Agriculture Reference
In-Depth Information
Table 17.8
The Most Economically Important Storage Diseases of Onions.
Name of disease
and organism
Symptoms
Biology and spread
Conditions favouring disease
Control methods
Neck rot
Botrytis allii
(
B. byssoidea and
B. squamosa
can
cause similar damage
but are less common
in storage.)
Grey-brown rot of flesh often
starting in neck area, not
usually detected at harvest
but shows up within 3
months of storage. Grey
mould on the surface
followed by large black
sclerotia.
Seed transmitted and may
also survive in field
rubbish and cull piles to
infect subsequent crops.
Latent in onion plants in
field; develops after
harvest. Can survive
unnoticed from the
seedling stage.
Untreated seed; cool wet field
conditions especially before
and at bulbing. Failure to cure
bulbs using heat. Coloured cvs
are more resistant than white
skinned ones. Can develop at
low temperatures but storage
at 0°C and 65-75% RH slows
development of the fungus.
Seed treatment with
benomyl + thiram. After
harvest, a relatively short
heat treatment at >25°C
to dry neck and kill
latent fungus in the bulbs
is effective.
Good ventilation and low
RH in storage
discourages sporulation.
Grow resistant cultivars
if possible.
Basal rot
Fusarium
oxysporum
f.sp.
cepae
A slowly spreading rot
infecting the base plate in
the field, later shows as
white mycelium and a
watery rot that spreads
upwards from the base in
the bulb.
Soil-borne disease which
may also be carried on
seed. Often associated
with pink root rot,
Pyrenochaeta terrestris
.
May survive on
weed hosts.
Warm conditions (14-32°C) in
soil and in store favour the
disease, with optimum
temperature at 26-28°: more
severe in Mediterranean
countries than in cooler ones.
Lack of rotation of onion crop.
Remove infected bulbs
before storing. Cool
storage at <15°C slows
disease development.
Black mould
Aspergillus
niger
(
A. fumigatus
, blue-green
mould, and
A.
alliaceus
, yellow
mould, also occur at
even higher
temperatures.)
A black powdery mould on or
between outer bulb skins
after a few days or weeks.
More common on veins
than between them. Can
lead to soft rot of the bulb
under hot humid
conditions.
Partly seed-borne and can
survive in plants until
harvest. Very common as
a saprophyte on field
debris and rubbish
around stores especially
in hot countries. May be
more severe on coloured
than on white bulbs.
Favoured by high humidity and
temperatures of 27-35°C, and
can develop on bulbs if curing
stage lasts too long at high
temperatures. Commonly
develops on stored onions in
the tropics if curing and
ventilation are inadequate.
Seed treatment may help to
reduce seedling damage
and storage problems.
Proper curing and good
ventilation in store help.
Cold storage should
prevent it from becoming
troublesome.
