 hours, and either destroy or
    transfer the agent to a registered entity within 7 calendar days. In
    compliance with this Act, if a diagnostic laboratory held back part
    of a screened sample for voucher purposes and the sample that was
    forwarded to the USDA Beltsville Laboratory came back as positive
    for a select agent, the diagnostic laboratory is required to notify
    the APHIS Select Agent Program immediately. If the determination of
    the laboratory is to destroy the sample, this must take place within
    seven (7) calendar days of results notification and a PPQ Officer
    must witness the destruction of the sample on or before the 7-day
    period expires. Clarification of this and other information related
    to adherence to the select agent regulations is available on the
    following APHIS website:
    <http://www.aphis.usda.gov/programs/ag_selectagent/index.html>, or
    call (301) 734-5960.

**VI. Response**

While this plan is focused primarily on recovery, certain facets of the
response to a new detection involve aspects of recovery with a continuum
of activities from response to recovery. The response is under
USDA-APHIS-Plant Protection and Quarantine's authority delegated from
the Secretary of Agriculture under the Plant Protection Act of 2000.

After a detection of *S. endobioticum* has been confirmed by a
USDA-APHIS-PPQ recognized authority, APHIS, in cooperation with the
appropriate state department of agriculture, is in charge of the
response. Race or pathotype determination is not necessary to initiate
the response as all races are under quarantine. The response is
immediate in the form of advance assessment teams of state and federal
experts and survey personnel. They are sent to the site of initial
detection to prevent the movement of regulated articles, conduct
investigations, and initiate delimiting surveys using trace back and
trace forward information, crop, tuber, and soil samples, shipping point
inspections, post-harvest field sampling and/or bioassay of crops.
Actions that may be taken include regulatory measures to quarantine
infested or potentially infested production areas, stop movement of
infected or potentially infected articles in commerce, and other control
measures which may include host removal and destruction, and/or insuring
adherence to required sanitary practices. APHIS imposes quarantines and
regulatory requirements to control and prevent the interstate movement
of quarantine-significant diseases or regulated articles, and works in
conjunction with states to impose these actions parallel to state
regulatory actions which restrict intrastate movement.

The field(s) known to be positive (index sites), all adjacent fields,
and fields within a ½ mile (0.8 km) core (regulated) area around the
index site will be placed under quarantine. All fields with previous
linkages to index field(s) by shared harvesting equipment, vehicles, or
machinery are called contact fields and will also be placed under
quarantine. Regulated articles include host plants and plant parts
(potato and tomato), soil and crops contaminated with soil, machinery,
equipment and vehicles from the regulated area. Also included are bags,
tools, seed houses, and storage sheds with links or associations to
infested fields or tuber lots. Host material and soil are prohibited
movement outside the regulated area and any regulated equipment, tools,
vehicles, machinery, or storage areas must be disinfested with
prescribed formulations of quaternary ammonium.

Fields that are positive for *S. endobioticum,* and all host crops in
the regulated area must have the current crop destroyed with an approved
herbicide. The dead plant material is subsequently destroyed by burning,
incineration, or removed and sent to an approved landfill. Harvested
potatoes from regulated fields are also destroyed by approved methods.
Prohibition of host crops in the regulated area remains in effect until
the disease is considered eradicated. An exception to this is allowed if
the race is determined to be one with which immune varieties of potato
are available, and they can be planted. Potatoes from these plantings
can only be used for processing under compliance agreement, and are not
allowed for use as seed or tablestock. In a minimum of ten years the
fields are resurveyed requiring planting with a susceptible variety for
three consecutive years with conducive environmental conditions for the
disease. If no evidence of potato wart is detected in those three years,
the fields can be released from quarantine.

