Agriculture Reference
In-Depth Information
secondary fungal growth (Plate 16.3). Eventually infected
tubers shrivel and mummify. The disease may arise from
infected seed or soil inoculum, but wounding at harvest
leads to dry rot development in storage. The rot is generally
promoted by warm, dry conditions and warmer storage
temperatures. Dry rot is minimised by ensuring good skin
set before lifting, preventing damage, good store hygiene,
promoting wound healing and the application of post-
harvest fungicide.
drained soils and the disease can progress very quickly
during warm weather. The disease can spread to healthy
tubers during harvest and storage. Infected tubers exude a
clear, watery liquid. The skin turns dark, the tissue remains
intact with a firm texture. When infected tubers are cut open,
a pinkcolour develops within 30 minutes, later turning black.
A line demarcates healthy and infected tissue (Plate 16.6).
Pink rot in storage is often accompanied by a distinctive
odour. Fungicides are available for pink rot control. If pink
rot infected areas of fields are identified or suspected, tubers
need to be marketed directly after harvest. If this is not pos-
sible, they should be stored separately from healthy tubers.
Gangrene
Gangrene is caused by Phoma spp . in particular Phoma
exigua foveata , in Europe and Australia and the closely
related, less virulent species P. exigua var. exigua being more
widespread throughout potato growing areas of the world.
Severe losses can occur during storage, particularly when
the potato crop has been harvested during cold weather.
Symptoms on tubers appear as small dark depressions
associated with wounds, eyes or lenticels enlarging into
sunken areas. Below the superficial depressions there may
be extensive internal rotting and well defined infected
areas of dark tissue with extensive cavities (Plate 16.4).
Infection of tubers occurs mainly through mechanical
damage at harvest and grading. Storage at low temperature
encourages rot development. Cold and wet conditions
during handling encourage disease development, so control
of temperature and humidity is important. Treatment at
harvest with a fungicide may lessen subsequent disease
development in store.
Pythium “leak”
Leak is caused by Pythium spp., principally Pythium
ultimum . The description “leak” comes from the extreme
wetness of rotted tissues; gentle pressure of a rotten tuber
will result in a clear liquid being expelled. In less advanced
stages, infected tissue may only be seen when tubers are
cut  to reveal an internal grey/brown/black rot whereas
the outer cortex of the tuber may be intact (Plate 16.7). There
is usually a distinct line between healthy and diseased tissue.
Frequently the tubers may be secondarily infected particu-
larly by soft rot bacteria. The fungus is present in many soils
and can survive over winter in plant debris. Typically tubers
become infected through wounds so minimising tuber dam-
age is important to avoid infection. Grading out infected
tubers prior to storage reduces spread to healthy tubers.
Rubbery rot
Rubbery rot ( Geotrichum candidum ) is a soil borne patho-
gen. Infected tubers have irregular brown patches on skin
with dark margins with occasional white tufts of mycelia.
If cut open the flesh has a rubbery texture and turns a dirty
pink in about four hours (Plate 16.8). There are no haulm
symptoms and usually only a few tubers per plant are
affected, which is why the problem is often overlooked
until storage. A fishy smell and dark exudates are typical
characteristics of infected potato. It is usually only a prob-
lem in exceptionally warm and wet conditions. Prevention
is best effected by improving field drainage.
Late blight
Late blight, one of the most devastating diseases of potatoes,
is caused by the oomycete Phytophthora infestans .
It is capable of infecting all parts of a potato and tubers can
be infected if exposed near the soil surface or if spores are
washed from infected aerial tissues into the soil. Infected
tubers tend to have a rusty brown, dry rot extending inwards
from the skin, the tissues being firm to the touch (Plate 16.5).
Generally P. infestans survives only in living host tissue
although the sexual spore stage can survive dormant in soil.
Eventually, the infected tubers become secondarily infected
with bacterial rots that can spread, tuber to tuber, in store.
Follow a recommended fungicide spray program prior to the
arrival of the pathogen. Local or national crop protection
agencies should be consulted for registered fungicides.
Tuber Blemish Diseases
Blemish diseases are pathologically minor fungal infec-
tions of skin but, where potatoes are sold washed, they
affect appearance and therefore impact on crop value.
Pink rot
Pink rot is caused by the oomycete Phytophthora
erythroseptica, a soil living organism which can infect sub-
terranean parts of potato. The pathogen thrives in poorly
Black dot
Colletotrichum coccodes , the fungal agent of black dot is
widespread in soil and is a weak pathogen of many plant
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