Fungal and Bacterial Disease Aspects in Potato Production - The Potato: Botany, Production and Uses - page 177

tubers (Wale et al ., 2008). Although soilborne
inoculum appears to be more important than
seedborne inoculum in black dot disease dissem-
ination, infected seed is the principal way the
pathogen is introduced into uncontaminated
soil in the first place. As a result, in some coun-
tries, table-stock (ware) producers have imposed
tight seed tolerances on the level of black dot in-
fection in order to limit soil contamination.
Rhizoctonia diseases of potato are caused
by the fungus, R. solani Kühn (teleomorph Than-
atephorus cucumeris (A.B. Frank) Donk), and can
be found on all underground parts of the plant
at different times during the growing season.
R. solani has many synonyms and is divided into
subgroups called anastomosis groups (AGs), in
which isolates are categorized according to their
ability to anastomose with one another (Johnk
et al ., 1993; Carling et al ., 2002; Woodhall et al .,
2007). Rhizoctonia isolates can be assigned to
one of thirteen AGs; AG-3PT is the group of
Rhizoctonia isolates that is most commonly as-
sociated with disease in potato. AG-3PT is rela-
tively specific to potato, and sclerotia on tubers
belong almost exclusively to this AG. However,
AG2- 2 IIIB and AG4 HGII, which are more com-
monly associated with sugarbeet crown and root
rot, have recently been shown to cause stem
canker on potato stems in Idaho and Michigan,
USA (Woodhall et al ., 2012). AG2- 1, 4, 5, and 8
have also been isolated from potato in the UK
(Woodhall et al ., 2008; Ritchie et al ., 2009).
R. solani causes black scurf on tubers ( Fig.
11.1 ) and stem and stolon canker in potatoes,
and occurs wherever potatoes are grown. How-
ever, R. solani causes economically significant
damage primarily in cool, wet soils (Banville,
1989). Losses from Rhizoctonia are sporadic and
only occur when the weather is cold and wet in
the weeks following planting. In northern areas,
where growers must often plant in cold soils,
Rhizoctonia is a more consistent problem (Bandy
et al ., 1988). Poor stands, stunted plants, reduced
tuber number and size, and misshapen tubers are
characteristic of diseases caused by R. solani . The
symptoms of the disease are found on both
above- and belowground portions of the plant.
Black scurf ( Fig. 11.1 ) is the most conspicuous
sign of Rhizoctonia disease. In this phase of the
disease, the fungus forms dark brown to black
hard masses (sclerotia) on the surface of the
tuber. These structures are the resting bodies of

the fungus. Sclerotia are superficial and irregu-
larly shaped, ranging from small, flat, barely vis-
ible blotches to large, raised lumps. Although
these structures adhere tightly to the tuber skin,
they do not penetrate or damage the tuber, even
in storage. However, they will perpetuate the dis-
ease and inhibit the establishment of potato
plants if infected tubers are used as seed. Cur-
rently, it is not possible to control Rhizoctonia dis-
eases completely, but severity may be limited by
following a combination of cultural and crop
protection strategies. Effective management of
this disease requires the implementation of an
integrated disease management approach and
knowledge of each stage of the disease. Although
the most important measures are cultural, chem-
ical controls should also be used.
The potato early die complex (PED) has
been recognized as a key limiting factor in potato
production since the early 1970s (Davis et al .,
1972) and remains a challenge in current pro-
duction (Davis et al ., 2010a), especially in the
sandy soils of north central USA (MacGuidwin
and Rouse, 1990; MacGuidwin et al ., 2012).
This infectious disease is caused by an inter-
action between the root-lesion nematode ( Praty-
lenchus penetrans ) and the Verticillium wilt fungus
( Verticillium dahliae) (Davis et al ., 1972), and is
often complicated by interaction with other
pathogens such as the black dot pathogen
( C. coccodes ) (Davis and Howard, 1976; Azad
et al ., 1985; Barkdoll and Davis, 1992) and fer-
tility regimes (Davis et al ., 1990, 1994). In vari-
ous studies, the impact of green manures and
maize has resulted in suppression of PED in
Idaho (Davis et al ., 2010a,b), and may be useful
in other regions. Yield losses range from 10 to
50% and about half of Michigan's potato acre-
age is known to have the problem. Throughout the
years, PED has been managed with soil fumiga-
tion ( 1,3- D or metam) and non-fumigant
nematicides (aldicarb, oxamyl, ethoprop, or
phenamiphos), while chloropicrin is used in
Wisconsin. Currently, metam sodium applied at
a nematicidal rate is used extensively for PED
control (MacGuidwin et al ., 2012). Crop rotation
has not been successful for PED management,
and multi-year searches for PED-resistant ger-
mplasm have also not been successful. Currently,
there is an urgent need to identify new PED
management practices for use in potato produc-
tion because of the cost of soil fumigants, their