Compendium of Potato Diseases - (PDF, 101 mb) - USAID

identifying tissue bruised by black spot without rupture of the
surface, nor is the reaction obtained after wounded tissue has
hea led.
Corntrol
I) Little can be done to avoid cracking during the growing
period except by judicious irrigation, fertilizer application,
plant spacing, and cultivar selection.
2) Delay harvest until vines have been dead for some time and
tuber periderm has matured. Avoid harvesting from cold soil.
3) Avoid sudden impact on tubers, and protect them from
rapid drying after digging and during transit from field to
storage.
Fig. 17. Cracking: A,crack caused by impaction of turgid tuber
during digging; B, growth crack that healed over before digging.
- . •.
Selected References
IRIIANI. W. M. 1968. [he use of catechol for enhancing bruise
detection. Am. Potato J. 45:312.
PAINTER. C.G..and .. AUiUS I N. 1976. the effect of soil moisture
and nitrogen on yield and quality of the Russet Burbank potato.
. ' ,: L ', jP
.,
Am. Potato .1.53:275-284.
SM IT I.E., 1). A., R. E. TIIORNION. C. IL.PETERSON, and B. B.
IFAN. 1974. ltarscsting potatoes with minimum damage. Am.
Potato .1.51:152-164
WER NER. II.0.. and .1.0. I)FT T. 1941. Reduction of cracking of late
crop potatoes at harvest time b\- root cutting or vine killing. Am.
ota t o .1. 18: 189- 208.
(Prepared by W. .1. Hooker)
Blackspot
Fig. 18. Harvest or thumbnail cracks following mild bruising and
surface drying. (Courtesy W. C. Sparks) Blackspot is alays caused by bruising injury. either from
ctied bfotr hirvcst o requetit wound heal aid. as tubers
impact during harvest, handling, and grading, or from pressure
during storage. File
and
disorder
northern Europe
is well known
and has
in
become
North America
atn increasingly serious
consttl to groi. becotie relanets shallow a nd of little problem in most potato-growing areas that have adopttd
contisequtenice (Fg. 17131. Wound-healed cracks seldom become mechanical harvesting and handling techniques.
infected. Potato culti\ars differ in susceptibility to injury.
(Itorwth cracks may also develop in tubers of plants with tile Symptoms
yello\\ dwarf \irus. potato mop-top virus, or certain strains of Blue-gray to black discolored areas develop just beneath
the
the
spindle tuber viroid. tuber skin (Fig. 19). Internal symptoms do not appear
Mecha nical cracking during harvesting may follow sudden immediately after brusing but develop to full intensity
impacts
over
(Fig.
a
17A). Cracking is dependent upon varietal period of 1-3 days as flattened, spheroidal blue-gray patches
response. tuber maturity, internal tuber turgor, and degree of centering in the vascular region. Margins are diffuse and grade
mechanical cmnpressiin during Iaiest and iovetntient into into the unaffectcd tissue. Blackspot is usually
storage.
more
Imutture
noticuablc
tubers and large tubers are most easily at the stolon end of the tuber than at the apical
inilured.
end. Tubers
Severit\
with
is greatest when tuber temperatures are low internal blackspot frequtentilv show no external sylmptols.
and tissue is turgid. Iu bers s\ith high internal turgor are easily Melanin is present on intracellular
cracked
protoplast
to a depth
surfaces
of 5
and
min or
on
more. Extreme turgidity results inner wall surfaces of affected cells. The absence of wound
\when soil moistuire le\cls are hlgh :atd tuts contitnue to function periderm in lesions is characteristic of blackspot
after
and helps
\ines
to
hase been suddenly killed by frost, by herbicides, or differentiate this disorder from other internal defects such
b\ harvesting
as
tubers when vines are green. Root pruning. internal brown spot, heat necrosis, and certain internal lesions
undercutting, or prc-har\est vine killing reduces incidence of caused by pathogens.
cracking. I)eiayingdigging fora few htlrsCarhlin thledaV until
the soil is %\ar ied may also reduce cracking considerabyh. Histopathology
Seserel\ cracked tubers are of little %,alue because wound Bruising injury initiates a series of biochemical oxidations
healing
in
is incon,pletc, deh.ydration is rapid. and incidence of damaged cells. Phenyl substrates such as tyrosine are oxidized
tuber rot may be high. Paradoxically. shatter bruising becomes to conjugated quinones by polyphenol oxidases. The
no10
quinones
se crc \kith high tuber turgor and black spilt intensifies polymerize to produce the black pigment. Oxidation reactions
%%11 lth Io tuber turuor.
are usually completed within 24 hr of bruising. and spots neither
Ilaivest cracks are crescent shaped. resembling cracks made enlarge nor disappear during subsequent storage.
with a 0humbriail. lhrev are usually shallow, 1-2 Inm deep. aInd
result rom ough handling and drying of the tuber surface Epidemiology
tissue after digging. particularly when tubers are turgid (Fig. Severity if' blackspot is determined by both the number of
18). Severity 10 injury depends on intensity of bruising and damaged
rapidity
cellsand
ol deh.dration.
the amount
Direct
of melanin
harvesting
produced
by iachinery
in each one.
often An impact will damage more cells in susceptible tubers
reduces
than
incidence
it
of harvest cracks as compared to harvesting will in resistant ones, and larger, deeper spots will form. Tuber
by lifting tubers to the soil surface and gathering them later, susceptibility is influenced
[Hie
by a
catechol
number of
test
factors.
(I- 1.5"1 practical grade) reveals areas of Tubers with low turgor pressure arc more likely
injury
to
by
have
turii ig
severe
them dark red to purplish within 3-5 min of blackspot. Iherclore, blackspot is usually nmore serious
treatment.
after
Catcchol,
a
a polyphenol, is oxidized by phenolase, dry growing season in nonirrigated growing areas. Conditions
which isreleased from recently broken cells. It is net effective in such as low soil moisture, poor root development, or hot dry
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