**VII. Infrastructure and Experts**

Much of the infrastructure, outside the regulatory community, for
detecting and diagnosing a pest of national concern such as potato wart,
has been organized and developed in the past four years. The National
Plant Diagnostic Network (NPDN) was established by USDA-CSREES in 2002
to enhance national crop security by quickly detecting and diagnosing
introduced plant pests and pathogens. To date, more than 12,000 growers,
crop consultants and extension educators have received NPDN training to
be aware of unusual or suspicious plant symptoms for many different
pathogens and pests, to collect and package suspicious plant samples
properly, and quickly submit them to the nearest state land grant
university diagnostic lab. More information about NPDN First Detector
training and training materials can be found at <http://www.npdn.org/>,
Click on First Detector Information under NPDN portal.

The NPDN has also updated lab equipment and linked all land grant
university diagnostic laboratories electronically, giving labs the
ability to visually compare samples over a secure network. Standard
Operating Procedures are being developed for the identification of many
plant pathogens, including potato wart. National training sessions are
regularly provided for diagnosticians to upgrade diagnostic techniques.

Standardized protocol for handling specimens, securing identification
and facilitating communications has been developed jointly between NPDN
and APHIS-PPQ. This protocol has been practiced in 43 of 50 states,
using simulated diagnostic events of high risk pests. The diagnostic
exercises have involved extension educators, extension specialists, land
grant university diagnostic labs, and both state and federal regulatory
officials.

Each region of the U.S. where potatoes are grown has at least one land
grand university plant pathologist specializing in potato diseases. U.S.
plant pathologists that specialize in potato diseases can be queried
from the American Phytopathological Society website
([http://www.apsnet.org/](http://www.aps.net.org/)) under membership
directory, advanced search.

North American scientists with expertise in potato wart include:

> Lawrence G. Brown -- USDA-APHIS-PPQ, Center for Plant Health Science
> and Technology, North Carolina State University,1730 Varsity Dr. 
> Suite 300, Raleigh, NC, 27606. 919-855-7503,
> <lawrence.g.brown@aphis.usda.gov>
>
> Russ Bulluck - Center for Plant Health Science and Technology (CPHST),
> USDA- APHIS- PPQ, 1730 Varsity Drive, Raleigh, NC 27607, 919-855-7646,
> <russ.bulluck@aphis.usda.gov>
>
> Terry Bourgoin - USDA-APHIS-PPQ, 15 Iron Road, Suite 1, Hermon, Maine
> 04401, (207) 848-5199, <Terry.L.Bourgoin@aphis.usda.gov>
>
> Steven B. Johnson - University of Maine Cooperative Extension, PO Box
> 727, 57 Houlton Road, Presque Isle, ME 04769-0727, (207) 764-3361,
> <sjohnson@umext.maine.edu>
>
> Jim Stack -- National Plant Diagnostic Network (Director), experts in
> diagnosis and detection. Kansas State University, 4024 Throckmorton
> Hall, Manhattan, KS 66506-5502, 785-532-1388, <jstack@ksu.edu> .

The Canadian Food Inspection Agency (CFIA) maintains the following
centers involved with potato wart research for Canada:

-   CFIA Laboratory - Charlottetown, Prince Edward Island, Center of
    Expertise for Potato Diseases.

> The Center provides testing, and reference services for CFIA's Seed
> Potato Certification program. Activities include research to improve
> tests for potato diseases of quarantine and trade significance. The
> Center also provides audit and technical support for the accreditation
> of non-federal labs to test seed potatoes for import, export and
> disease control programs.

-   CFIA Laboratory -- St. John's, Newfoundland.

> Provides testing for potato wart disease and cyst nematodes;
> collaborates with Agriculture and Agi-food Canada and the Newfoundland
> government in the development of potato cultivars resistant to potato
> wart and potato cyst nematodes.

The European and Mediterranean Plant Protection Organization (EPPO) is
an intergovernmental organization responsible for European cooperation
in plant health. Founded in 1951 by 15 European countries, EPPO now has
48 members, covering almost all countries of the European and
Mediterranean region. Its objectives are to protect plants, to develop
international strategies against the introduction and spread of
dangerous pests and to promote safe and effective control methods.

