Agriculture Reference
In-Depth Information
4
AVOCADO
(
Persea americana
)
The genus
Persea
belongs to the family Lauraceae. The
avocado originated in Central America and has three
ecological races, Mexican, Guatemalan and West Indian
(Lowland) with different climatic tolerances, allowing fruit
production in diverse environments. The most important
diseases of avocado in Australia are root rot caused by
Phytophthora cinnamomi
, and two fruit diseases:
anthracnose and stem-end rot. Minor diseases can be
significant in certain production areas. The Guatemalan
cultivar 'Hass' is the major cultivar grown in Australia.
Most avocado trees in Australia are grafted onto seedling
rootstocks. A small percentage is on vegetatively cloned
rootstocks.
Source of infection and spread
The bacteria are common saprophytic epiphytes on leaves,
stems and fruit. Infection may occur in the field, but occurs
more commonly after harvest. Infection is favoured by cool,
wet weather as the fruit nears maturity, and by harvesting on
rainy days.
Importance
This disease occurs sporadically and causes significant
postharvest losses when fruit is harvested during wet
weather. Fuerte and Sharwil cultivars are more susceptible
than Hass.
Management
•
Harvest fruit carefully to prevent surface injuries.
BACTERIA
Avoid harvesting fruit when they are wet, or washing
them in dip tanks before packing.
•
BACTERIAL SOFT ROT
■
Clipping the fruit and leaving the fruit stalks intact
during packing will reduce the disease.
•
Cause
The bacteria
Erwinia carotovora
and
E. herbicola
.
Chlorinate the water if washing is unavoidable.
•
Symptoms
Externally, the fruit has a darkened metallic sheen. Internally,
the f flesh is grey, softened and sometimes liquid, and there is a
rancid smell.
FUNGI
ANTHRACNOSE
■
Cause
The fungus
Colletotrichum gloeosporioides
.
Symptoms
Symptoms may occur in ripe fruit or in immature fruit while
still on the tree. In ripe fruit, symptoms initially appear as
small, light brown, circular spots. These spots enlarge rapidly
and change colour to dark brown or black. Centres of spots
may be slightly sunken and, if fruit are kept in a moist
atmosphere, pink spore masses will develop on the spots.
One or more spots may enlarge to cover the side of a fruit.
The rot may penetrate deeply into the f flesh in a
hemispherical shape.
Fig 4.1 Bacterial soft rot in fruit, cut to show blackening and
complete breakdown of affected tissues.
