Tipburn
Tipburn is the marginal collapse and necrosis, at or
near the leaf margins. The disorder can occur outside or inside the
head. Tipburn is a calcium related discorder and is associated with
low levels of calcium in leaf tissue. Warm temperature, excessive
fertilization and increase in light intensity, and other factors
that contribute to rapid growth of lettuce, can enhance the
development of Tipburn.
 Other factors that reduce the uptake of calcium
such as high salt
concentrations and high
humidity can intensify
the problem. Good
cultural & management
practices which reduce
rapid and excessive
plant growth, limit the
incidence of Tipburn.
Additional applications of calcium fertilizer are generally
ineffective on crisphead lettuce.
 Bolting
Bolting is the premature
elongation of the stem,
due to premature
initiation of the
reproductive (flowering)
process. Head formation
will be prevented and
the plant will not be
harvested. Resistance to bolting is inherited, and it is therefore
possible to select
appropriate varieties for
climate zones and seasons. A
variety grown in the season
and location for which it was
bred usually does not bolt.
However, if temperatures are
higher than usual for that
region, bolting can occur.
Downy mildew
Cause: Caused by the fungus Bremia lactucae.: Bl. Symptoms: Light green patches
develop on leaves, which turn yellow then brown.
A fluffy growth can often be
seen on the underside of
the leaves below the
discoloured patches.
Source of infection: The
primary source of infection
is from spores in paddock
debris, spores from wild
lettuce weeds and from
nearby fields. Control: Use resistant
varieties. When growing a variety that is susceptible to
downy mildew, the strategic use of fungicides is the best
option. Both protective and curative (systemic) fungicides
are available.
Sclerotinia drop
Cause: Caused by the soil borne fungi Sclerotinia minor
and S. sclerotiorum.
 Symptoms: The early signs of the
disease are wilting and browning of leaves followed by
eventual death. Source of infection: Both species produce
hard, black reproductive survival structures called sclerotia. These can be seen on the
lower leaves and at the base
of lettuce infected by S. minor
and higher in the head when
infected by S. sclerotiorum.
Excess rainfall or irrigation
which results in soil moisture
saturation predispose lettuce
crops to drop. Control:
Rotation with non-host crops
helps in controlling both types
of fungi. Drip irrigation has also been shown to reduce
levels of drop.
Big Vein Cause: Big Vein or Mirafiori lettuce virus: MiLV.
Symptoms: The lettuce develops leaves with large veins
that appear stiff with ruffled margins. Depending on
 the
severity of infection, some hearts will make it through to
harvest, whereas other hearts are stunted and not
harvestable. Cooler weather
and low light levels favour
lettuce Big Vein development.
Source of infection: The
soil- borne fungus, Olpidium
brassicae spreads this virus.
The fungus itself has no effect
on the lettuce crop and can
survive in the soil for many
years. Control: There is no
chemical control available for
Big Vein. Irrigation management to reduce water logging
will help reduce the likelihood of this disease. Where
possible, growers should use varieties that show a useful
degree of resistance.
Corky Root Cause: Caused by the corky root bacterium, Rhizomonas
suberifaciens. Rs. Symptoms: Early symptoms of corky
root are yellow bands on tap
 and lateral roots of lettuce
seedlings. These yellow areas gradually expand, taking on
a green brown colour and developing cracks and rough
areas on the surface of the root. As disease severity
increases, the entire tap root may become brown, severely
cracked, and non-functional; the feeder root system will also
be reduced and damaged.
When the root is severely
diseased, aboveground
symptoms consist of wilting
during warm temperatures,
stunting of plants, and
general poor and uneven
growth. Control: Rotate
crops out of lettuce; do not
grow lettuce consecutively.
Avoid over fertilizing with nitrogen fertilizers. Some corky
root resistant cultivars are now available. For corky root
infected crops, growers may need to add additional fertilizer
and water in order to bring the crop to maturity. High, welldraining
beds may sometimes reduce
corky root severity. |
