Agriculture Reference
In-Depth Information
EYE SPOT
FUSARIUM WILT
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Cause
The fungus
Mycosphaerella fragariae
.
Cause
The fungus
Fusarium oxysporum
f. sp.
fragariae
.
Symptoms
In warm weather, the outer leaves wilt and then the whole
plant wilts suddenly and dies. However, in cool weather,
some of the central leaves remain healthy with others
slightly rolled and yellow. When the crown of an affected
plant is cut lengthwise, a brown discolouration of the water
conducting tissues is seen and often extends into the leaf
bases. In advanced stages, complete rot of the crown occurs.
Symptoms
On the upper side of leaves, the fungus produces small,
reddish to purple spots, the centres of which dry out to
a light grey or white colour. Small, black, fruiting bodies
(perithecia) of the fungus may appear in the centre of older
spots. When numerous, spots may cause a rapid scorching
of the leaves and a gradual defoliation of the plant.
Leaf and fruit stalks, stolons and calyces may show similar
spots. On fruit, firm, dark brown, sunken spots up to
3 mm wide occasionally occur.
Source of infection and spread
The fungus is a soil inhabitant and can survive there for
many years as chlamydospores. It enters the plant through
the roots and grows into the water-conducting tissues of the
crown and leaves. It is most serious in summer. If an affected
plant produces runners, the fungus may spread through the
stolon from the parent plant into the runner. The disease is
frequently spread by planting infected runners. The fungus
can also be dispersed in irrigation water. Although spores
may survive in dams for at least three months, they will sink
to the bottom after one or two days.
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 in fruiting bodies are spread during wet weather
by rain splash and wind. Cool conditions favour the
disease. Eye spot is usually more severe in low-lying areas
or where plants are shaded.
Importance
Eye spot is a minor disease in well-managed crops.
Importance
Fusarium wilt became a minor disease with widespread use
of approved runners and fumigation with methyl bromide.
The disease is increasing in importance since methyl
bromide is no longer used as a soil fumigant.
Management
•
Reject severely affected runners and remove diseased
leaves from less-affected plants before planting.
Management
•
Do not plant strawberries in low-lying or shady areas.
•
Plant runners from approved or certifi ed strawberry
runner schemes.
Use the recommended fungicide program.
•
Do not use second-year plastic mulch if the disease
is known to be present.
•
Fig 18.16 Eye spot on strawberry leaves. Spots are usually round
with a white centre and a purple border.
Fig 18.17 Strawberry plants showing wilting caused by Fusarium wilt.
