Agriculture Reference
In-Depth Information
vegetative growth tested for virus and bacterial
pathogens (postharvest testing). Laboratory test-
ing has become one of the most effective mechan-
isms to assist certification programs in making
good science-based decisions about the health of
the certified seed lots.
seed potatoes at 10.0% for viruses, with no more
than 2.0% severe, 2.0% for blackleg, 0.5% for
variety mixture, and zero tolerance for bacterial
ring rot, potato cyst nematode, potato wart, brown
rot, and potato spindle tuber viroid (UNECE, 2013b).
The term “zero tolerance” means none of the
disease or pest of interest was found during normal
visual inspections of the crop, or during routine
testing of the soil and foliage. If a zero tolerance
disease or pest is found at any time during a seed
lot's life, the lot will be de-certified, and often the
farm or certain production fields put on quaran-
tine status. Certification, however, is not a guar-
antee that the lot inspected is free from any zero
tolerance disease or pest (Sather
et al
., 2013).
The Specialized Section on Standardization
of Seed Potatoes within the United Nations Eco-
nomic Commission for Europe (UNECE) has
spent many years developing tolerances and
seed certification practices to help standardize
the certification process (UNECE, 2013). To quote,
“It is a unique international frame of reference,
covering all aspects related to seed-potato certifi-
cation: (a) varietal identity and purity; (b) ge-
nealogy and traceability; (c) diseases and pests;
(d) external quality; (e) sizing; and (f) labeling”
(UNECE, 2013, p. 1). The UNECE section has
participants from most regions of the world, and
members representing all European countries,
the USA, and Canada. This is an important
framework within the world's seed certification
community because it is a global representation.
Disease tolerances
Disease tolerances are established for varietal
mixtures and disease content. They tend to be
highest in early inspections and ratchet down
for later inspections. This gives the seed produ-
cers the opportunity to remove diseased plants
from the field between inspections.
Roguing, or the removal of diseased plants
from the field, can be quite effective for certain
virus problems such as leaf roll and the mosaic vir-
uses (PVY, PVA, PVM), but less effective to non-
effective for viruses that may be latent or mild in
the crop, depending on strain or cultivar character-
istics, including certain PVY strains, PVX, and PVS
(Hane and Hamm, 1999; Whitworth
et al
., 2010;
Davidson
et al
., 2013). Additionally, roguing is in-
effective for bacterial diseases such as blackleg/soft
rot and bacterial ring rot.
During the inspections, the certification
personnel make precise plant counts (plants per
hectare) within the seed lot, and often have min-
imal count numbers like 2500 plants or 100%
of the plants grown for small lots of
1
ha or less,
or 2500 plants ha
-
1
for larger lots. Inspections
are conducted across representative portions of
the lot, during which time the number of plants
that have disease symptoms, varietal mixtures,
or other problems are recorded during the count.
The inspector may make a percentage count on
each of the problems observed and match the
percentage to the tolerances by seed classifica-
tion to ensure the lot qualifies as certified seed.
As part of the certification process, each
country has a set of standards regarding disease
and pest tolerances. For example, in the USA and
Canada, the State/National Harmonization Pro-
gram sets the maximum tolerances for certified
seed potatoes after the final field test. The toler-
ance is 1.0% for leaf roll, 2.0% for mosaic, with a
total virus content of 3.0%, and 0.5% for variety
mixture, and zero tolerance for bacterial ring rot
(USDA, 2006). In comparison, the European
Union allows maximum tolerances for certified
Postharvest testing
When lots have been found to meet the applic-
able rules, regulations, and disease tolerances,
the grower is allowed to harvest and move the
potatoes into an approved storage. Certification
officials will conduct a storage inspection to
ensure the lots have been stored in a manner
consistent with good seed practices, have appro-
priate isolation from non-certified stocks, can be
verified by location information, and have no
obvious disease issues. Also, growers will select a
subset of tubers at harvest representative of
each individual seed lot for additional posthar-
vest testing.
Evaluation of tubers for additional testing
may be carried out in a number of ways. Tubers
may have dormancy broken and be planted in a
