Agriculture Reference
In-Depth Information
White rot
Sclerotium
cepivorum
A white mycelium affects the
roots and base of the bulb,
followed by small pin head
size black sclerotia.
A long-lasting soil-borne
fungus requiring very
long rotations once
established. However
some fields may have
some natural biological
control agents. Easily
spread by workers,
machinery or animals.
Favoured by moderate
temperatures of 10-20°C, can
kill plants in field or start
rotting infected bulbs in store.
Long rotations; use of
special preparations to
induce germination of
the sclerotia has shown
some promise. Some
protective chemicals
available.
Blue-green fungus
Penicillium
spp.
Fluffy coating of blue-green of
greyish fungus on the outer
damp skins of onion bulbs
especially after transport in
damp conditions.
Very common saprophytic
fungi which can attack
outer skins of onions if
they are damp for a
prolonged period.
High RH of the air and lack of
ventilation favour these fungi.
Drying and shedding of the
outer skins usually gets
rid of the unsightly
fungus coating.
Bacterial rots of onion:
several species
including
Burkholderia
(Pseudomonas)
alliicola, B. (Ps).
cepacia, Ps.
aeruginosa, Ps.
viridiflava, Erwinia
spp., Lactobacillus
spp., Pantoea
ananatis,
Xanthomonas spp
.
Symptoms of the various
bacterial infections are soft
rots called 'slippery skin',
'sour skin', 'brown rot,',
'bulb rot' 'centre rot' etc.
Often only the outside
fleshy rings or certain inner
rings are affected by the
rots. May be accompanied
by odours which can be
diagnostic.
Sometimes bulbs in store rot
then dry and shrivel.
Some species suspected to
be seed-borne. Often
spread between plants by
rain splashing, may enter
through surface wounds
including at harvest and
by topping blades. Some
species are harboured by
common field weeds.
Some bacteria can spread at a
wide range of temperatures
(5-41°C) but in general warm
or hot (>25°C) and damp
conditions favour bacterial
rots. Under very hot climates,
other normally saprophytic
bacteria and yeasts can also
attack onion tissues. Care to
avoid damage, rotations,
cleanliness in field operations
are all recommended.
Overhead watering should
be avoided in the run-up
to harvest.
Copper fungicides,
sometimes with
antibiotics (if permitted)
have been used, but
bacterial diseases are
hard to control especially
in wet seasons. Less of a
problem in dry climates
and dry season crops.
If possible, allow foliage
to dry completely before
trimming, to prevent
spread. Good weed
control removes the
alternative hosts.
Cool storage slows
development of bacterial
infections.
Sources: Chupp and Sherf (1960), Anon. (1986), Maude (1990), Snowdon (1991), Hayden and Maude (1997) and Mark
et al
. (2002).
