Agriculture Reference
In-Depth Information
ALTERNARIA LEAF BLOTCH AND FRUIT
SPOT OF APPLE
Cause
Species of the fungus
Alternaria
.
Internationally,
Alternaria
species causing leaf infection on
apples are referred to as
Alternaria
mali
. In Australia, many
Alternaria
species isolated from apple leaves and fruit have
been identified as species of
Alternaria
other than
A. mali
.
It is unclear whether other species of
Alternaria
can cause
leaf blotch and fruit spot symptoms.
Symptoms
Leaf blotch: Alternaria leaf blotch is characterised by
irregular light brown-reddish shaped lesions, often with
purple borders on leaves. Initially, the lesions are roughly
circular. These symptoms may be very similar to symptoms
of physical damage or other fungal pathogens. This means
a diagnosis of
Alternaria
based on observation of leaf
lesions alone is not conclusive.
Under favourable conditions the leaf blotches will
continue to expand, often causing considerable
defoliation.
Fig 3.9 Black spot symptoms on pears.
Importance
Black spot is a major disease of apples worldwide, and is
particularly significant in areas with high spring/summer
rainfall. Fruit affected with black spot is unacceptable
to consumers and is unmarketable.
Management
•
Plant resistant varieties.
Fruit spot: small, slightly sunken, light to medium brown
spots appear on fruit lenticels. This often occurs after
rainfall and usually no earlier than six to eight weeks
before harvest. Fruit spots do not appear during storage,
and preharvest Alternaria fruit spots do not appear to grow
significantly in size during cold storage. However, once
removed from cold storage, existing spots can continue to
increase in size, and new spots can develop, providing an
excellent entry point for other secondary fruit rots.
Spray susceptible varieties with recommended protectant
fungicides from green tip, and apply eradicant fungicides
as advised by apple black spot warning services.
•
Apply urea during leaf fall to hasten defoliation and
encourage leaf litter breakdown, which will reduce
carryover of disease inoculum.
•
During winter, cultivate diseased leaves, fruit and prunings
into the soil to increase tissue breakdown and reduce
carryover of inoculum from one season to the next.
•
Fig 3.10 Leaf symptoms of apple black spot.
Fig 3.11 Symptoms of Alternaria leaf blotch.