**VIII. Economic Impact and Compensation**

Member nations of the World Trade Organization are entitled to ban
imports of plant material that may introduce exotic diseases into their
territories. Thus, importing countries that are themselves free of a
particular highly contagious plant disease will routinely impose
sanitary or phytosanitary restrictions on trade with countries in which
that disease occurs. This can result in billions of dollars of lost
trade.

In 1987, a senior policy advisor (D. Karamchandani) with Agriculture and
Agrifood Canada, did an analysis of the putative economic losses of a
potato wart discovery to Atlantic Canada and the far-reaching
consequences for the rest of the country. At that time there had not
been an incidence of potato wart in a commercial Canadian field. The
analysis proved prophetic when the economic impact from the 2000-2001
discoveries of potato wart on P.E.I. closely followed her assumptions.
Karamchandani assumed new outbreaks would be small and sporadic and
would be detected before they caused major crop damage. She concluded
that the impact would not be caused directly by the pathogen but would
result from the imposition of various measures such as embargoes,
quarantines, long crop rotations, increased research activity and
incentives to grow only resistant cultivars. In Atlantic Canada's major
potato growing region, farm revenues would fall, potatoes would be
consumed locally at lower cost to the consumer, and supplies to central
and western Canada would fall, causing increased costs in potato-deficit
areas. Governments would be under pressure to bear most of the adverse
economic consequences, and there would be repercussions on world trade
as other countries would most likely cease to import Canadian potatoes
(Hampson, 1993).

***Federal Crop Insurance***

Federal crop insurance compensation is provided for potato losses due to
plant disease but only if good farming practices are followed that
include disposing of any infected crop from previous years, use of
appropriate crop protection chemicals, and using recommended rotation
practices for the specific disease. The various policies for potatoes
can be accessed by policy number from the website listed below.

  ----------------------------------------------------------- -----------
  **POTATO**                                                  

                                                              
  ----------------------------------------------------------- -----------

  ----------------------------------------------- -------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------
  **Policy Name**                                 **Policy \#**                                                                                **Ref \#**

  Northern Potato                                 [[98-184]{.underline}](http://www.rma.usda.gov/FTP/Policies/1998/crops/PDF/98184POT.pdf)     [[3]{.underline}](http://www.rma.usda.gov/policies/2001policy.html#3%233)

  Northern Potato Quality Endorsement             [[98-84A]{.underline}](http://www.rma.usda.gov/FTP/Policies/1998/crops/PDF/98-84A.pdf)       [[3]{.underline}](http://www.rma.usda.gov/policies/2001policy.html#3%233)

  Northern Potato Processing Quality Endorsement  [[98-84B]{.underline}](http://www.rma.usda.gov/FTP/Policies/1998/crops/PDF/98-84B.pdf)       [[3]{.underline}](http://www.rma.usda.gov/policies/2001policy.html#3%233)

  Northern Potato Certified Seed Endorsement      [[2001-84C]{.underline}](http://www.rma.usda.gov/FTP/Policies/2001/crops/PDF/2001-84c.pdf)   [[3]{.underline}](http://www.rma.usda.gov/policies/2001policy.html#3%233)

  Northern Potato Storage Coverage Endorsement    [[98-84D]{.underline}](http://www.rma.usda.gov/FTP/Policies/1998/crops/PDF/98-84D.pdf)       [[3]{.underline}](http://www.rma.usda.gov/policies/2001policy.html#3%233)

  Central & Southern Potato                       [[99-284]{.underline}](http://www.rma.usda.gov/FTP/Policies/1999/crops/PDF/99284POT.pdf)     [[3]{.underline}](http://www.rma.usda.gov/policies/2001policy.html#3%233)
  ----------------------------------------------- -------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------

<http://www.rma.usda.gov/policies/2001policy.html>

**IX. Mitigation and Disease Management**

***Modeling***

The North Carolina State University/APHIS Plant Pest Forecast (NAPPFAST)
([www.nappfast.org](http://www.nappfast.org/)) System is an internet
tool for plant pest modeling using geo-referenced climatological weather
data. NAPPFAST has developed risk maps representing the influence of
weather and climate on potato wart development. The probability risk
maps are estimated from 30 years of daily climatic data using two simple
models. The first model represents the influence of climatic conditions
upon overwintering and the second represents epidemic development of the
disease during the growing season (Magarey et al., 2004).

