Agriculture Reference
In-Depth Information
Importance
Black rot is a common ripe-fruit rot in Australia, often
causing serious losses.
Management
•
Avoid very susceptible lines when selecting seed.
Apply the recommended fungicides to fruit in
plantations.
•
Treat fruit with fungicides after harvest.
•
Remove and destroy old, dying leaves and infected fruit.
•
BLACK SPOT
■
Cause
The fungus
Asperisporium caricae
.
Symptoms
Leaf spots develop on older leaves and are 1-3 mm in
diameter, circular and initially water-soaked, later becoming
brown in colour. The centres of the spots become bleached
with age. Masses of dark brown to black spores form on the
undersides of leaves. Severely affected leaves curl, turn
brown and die, often resulting in extensive defoliation.
Slightly raised spots, 2-6 mm in diameter, develop on
trunks. Fruit spots are slightly sunken, 2-6 mm in diameter
and brown to black in colour. The tissue beneath these spots
becomes corky, but internal decay does not occur.
Fig 14.15 Black rot (
Phoma caricae-papayae
) on ripe papaya fruit.
contact with a dead leaf. Infected fruit eventually wither
and fall. Small, brown, slightly sunken water-soaked
lesions develop on ripening fruit; these later turn black
and grow up to 4 cm in diameter. The margins of the
lesions are light brown and translucent, and their surface
dries and wrinkles with age and are covered by fungal
growth and fruiting bodies.
Source of infection and spread
Fungal spores are spread by wind and wind-driven rain.
Cooler temperatures and intermittent moisture (e.g. dews
and rain showers) favour disease development. Hot, dry or
very wet conditions inhibit the disease. Papaya is the only
known host of the fungus.
The fungus also causes a stem-end rot on ripening fruit.
Initial infection is through broken fruit stalks. As the
disease develops, the infected areas become wrinkled, dry
and black.
The fungus can also invade the upper stem through
wounds or infected fruit, causing an extensive dark,
sunken rot and eventual death of the upper portion of
the trunk.
Source of infection and spread
The fungus survives on old fruit and leaf stalks and
generally invades trees whose tops have been damaged, for
example by frost. Fungal spores are spread to developing
fruit during wet weather but require wounds to infect the
fruit. Once established, the disease may develop slowly in
green fruit or remain latent until fruit ripening.
Fig 14.16 Black spot on the upper and lower surfaces of papaya leaves.
