Agriculture Reference
In-Depth Information
GREASY SPOT
■
Cause
The fungus
Mycosphaerella citri
(anamorph
Stenella citri-
grisea
).
Symptoms
Leaves are most commonly affected by this disease. Greasy
spot first appears as yellow spots on the upper side of the
leaf that coincide with slightly raised pale yellow blisters on
the underside of the leaf. With time, the spots take on a
darker, oily or greasy appearance. Lesions typically appear
three to six months after infection. Severe infections can
lead to defoliation, particularly under conditions of stress
(e.g. poor nutrition). Infection and production of
symptoms on fruit is rare; however, lesions that do occur
begin as dark specks that coalesce to form a greasy spot on
the fruit, often referred to as 'greasy spot rind blotch'. Fruit
yield and size may be reduced under high disease pressure.
Source of infection and spread
The primary agents of infection are the sexually produced
spores (ascospores) of the fungus produced in specialised
fruiting bodies (pseudothecia) on decomposing citrus leaves.
Wetting and drying of the leaf litter stimulates pseudothecia
development and release of the ascospores that, under warm
humid conditions, can infect leaves and fruit. Infection occurs
through the stomata, facilitating slow invasion of the leaf
tissues. The fungus can also produce an asexual type of spore
(conidiospore) on the underside of hanging leaves; however,
these spores are considered less important.
Fig 6.25 Greasy spot symptoms on leaves.
Management
•
Apply recommended registered fungicides during
weather events conducive to infection, particularly
during the mid-summer to late-autumn period.
Applications of oil targeting scale insects may reduce
greasy spot if the timing coincides with infection.
•
Importance
Greasy spot is most common in areas that receive autumn
rainfall and frequent overnight dews. Losses to the disease
are more likely when trees are suffering stress (e.g. salt,
nutrient deficiency/toxicity, root rot).
Remove completely, or apply mulch over leaf litter to
reduce the presence of inoculum.
•
Avoid or remedy any factors that stress the trees (e.g.
nutrient defi ciency, water stress during ripening).
•
MAL SECCO - BIOSECURITY THREAT
Cause
Fungus
Phoma tracheiphila
(teleomorph unknown).
wilting, leaf fall and then twig and branch dieback.
Often the petioles remain after infected leaves have fallen
from the branch. Fallen leaves may show red colouration
of the midrib and secondary veins. Infected bark often
becomes silver or grey, rupturing to reveal black fruiting
bodies (pycnidia) on affected twigs and branches. The
pathogen moves slowly downwards from the shoots to
the limbs, trunk and roots, eventually leading to tree
death. A common response to infection is the sprouting
of shoots from the base of affected branches and
production of rootstock suckers. When the stem is
Symptoms
Symptom expression and disease development depend
on whether infection starts in the canopy or roots, but
are most severe in spring and autumn.
When infection starts in the canopy, the first signs are
leaf and shoot yellowing (veinal chlorosis) followed by