***Chemical control options***

It the past, infested soils were chemically treated to eradicate
surviving soil inoculum of potato wart. This approach is no longer
suitable. The drastic ecological impact of the type and high rates of
chemical treatments (mercury, sulfur, copper, chlorine-based chemicals
and formaldehyde) that were used left the soil barren. This is
unacceptable because the intention of soil treatment is to resume potato
cultivation (Reigner, 2006; De Boer, 2005).

There is zero tolerance for potato wart. For any crop protection
chemical to be considered effective, it must be eradicative, not just
suppressive. More than 400 compounds have been tested for controlling
potato wart. To date, no chemicals are recommended for managing the
disease (Hampson, 1993).

***Biological control***

Several biological control theories and non-chemical treatments, such as
the use of chitin, urea, organic fertilizers and crab shell meal have
been proposed and investigated in areas of known infestation of the wart
pathogen. None have been widely adopted in field use, and efficacy
information is limited.

***Disease management***

To date, the best method of managing an initial outbreak of potato wart
disease is quarantine and movement restriction on fields that are within
a buffer area and associated with the index field.

***Prince Edward Island (P.E.I.) Potato Wart Disease Case Study***

Potato wart was first discovered on P.E.I. on October 24, 2000.
Originally, one field of potatoes was found to be infested in New Annan,
P.E.I. in 2000, and in 2002 five additional infested fields were
discovered. The Canadian Food Inspection Agency (CFIA) and the United
States Department of Agriculture (USDA) developed a three-year working
plan including restricted movement of host material and equipment within
the quarantine area, surveillance activities, and a certification
process for potatoes produced on P.E.I. Potato wart was detected in one
P.E.I. field during 2003, indicating that the conditions were favorable
for the disease in that location. One indication that the 2003 potato
growing season on P.E.I. was conducive for potato wart was that there
were symptomatic volunteer plants in an index field near the New Glasgow
area.

Following is a narrative of how the disease outbreak was handled by the
Canadian government, and the regulatory requirements imposed by USDA
before potatoes could again be imported into the United States from the
island.

In October 2000, the United States closed its border to shipments of
potatoes from P.E.I., Canada because potato wart had been identified in
a field on the island. Harvesting was stopped as soon as the first
infected potato was seen, and harvesting equipment was sanitized.
Canadian regulatory actions to delimit the infestation included
immediate quarantine of the area where the pathogen was found, a
prohibition on the movement of potatoes and machinery from the index
field as well as surrounding fields, collecting and analyzing thousands
of soil samples in the index field and surrounding area, conducting
trace-backs and trace forwards on the seed potatoes, and investigating
past farming practices in the index field. The field where the infected
potatoes were found was enclosed with a chain-link fence and security
guards were posted 24 hours a day. By mid-November 2000, the delimiting
survey determined that the initial detection was restricted to the
section of the original index field.

The U.S. border remained closed to P.E.I. potatoes for six months, until
April 2001, while a scientific risk analysis was completed (Brown,
2006). The objective of the assessment was to recommend a general
surveillance survey to reduce the risk of *S. endobioticum* moving from
P.E.I. into the United States. The risk in need of surveillance was the
independent existence of undetected infestations (cryptic fields)
occurring on P.E.I.

