Agriculture Reference
In-Depth Information
Importance
Apical bud necrosis can be important in areas of cool
climate in Australia, particularly south-east Queensland,
Northern New South Wales and southern Western
Australia. It has been reported in Spain, Portugal and Israel.
Management
There is little information about management strategies for
this disease. Overseas work suggests some resistance in
varieties including Sensation and Keitt. Being systemic,
standard topical fungicides/bactericides have little or no
effect. New infections may be prevented by heavy copper
applications before and immediately after late summer
pruning when the bacterial populations are at their lowest.
Rapid recognition of infections and resultant good
phytosanitary practices should limit the spread of the
disease through orchards.
Fig 12.3 Vascular discolouration and bacterial ooze associated with
apical bud necrosis.
BACTERIAL BLACK SPOT
Cause
The bacterium Xanthomonas campestris pv. mangiferaeindicae .
It is speculated that environmental conditions, such as
wind and hail, may promote infection by causing physical
damage to the plant. It is not known whether the infection
can be spread by f flower-visiting insects or other animals.
New infections become systemic as the bacteria colonise the
stems and panicles, presumably leading to later outbreaks.
Evidence from P. syringae infections on other plants suggest
that spatter spread may occur during rainfall events.
Pruning operations and any actions that damage plant
surfaces may help spread the disease. Cool, wet conditions
are the most favourable for infection and optimal growth
of the bacteria occurs under cooler conditions with
populations appearing to peak during winter.
Symptoms
Leaves: slightly raised, black, elongated lesions with greasy
margins, frequently delineated by veins, appear on young
leaves or abraded, mature foliage. Lesions are often
surrounded by a chlorotic halo and become grey and
cracked with age. Compared with anthracnose lesions,
bacterial black spots are darker and more angular with
distinct, usually raised, margins. Lesions also occur on
petioles and twigs. Adjacent lesions may coalesce,
destroying large areas of the leaf.
Fruit: initial symptoms appear around the lenticels as
small, irregular, water-soaked specks from which a bead
of bacterial ooze may develop. These symptoms closely
resemble fruit f fly stings. Raised, black spots with greasy
margins develop later. These disfiguring, skin-deep
symptoms are visible on fruit at harvest and lead to
downgrading or rejection. These fruit lesions are the most
important economically.
Cracks, from which bacteria-laden sap may ooze, frequently
develop. Anthracnose and secondary rots commonly develop
from bacterial black spot lesions as the fruit matures, causing
deep, extensive decay. Bacteria from fruit, twig or leaf lesions
may infect the fruit in a tear-stain pattern. Lesions can also
occur on f flower and fruit peduncles.
Source of infection and spread
Bacteria from diseased tissue are dispersed by wind-driven
rain, water splash and insects, and transferred from
Fig 12.4 Typical foliar symptoms of apical bud necrosis showing the
absence of a halo around lesions (left). Right: bacterial black spot
showing lesions with yellow halos.
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