Agriculture Reference
In-Depth Information
appear first on the exposed sides of fruit, on the warmer
northern side of the tree. Fruit is susceptible to infection for
several months from just after petal fall; the period of
susceptibility depends on the cultivar and the age and
vigour of the tree. Secondary spores (conidia) produced on
the fruit lesions and spread by water are considered of little
importance in the disease cycle, with the exception of
varieties or circumstances that lead to symptomatic fruit
remaining in the tree when young, susceptible fruit are
developing (e.g. lemons and Valencia oranges).
Importance
The disease is common in humid growing regions subject to
summer rainfall, and has not been recorded in arid growing
areas subject to winter rainfall. It mainly occurs in late-
maturing cultivars, such as Valencia oranges, Murcott tangors,
lemons and grapefruit. Speckled blotch can be a serious
disease on maturing fruit (e.g. early Navel oranges) when late
summer and early autumn weather is abnormally hot and dry.
Fungicide application provides satisfactory control for the
domestic market requirements; however, more stringent
control measures are required for the export market. In
Australia, several citrus growing regions (e.g. Sunraysia,
Riverland and Riverina) are not suitable, climatically, for
G.
citricarpa
development, preventing black spot from occurring.
Fig 6.20 Hard spot symptoms of black spot on orange (right) and pink
grapefruit (left). Note the distinct black margin, and light coloured
centre containing the small black fruiting bodies of the fungus.
Management
•
Apply recommended registered fungicides during spring
and summer. Spray fl owers, young fruit and young
leaves to protect them from infection.
Manage leaf litter (e.g. cover with mulch or completely
remove) to reduce inoculum levels.
•
Harvest fruit as soon as possible, and avoid leaving fruit
on the tree long enough for symptoms to develop.
•
After harvest, avoid exposing fruit to temperatures above
20°C, which might trigger symptom expression.
•
Fig 6.21 Virulent or galloping symptom of black spot occurs when
spots coalesce. Note the presence of the fungal fruiting bodies.
BLUE AND GREEN MOULDS
■
Cause
The fungi
Penicillium italicum
(blue mould) and
Penicillium digitatum
(green mould).
Source of infection and spread
Most infection results from primary sexual spores
(ascospores) ejected from fruiting bodies (perithecia) that
develop on fallen citrus leaves on the orchard f floor after
several cycles of wetting and drying. Carried by wind, the
spores then infect young leaves and fruit under moist
conditions. The fungus then stops growing and remains
dormant in the tissues. Leaf symptoms rarely show until
leaf fall when the fruiting bodies begin to develop. Fruit
generally first show symptoms at ripening, particularly
during warm to hot weather. Black spot symptoms typically
Symptoms
Small, soft, water-soaked areas occur on fruit and develop
rapidly in warm, humid conditions, eventually rotting the
whole fruit. Rotted areas become covered with a dense,
white, fungal growth from the centre of which masses of