The U.S. border reopening came with strict USDA-APHIS provisions for
Canadian inspection and handling of the 2000 potato crop. A 3-year plan
was put in place whereby Canadian inspectors must inspect every P.E.I.
potato field after harvest to determine if there was a detectable level
of potato wart disease. Officials also had to inspect random samples of
potatoes heading into and out of storage (Herglotz and Jones, 2001).

In September 2002, two more incidences of potato wart were detected from
two different fields (independent of the original infestation)
approximately six and 15 miles from the original find in 2000. Both
infections were discovered during a Canadian Food Inspection Agency
post-harvest field inspection required under the 3-year Canada/US
working plan. The new finds did not negatively affect trade. The CFIA
regulations put into place for P.E.I. after the initial detection,
mitigated this expanded risk. The inspection was instrumental to
safeguarding U.S. trade with Canada.

In the four years immediately following the disease outbreak, 126,000
hectares were surveyed for evidence of the disease and 26,000 soil
samples were processed for pathogen extraction. The overall cost of the
P.E.I. outbreak was estimated at over \$83 million. It is estimated that
the loss to the island's economy and its 600 potato growers was between
\$30 and \$50 million (Canadian Phytopathological Society website:
<http://www.cps-scp.ca/pathologynews/potatowart.htm>).

**X. Research, Extension, and Education Priorities**

***Research Priorities***

**[Evaluation of U.S. potato varieties for resistance/susceptibility to
potato wart]{.underline}**

North American potato varieties differ from those used in Europe and
have not been included in the virulence testing there, so very little is
known about the resistance/susceptibility of North American varieties.
There is no program in the United States for evaluating the resistance
of potato varieties to *S. endobioticum*. However, there is a program at
the CFIA Laboratory in Newfoundland that could be a useful model, a
starting point, or a resource for beginning a new or cooperative program
of resistance evaluation for U.S. potato varieties.

In countries with known infestations of potato wart, cultivation of
resistant (immune) potato varieties has been the prime measure for
managing the disease. The European Union (EU) member states are obliged
annually to send the EU commission a list of potato cultivars which have
been officially tested and found to be resistant to *S. endobioticum*.
The EU directive does not specify the methodology to be used for
testing, only describes the phytosanitary goal that is to be reached in
terms of risk. Complete resistance to *S. endobioticum* is not required.
Rather, resistance should be such that secondary infection within the
crop, or of a subsequent crop of the same cultivar, need not be feared
(Baayen et al., 2005).

The Canadian Food Inspection Agency (CFIA) maintains a website listing
descriptions of all commercial potato varieties used in Canada
(<http://www.inspection.gc.ca/english/plaveg/potpom/var/indexe.shtml>).
The list can be queried by specific disease. Fifty potato varieties are
included that range from immune to highly resistant to potato wart, with
pathotype resistance listed where known. Only three varieties on this
list are grown in the United States.

Over 20 different pathotypes of potato wart have been described, mainly
from Europe where resistance testing has been on-going since 1920. Each
pathotype is characterized by its pattern of infectiousness on different
potato varieties. Because the pathotypes work in subtly different ways
within potatoes, genetic resistance must be specific to each pathovar.
To date, there is no international harmonization of pathotype codes
(Baayen et al., 2006).

Pathotyping is a time-consuming and difficult procedure. One of the *S.
endobioticum* isolates from the 2000/2001 P.E.I. potato wart outbreak
has been identified as the strain consistent with German pathotype 6,
and typing of two other isolates has not been successful thus far.
Newfoundland, Canada is known to harbor pathotypes 1, 2, 6 or 7, and 8
(Stachewicz and De Boer, 2002).

The United States would benefit from the development of cooperative
projects with Canada and European countries for the purpose of including
U.S. potato varieties in potato wart resistance screening programs.
While the U.S. does not have a national public database for potato
varieties and pest resistance, there are regional websites such as the
Potato Information & Exchange (<http://potatoes.wsu.edu/>) at Washington
State University that could benefit from the inclusion of disease
resistance/susceptibility information in their varietal lists.

