Agriculture Reference
In-Depth Information
when tuber pulp temperature is in the range of
7-
18°C, to eliminate temperature gradients that
can promote the development of condensation,
which in turn enhance the conditions that are
conducive for the development of soft rot. Some
other important factors that reduce the risk of
soft rot developing during the early storage
period were described by Knowles and Plissey
(2007) and included:
acidiscabies
, and
Streptomyces
turgidiscabies
. In
particular,
S. scabies
has been well documented in
causing scab lesions (Loria
et al
., 1997, 2006;
Wanner, 2005, 2006, 2009; Bignell
et al
., 2010).
S. scabies
infects a number of root-grown crops
(Loria
et al
., 1997), including radish (
Raphanus
sativus
), parsnip (
Pastinaca sativa
), beet (
Beta
vulgaris
), carrot (
Daucus carota
), as well as potato
(
S. tuberosum
L.). The disease may be found
wherever potatoes are grown worldwide. Since
the marketplace for potatoes is quality driven,
the presence of scab lesions, especially those that
are pitted, significantly lessens the marketability
for both table-stock and processing varieties. The
genus
Streptomyces
is also noted for its ability to
produce medicinally important antibiotics. More
than two-thirds of naturally derived antibiotics
currently in use today are produced as second-
ary metabolites by
Streptomyces
species.
The symptoms of common potato scab are
present on the surface of the potato tuber. The
disease forms several types of cork-like lesions
including surface, raised, and pitted lesions
(Fig. 11.5
). Sometimes, surface lesions are also
referred to as russeting, particularly on round
white tubers, as the general appearance resem-
bles that of a russet tuber skin. Pitted lesions
vary in their range of depth, although on aver-
age they extend
3-
4 mm deep. Infection begins
with the onset of tuberization. Once the patho-
gen penetrates through several layers of the cells
of a young tuber, it is able to derive its nutrient
needs from the dead cell material it has infected.
This becomes the source of a scab lesion. Under
extreme infection, a noticeable reduction in vine
growth and yield may be observed.
Potato common scab is a saprophyte that
may overwinter either in soil or on the surface of
tubers, thereby serving as next year's inoculum
source
(Fig. 11.6
). The inoculum is spread by
water, wind, on seed tubers, and on farm equip-
ment with leftover soil residue. Once present in
the soil,
S. scabies
may survive indefinitely, since
it finds refuge in organic matter. The disease is
known to persist in soils with a pH between 5.2
and 7.0. There are other species that may sur-
vive and function at even lower pH.
S. acidiscabies,
as its name implies, has the ability to cause
symptoms identical to those of
S. scabies
at a pH
as low as 4.5.
S. scabies
has branched mycelium, and spor-
ogenous hyphae develop into corkscrew-like spiral
•
Limiting the pile size to a height of
5-
5.5 m.
•
Quickly cooling the tubers to the final stor-
age temperature (
3,
5,
7,
and 10°C for seed,
table-stock, French fry and chip (crisp) pro-
cessing, respectively).
•
No humidification.
•
Fans run to dry the tubers as much as
possible.
•
Daily monitoring for high-risk areas with
elevated temperature and/or moisture.
The effective management of soft rot en-
sures that tubers destined for consumption are
high quality and profitable. Healthy potato seed
tubers are vital for sustaining the continued supply
of potatoes between seasons (Toth
et al
., 2003).
Potato common scab
The potato crop is produced mainly for fresh,
chipping, frozen, or starch markets. The fresh
and chipping markets require a high quality in
finished appearance. Recurrent and persistent
bacterial soilborne diseases have long plagued
potato production systems, such as common
scab (PCS,
Streptomyces
spp.; Stevenson
et al
.,
2001). Typically, PCS impacts tuber quality and
marketable yield, but in seed can cause poor
plant vigor, or even death (Stevenson
et al
.,
2001; Loria
et al
., 2006; Wanner, 2009). PCS is
an annual production concern for commercial
potato growers (Loria
et al
., 1997; Slack, 1992)
and has been identified as the highest priority for
research, and as an intractable and endemic
problem by potato research and commodity
groups.
Of the more than ~800 identified species in
the genus
Streptomyces
, only a fraction are con-
sidered plant pathogenic. PCS may be caused by
several soil-dwelling plant pathogenic bacterial spe-
cies in this genus (Wanner, 2007; Hao
et al
., 2009).
These include
Streptomyces scabies
,
Streptomyces
europeaiscabiei
(Wanner, 2009),
Streptomyces
