Agriculture Reference
In-Depth Information
LEAF SCORCH
■
Source of infection and spread
The main sources of infection are diseased ratoon crops or
undecomposed strawberry residues in the soil. The fungus
may also be introduced on diseased runners. Spores
produced on the spots are spread by rain splash and wind.
Warm, wet weather favours the disease.
Cause
The fungus
Diplocarpon earlianum
.
Symptoms
The fungus attacks old and middle-aged leaves in
preference to young leaves and progresses rapidly on
the oldest leaves. Scorch shows first as small, purple
spots on the outer, older leavers of the plant; the spots
gradually enlarge until they are up to 6 mm in diameter.
They either remain a uniform, dark purple colour, or
dry out in the centre to a dark brown. The centre never
produces the light colour that is characteristic of
eye spot.
Numerous blotches may eventually join, and the entire
leaf becomes purplish to reddish. Dark, glistening,
fruiting bodies (acervuli) of the fungus develop thickly
over the upper surfaces of the spots. The margins of
affected leaves subsequently curl, giving the leaves a
scorched appearance.
Under heavy leaf scorch infection, typical symptoms
develop. Many small blotches may form and join into large,
irregular, purplish-to-reddish areas covered with minute
greyish dots. These symptoms should not be confused with
those produced by eye spot.
On leafstalks and fruit, scorch appears as
elongated, sunken streaks. If leaf and fruit stalks
are encircled completely by scorch, the leaves and fruit
will die.
Importance
Leaf scorch is usually a minor disease in well-managed
crops and is rarely seen.
Management
•
Reject severely affected runners and remove diseased
leaves from less-affected plants before planting.
Do not plant strawberries in low-lying or shady areas.
•
Use the recommended spray program.
•
LEAK (RHIZOPUS ROT)
■
Refer to the description of transit rot (Rhizopus
soft rot) in
the chapter on Common diseases of perennial fruit crops.
POWDERY MILDEW
■
Cause
The fungus
Podosphaera aphanis
.
Symptoms
The fungus can attack petioles, leaves, f lower calyces, fruits
talks and fruit.
Leaves: white patches of fungal growth develop on the
lower leaf surface. The patches enlarge and merge to cover
the entire lower surface. The leaf edges roll upwards,
exposing the white, powdery fungal growth. Purple to
reddish blotches may also develop on the lower leaf surface.
Necrotic spots may also develop on leaves.
Fruit: a white, powdery coating develops on fruit,
fruitstalks and calyces. The surfaces of the fruit harden
and, with ripening, generally crack. This is often confined
to the stem end of the fruit.
Source of infection and spread
The fungus survives on diseased ratoon crops and can be
introduced on diseased runners. The white mildew
contains large numbers of spores, which may be carried
long distances by wind. Dry weather and temperatures
between 15°C and 27°C favour the disease.
Fig 18.23 Leaf scorch disease produces small, dark, purplish,
irregular spots.