**[Breeding for Resistance]{.underline}**

Potato resistance breeding programs are crucial in the control of potato
wart disease. Potato wart resistance/susceptibility is not a
consideration in U.S. potato breeding as there is no program available
to test against North American pathotypes and it is an uncommon practice
to breed for resistance to pests that do not occur in the country.

Resistant germplasm is available for use in breeding. Researchers in
Germany at the Max-Planck Institute for Plant Breeding Research have
developed potato lines that harbor multiple resistance genes by marker
assisted selection. Knowledge of the genetic position of the desirable
traits and of closely linked DNA-based markers allows the targeting of
specific genes for introgression, and provides a fast track to increase
genetic gain in crop breeding. The plant lines generated are resistant
to four important potato pathogens, Potato Virus Y, *Synchytrium
endobioticum*, and the root cyst nematodes *Globodera rostochiensis* and
*Globodera pallida*. The selected plants can be used as sources of
multiple resistances and are available from the Institut für
Pflanzengenetik und Kulturpflanzenforschung (IPK) potato germplasm bank
in Germany. Selection of *Synchytrium*-resistant potato cultivars may be
facilitated by using markers closely linked with a resistance gene or by
transferring a cloned gene for resistance into susceptible cultivars. A
single, dominant gene for resistance to *Synchytrium endobioticum* has
been located on potato linkage *group XI* (Gebhardt et al., 2005).

**[Developing improved molecular diagnostic tools for detection of *S.
endobioticum* in both plants and soil]{.underline}**

A discovery of potato wart infection is soon followed by intensive
testing of soil and tissue for evidence of the pathogen. Time is of the
essence in order to determine the extent of the infestation and
delineate the quarantine area. The best qualitative and quantitative
testing procedures must be continually scrutinized and updated to the
standards of new molecular technology.

A technique developed in Canada during the 2000-2001 outbreak, used the
DNA of several potato wart-diseased potatoes including one specimen from
the original outbreak in Newfoundland in 1903. The DNA extraction and
subsequent development of probes for detection made possible the
creation of a highly sensitive test for detecting as few as one to three
pathogen spores per gram of soil (Levesque, 2001).

Recent work in Germany demonstrated the utility of 18S rDNA sequence of
*S. endobioticum* in microarrays for the simultaneous detection of
fungal and viral pathogens of potato. The study suggests that
oligonucleotide-based microarray has great potential, providing a very
convenient option for multiple pathogen detection, especially in
quarantine laboratories where plant materials are routinely screened for
both characterized and uncharacterized disease-causing organisms
(Abdullahi et al., 2005).

**[Delimiting surveys and the potato wart surveillance
model]{.underline}**

The North Carolina State University (NCSU)/APHIS Plant Pest Forecast
(NAPPFAST) System, a web-based interface used for mapping the
distribution of invasive pests, recently completed a project on potato
wart (described in the Disease Management and Mitigation section under
*Modeling*). The project was a general assessment for the United States,
as a whole. The next step would be to refine the model for the important
potato production states. It is essential to determine the likelihood of
conditions being conducive for survival of *S. endobioticum* -- as an
individual state may have some areas that are less likely to support the
disease than other areas in the same state. The information would be of
assistance when planning detection and delimiting surveys for potato
wart. NAPPFAST allows a wide range of users to rapidly create
weather-based development/incidence models to support predictive pest
mapping needs, and to assist in pest prediction and occurrence (Magarey
et al., 2004).

***[Ranking research priorities]{.underline}***

Priority ranking for research projects could change depending on the
current situation with *S. endobioticum*. Included here are three
scenarios that would affect the priority of the ranking.

