Agriculture Reference
In-Depth Information
FUNGI
growth f flushes are sparse and lack vigour, but f lowering
and fruit set may be prolific. However, these fruit remain
small and fail to reach a marketable size.
ANTHRACNOSE
The internal symptoms are most evident immediately
under the bark in the cambium and outer wood tissues of
the basal trunk and taproot. The dark, systemic vascular
discolouration can be traced from the feeder and primary
roots into the taproot and above ground level into the
trunk. Dark streaking of the wood is rarely seen in the
branches above the graft union. In transverse sections of
the trunk at about ground level, the outer growth rings are
extensively discoloured; crescent-shaped sectors of the
inner growth rings are often discoloured from infection in
previous years. The secondary symptoms of collar rot,
decay of bark around the crown roots and extensive feeder
root decay become progressively more evident as the tree
declines.
Cause
The fungus Colletotrichum gloeosporioides .
Symptoms
Anthracnose causes two symptoms:
a necrotic spot 2-10 mm in diameter (the symptoms
are seen in fruit as dark brown to black spots on the
unripened fruit, not unlike black canker)
pepper spot specks on the surface of the fruit (these
spots coalesce on badly affected fruit).
Source of infection and spread
Wet and windy conditions favour the disease especially
in the many orchards in New South Wales and
Queensland that are now 20-25 years old. There is likely
to be a high anthracnose inoculum load in these large
trees. Old fruit left hanging on the trees will provide a
further inoculum source. Prunings may also be a source
of inoculum, if placed under the tree and tree skirts
are kept low or the weight of the fruit brings branches
down to ground level. This prevents the movement of
drying air.
Source of infection and spread
The bacterium survives in the soil and on a wide range of
crop plants and weeds including tomato, potato, capsicum,
eggplant, blackberry nightshade ( Solanum spp.) and wild
tobacco tree ( Solanum mauritianum ). The bacterium may be
introduced in soil or infected rootstocks. Once established, it
can spread rapidly through the crop by irrigation, rainwater
and soil adhering to farm machinery and animals.
Importance
The disease is widespread in Queensland, causing death
of trees of all ages.
Management
Use resistant cherimoya rootstocks, rather than
susceptible sugar apple or segregating atemoya
seedlings.
Avoid planting into soils known to be infested if
cherimoya rootstocks are not used.
Use disease-free grafted trees propagated in treated
potting mix.
During the orchard-establishment phase, avoid
intercropping custard apple trees with annual crops
susceptible to bacterial wilt.
Keep weeds, particularly hosts of bacterial wilt, under
control. Maintain a weed-free grass sod between rows
and a mulch cover along the row.
Avoid waterlogging by ensuring adequate surface
drainage, and constructing diversion drains and
mounding of soils of marginal depth.
Fig 8.3 Late symptoms of anthracnose on custard apple fruit.
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