Control of major onion and garlic diseases
O
nion and garlic are susceptible
to basal rot, white rot, purple
blotch, and bloat nematode. Below
are techniques to combat these diseases.
Basal rot
This disease is
primarily caused
by a soilborne
fungus. Most
infections occur
Fig. 1A. Fieldthrough wounds infected garlic
or old root scars
showing leaf
at the base of the dieback. (Photo:
bulb. Initially, the OMAFRA, Canada)
disease appears in
mid- to late-season
as yellowing
leaves and stunted
growth, followed
by leaves drying
Fig. 1B. Stored
up from the
garlic cloves
tip downward
showing basal
(Fig. 1A). Roots
rot from the field.
become pink
(Photo: MM
and start to
Rahman)
rot continuing
through the bottom of the bulb.
Postharvest rotting may affect one
clove or all cloves in the bulb (Fig. 1B).
Removing the outer leaf bases shows
soft, black, partially or fully rotten
cloves.
Control. Use disease-free seeds,
and minimize injury from cleaning
and weeding or soilborne insects and
nematodes. If done carefully, hot water
treatment decreases the contamination.
Wait three to four years before using
a field that had basal rot.
White rot
Infection of the scale stem plate and
roots is caused by a soilborne fungus
resting structure, known as sclerotia
(small spherical black bodies) (Fig. 2).
Leaf bases show semi-watery decay,
and older leaves turn yellow and wilt.
Mycelium (network of white filaments)
often develops
around the bulb
base.
Control. Once
soil is infested,
it is hard to use
Fig. 2. Visible
the field to grow sclerotia on
onion or garlic
onion infected by
without chemical white rot. (Photo:
treatment.
OMAFRA, Canada)
Applying natural
Allium or artificial products (e.g.,
diallyl disulfide) before planting
stimulates sclerotia germination.
Without a host, the mycelium from
sclerotia persists for up to several
weeks. After exhausting nutrient
reserves, sclerotia die.
Purple blotch
This is a fungal
disease. Initial
infection
produces small
water-soaked
Fig. 3. Purple blotch
spots on leaves.
on onion leaf with
Over time, these
zonate lesions.
spots turn brown
(Photo: www.
and enlarge
forestryimages.com)
to form zonate
(target) areas with a reddish-purple
margin surrounded by a yellow zone
(Fig. 3). In moist weather, the diseased
tissue may look moldy.
Control. Early disease detection and
fungicide spray control the disease.
Use single-site active products (e.g.,
Fenamidone @ 5.5 fl. oz. ܈