Condition A:      *Potato wart is not known to exist in the United
States*

Priority of research projects:

> 1\. Developing improved molecular diagnostic tools for detection of
> *S. endobioticum* in both plants and soil
>
> 2\. Delimiting surveys and the potato wart surveillance model
>
> 3\. Evaluation of U.S. potato varieties for resistance/susceptibility
> to potato wart
>
> 4\. Breeding for resistance

Condition B:      *Potato wart is discovered in the United State*s

Priority of research projects:

> 1\. Delimiting surveys and the potato wart surveillance model
>
> 2\. Developing improved molecular diagnostic tools for detection of
> *S. endobioticum* in both plants and soil
>
> 3\. Evaluation of U.S. potato varieties for resistance/susceptibility
> to potato wart
>
> 4\. Breeding for resistance

Condition C:      *Potato wart becomes established in the United States*

Priority of research projects:

> 1\. Breeding for resistance
>
> 2\. Evaluation of U.S. potato varieties for resistance/susceptibility
> to potato wart
>
> 3\. Delimiting surveys and the potato wart surveillance model
>
> 4\. Developing improved molecular diagnostic tools for detection of
> *S. endobioticum* in both plants and soil

*Extension and Educational Priorities*

Extension specialists and educators must continue to build awareness of
potato wart disease among their professional and nonprofessional
clientele through written and web-based learning materials, educational
programming and continued participation in the National Plant Diagnostic
Network's (NPDN) First Detector training program. It is also important
to promote awareness among home gardeners as part of the Master Gardener
curriculum.

Extension specialists and educators are an integral part of the NPDN
diagnostic exercise program. The exercises are providing practice in the
chain of custody and communication protocol involved with identifying
and diagnosing a high risk pest. Their input into the format and
function of the exercises is critical to the continued expansion and
awareness-building of the program among growers, extension personnel,
land grant diagnostic facilities as well as state and federal regulatory
officials.

**References**

Abdullahi, I., Koerbler, M., Stachewicz, H., and Winter, S. 2005. The
18s rDNA of *Synchytrium endobioticum* and its utility in microarrays
for the simultaneous detection of fungal and viral pathogens of potato.
Applied Microbiology and Biotechnology, 68 (3): 368-375. 

Baayen, R.P., Bonthuis, H, Withagen, J.C.M, Wander, J.G.N., Lamers, J.L,
Meffert, J.P., Cochius, G., van Leeuwen, G.C.M., Hendriks, H., Heerink,
B.G.J., van Boogert, P.H.J.F., van de Griend, P., and Bosch, R.A. 2005.
Resistance of Potato Cultivars to *Synchytrium endobioticum* in field
and laboratory tests, risk of secondary infection, and implications for
phytosanitary regulations. EPPO Bulletin 35: 9-23.

Baayen, R.P., Cochius, G., Hendriks, H., Meffert, J.P., Bakker, J.,
Bekker, M., van den Boogert, P.H.J.F., Stachewicz, H., and van Leeuwen,
G.C.M. 2006. History of potato wart disease in Europe -- a proposal for
harmonization in defining pathotypes. European Journal of Plant
Pathology, 116: 21-31.

Brown, L.G. 2006. *Synchytrium endobioticum* (potato wart): Analysis of
Surveillance Methods. USDA-APHIS-PPQ Center for Plant Health Science and
Technology. Raleigh, North Carolina 27606.

DeBoer, S. 2005. Emerging Potato Disease Challenges -- an Overview.
Potato Reporter Online.Com. -- the Monthly Online International Potato
Newsletter, July.
[http://www.potatoreporteronline.com](http://www.potatoreporteronline.com/)

European and Mediterranean Plant Protection Organization (EPPO)
Bulletin, Diagnostic Protocols for Regulated Pests: *Synchytrium
endobioticum*. 2004, vol. 34 (2): 213-218.

Gebhardt C., Bellin, D., Henselewski, H., Lehmann, W., Schwarzfischer,
J., and Valkonen, W.P.T. 2006. Marker-assisted Combination of Major
Genes for Pathogen Resistance in Potato. Theoretical Applied Genetics,
vol. 112 No. 8, May: 1458-1464.

