Fungal and Bacterial Disease Aspects in Potato Production - The Potato: Botany, Production and Uses - page 172

introduced, the range of management options
may decline because of quarantines that could
be imposed by government agencies. Examples
of this include potato wart caused by Synchytri-
um endobioticum (Franc, 2001), bacterial wilt
and tuber brown rot caused by R. solanacearum
race 3 biovar 2 (Norman et al ., 2009), which can
be introduced on imported cuttings of Geranium
(Swanson et al ., 2007), and more recently, Dick-
eya spp. in Scotland (Toth et al ., 2011). As many
diseases have tuber-borne phases, the increase
in trade between countries enhances the risk of
introducing new pathogens or genotypes of ex-
isting pathogens and the chance of pathogen
mutation. Seed certification and importation re-
gulations are therefore the first line of defense
against the introduction of new diseases. How-
ever, the increase in trade, coupled with the
decrease in investment in inspection services,
reduces the likelihood of intercepting infected
potatoes at ports of entry. Between 2010 and
2011, funding for inspection in the USA was cut by
US$73 million, and in 2012 it was cut by US$39
million ( http://www.obpa.usda.gov/budsum/FY12
budsum.pdf ) .

recertification. As an example, in Michigan,
USA, a limited seed tuber generation program
was adopted in 2002 and used tissue culture
material (Wang and Hu, 1985). The seed lots
entered for certification in Michigan are tested
in the winter in Hawaii, USA. On passing this
test, the seed lots are evaluated by Michigan De-
partment of Agriculture and Rural Develop-
ment (MDARD) seed inspectors prior to ship-
ping seed tubers from the Michigan seed farms.
The seed lots that pass this stage achieve “blue
tag” certification (US Standards for Seed Potato
Grades Regulation No 628) from the Michigan
Seed Potato Association (MPSA, PO Box 1865,
Gaylord, MI 49734, USA) and the MDARD.
Seed and commercial potato growers receive
seed as whole tubers to grow for recertification
as seed or for commercial production. Similar
systems of seed for the production of tubers for
processing, starch, ware, or table-stock are
used throughout the world (Janse and Wennek-
er, 2002; Simakov et al ., 2008; Hirpa et al .,
2010) and have been reviewed extensivley by
Frost et al . (2013). However, much of the
less-developed world, including China, a very
large producer, still does not have countrywide
certification programs, and the cost of using
certified seed is too high for most growers.
There are concomitant yield losses due to
increased pathogen (especially viral) load in
repeatedly saved seed.
As seed lots are grown in fields then stored
between growing seasons, the tubers are ex-
posed to soilborne diseases that can remain la-
tent on the seed. Seed tubers and daughter
tubers alike are exposed to pathogens that are
both seed- and soilborne, as well as propagules
of pathogens that are washed from the foliage
into the soil. Seed tubers are particularly vul-
nerable to diseases when they are cut into seed
pieces. Several studies have shown that infec-
tion can occur during seed cutting in the case
of potato late blight (Lambert et al ., 1998; Kirk
et al ., 1999; Glass et al ., 2001). The pathogen
can also be transmitted from the seed piece to
cause disease on the stems, stolons, and daugh-
ter tubers in the case of Rhizoctonia stem can-
ker (Secor and Gudmestad, 1999; Tsror, 2010)
and prevent emergence through the sprout rot-
ting in the case of Fusarium dry rot (Wharton
et al ., 2007b) and Rhizoctonia diseases (Woodhall
et al ., 2007).

Seed certification

Pathogens may infect foliage, roots, stolons,
and tubers; therefore, disease management is
an important factor throughout the crop cycle.
Although true seed is used increasingly in some
developing economies (Ierna and Tenorio, 2011),
vegetative seed tubers produced through meri-
stematic stem culturing systems and further
propagated by aeroponic or hydroponic systems
(Chang et al ., 2012) are the main initial source
of propagation material for commercial potato
production. Within the tissue culture system,
plants are produced under aseptic conditions,
tested for various bacterial and viral diseases,
and transplanted into greenhouses to produce
the nuclear class of seed, or what is commonly
known as minitubers. This involves meris-
tem-derived tissue culture seed stock develop-
ment, after which minitubers or tissue culture
plantlets are planted into a field(s) as the initial
source of certified seed potato lots. The seed lots
are then multiplied and increased to reach a
sufficient quantity for commercial use for up to
six generations (Field Year 1-6 Class Seed),
after which the seed lot is no longer eligible for