Agriculture Reference
In-Depth Information
Zebra chip
subsp.
sepedonicus
) and brown rot (
Ralstonia
solanacearum
race
3,
biovar 2), which is active in
temperate conditions, may develop into major
problems in potato production in the future as
trade in seed and commercial potatoes expands.
(Association between
Candidatus
Liberibacter
solanacearum and potato psyllid,
B. cockerelli
.)
A new defect of potato, “zebra chip”, was
first reported in 2007 in the USA, Mexico, and in
Central America (Munyaneza
et al
., 2007). The
disease was named for the characteristic symp-
toms that developed in fried chips (crisps) from
infected potato tubers. This defect was econom-
ically significant, causing several millions of dol-
lars in losses to potato producers and processors.
The symptoms of zebra chip in the plant resem-
ble those caused by potato purple top and psyllid
yellows diseases. Initially, it was thought there
might be an association between phytoplasmas
or viruses and the potato psyllid (Siller
et al
.,
2008), but further research indicated that the
association was between
Candidatus
Liberibacter
solanacearum and the potato psyllid (Li
et al
.,
2008; Abad
et al
., 2009; Henne
et al
., 2009;
Munyaneza
et al
., 2009; Secor
et al
., 2009).
Munyaneza
et al
. (2007) conducted experiments
to elucidate the association between the psyllid
B. cockerelli
(Sulc) (Homoptera:
Psyllidae
) and
zebra chip by exposing clean potato plants to this
insect under greenhouse and field conditions.
Potato plants and tubers exhibiting zebra chip
symptoms were tested for phytoplasmas by poly-
merase chain reaction. Potato psyllids collected
from infected potato fields were also tested. Re-
sults indicated that there was an association be-
tween the potato psyllid and zebra chip. Plants
exposed to psyllids in the greenhouse and field
developed zebra chip. In the greenhouse, 25.8
and 59.2% of tubers exhibited zebra chip symp-
toms in the raw tubers and fried chips, respect-
ively. In the field,
15
and 57% of tubers showed
symptoms in raw tubers and chips, respectively.
No zebra chip was observed in tubers from plants
that had not been exposed to psyllids, either in
the greenhouse or the field.
Epidemiology and management of
seed- and soilborne fungal diseases
A number of important potato diseases are trans-
mitted by seed- and soilborne inoculum. The epi-
demiology of these diseases has been discussed
above. Chemical control of pathogens including
C. coccodes
and
R. solani
can be inconsistent
(Wale, 2004). However, several products have
been developed specifically for the control of
seedborne fungal diseases of potato (Atkinson
et al
., 2010) and offer broad-spectrum control
for Rhizoctonia, silver scurf, and Fusarium dry
rot, and to some extent black dot. These seed
treatments include fludioxonil and flutolonil for-
mulations with or without mancozeb. The gen-
eral impact of these seed treatments is noted in
improved plant stand and crop vigor, but occa-
sionally application of seed treatments in com-
bination with cold and wet soils can result in de-
layed emergence. The delay is generally transient
and the crop normally compensates. The add-
itional benefit of the inclusion of mancozeb is for
the prevention of seedborne late blight (Wharton
et al
., 2012b). Applications of fungicides in-furrow
at planting have resulted in significant improve-
ment in Rhizoctonia disease control in potatoes.
Fungicides such as flutolonil and azoxystrobin
applied in-furrow at planting have given consist-
ent and excellent control of Rhizoctonia diseases
of potatoes (Atkinson
et al
., 2010, 2011).
Epidemiology and management
of foliar diseases
Other bacterial diseases
Late blight
Several other diseases and disorders may also be
associated with bacterial pathogens such as stem
rotting, initially caused by
Botrytis cinerea
and
Sclerotinia sclerotiorum
, and the disorder toxic seed
piece syndrome (Johnson, 1991). Although not
covered in this section, bacterial diseases such
as bacterial ring rot (
Clavibacter michiganensis
Potato late blight (
P. infestans
) is considered the
most important disease affecting potato world-
wide, and is also a major disease of tomatoes
(Fry and Goodwin, 1997b; Guenthner
et al
., 2001).
The resurgence of late blight has occurred des-
pite the introduction of improved fungicides and