Hampson, M.C. 1993. History, biology and control of potato wart disease
in Canada. Canadian Journal of Plant Pathology, vol. 15(4):223-316.

Herglotz, K., and Jones, B. 2001. U.S. and Canadian Agriculture
Officials Agree on Conditions for the Movement of Potatoes from Prince
Edward Island. Press Release from the Embassy of the United States of
America, Ottawa, Canada.\
<http://canada.usembassy.gov/content/textonly.asp?section=can_usa&subsection1=trade&document=potato_wart042601>

Levesque, A. 2001. New Species Specific Assay for the Causal Agent of
Potato Wart. Research Highlights 2000-2001, Canadian Eastern Cereal and
Oilseed Research Centre.
<http://res2.agr.ca/ecorc/highlights-faits_saillants_e.htm#5>

Lyman, G.R., Kunkel, L.O., and Orton, C.R. 1920. Potato Wart. U.S. Dept.
of Agr. Circular III, 19 pp.

Magarey, R.D., Borchert, D.M., and Brown, L.  2004. Pest Assessment:
*Synchytrium endobioticum*, (Potato Wart).  September, 2004 Project
Report. <http://www.nappfast.org/casestudies_files/potato_wart.pdf>

Putnam, M.L. and Hampson, M. C.. 1989. Rediscovery of *Synchytrium
endobioticum* in Maryland. American Potato Journal, vol. 66: 495-501.

Reigner, Nathan. 2006. Potential chemical controls & crop protection
industry contacts for recovery from establishment of select agent plant
diseases in U.S. agriculture. Report from the Croplife Foundation, 1156
15^th^ Street, NW, #400, Washington D.C. 20005.

Rossman, A.Y., Britton, K., Luster, D., Palm, M., Royer, M. and Sherald,
J. 2006. Evaluating the Threat Posed by Fungi on the APHIS List of
Regulated Pests. Plant Management Network. Published 5/5/06.
<http://www.plantmanagementnetwork.org/pub/php/perspective/2006/fungi/>

Stachewicz, H. and De Boer, S. H. 2002. Pathotype determination of
potato wart from Prince Edward Island, Canada. Nachrichtebl. Deut.
Pflanzenschutzd. 54:269.

Stevenson, W.R., Loria, R., Franc, G.D. and Weingartner (Editors). 2001.
Compendium of Potato Diseases, 2^nd^ Edition. APS Press, St. Paul, MN.
Pages 46-47.

Walker, JC. 1957. Plant Pathology, 2^nd^ Edition, New York: McGraw-Hill.

Weiss, F. 1925. The Conditions of Infection in Potato Wart. American
Journal of Botany, Vol.12, No. 7: 413-443.

**Web resources**

APHIS Pest List website: <http://www.aphis.usda.gov/ppq/pestlist/>

Canadian Food Inspection Agency (CFIA)-Plant Health Division-Potato
Section
<http://www.inspection.gc.ca/english/plaveg/potpom/var/indexe.shtml> .
The list of potato varieties used commercially in Canada can be queried
by specific disease.

Canadian Phytopathological Society website:
<http://www.cps-scp.ca/pathologynews/potatowart.htm>

EPPO Workshop on *Synchytrium endobioticum*, Paris, France. December
2005:
<http://archives.eppo.org/MEETINGS/2005_meetings/synchytrium_presentations/synchytrium_workshop#pres>

Maine Department of Agriculture, Seed Potato Certification:
<http://www.maine.gov/agriculture/pi/potato/index.htm>

National Agricultural Pest Information System (NAPIS):
<http://ceris.purdue.edu/napis/maps/pstsurvey.html#pwart>

USDA National Agricultural Statistics Service 2005 potato 2005 summary:
<http://usda.mannlib.cornell.edu/usda/current/Pota/Pota-09-21-2006.pdf>

Wisconsin Seed Certification Program:
<http://www.plantpath.wisc.edu/wspcp/>