Agriculture Reference
In-Depth Information
Fig 3.7 Apple black spot symptoms: sparse (left) and severe (right).
certain minimum time periods at temperatures ranging
from 6-24°C, the ascospores germinate and infect these
tissues, causing black spots. The duration of leaf wetness
in which infection may occur is called an apple black spot
infection period. In the Granite Belt region of south-east
Queensland, there are generally 6-10 infection periods
between mid September and early December, depending
on rainfall. All primary spores have been released by
this time.
Summer spores called conidia develop on the surface of the
black spots or scabs and are the major source of secondary
infection. These summer spores readily infect the adjacent
foliage and fruits whenever rainfall occurs. Secondary
infection by conidia is slightly slower (by three hours) than
infection by ascospores under the same conditions and
occurs at any time. It is important to obtain early control
of primary black spot infections to prevent secondary
infections.
Fig 3.6 Early and late symptoms of apple black spot.
Symptoms
Leaves: light, olive-green spots, about 3 mm in diameter,
with a diffuse, irregular, velvety growth, appear on young
leaves. The spots darken with age, eventually becoming
black, and may coalesce to cover most of the leaf. With
age, the spots become scabs and the leaves become
distorted.
Fruit: small, dark spots occur, which enlarge, becoming
brown and corky with a black broken margin.
Source of infection and spread
During winter, the fungus survives within old, diseased
apple leaves beneath the trees. The fungus forms minute
fruiting bodies (perithecia), which produce large numbers
of primary spores (ascospores). With the onset of spring
rains, these eject forcibly into the air during daylight and
are carried by wind and rain to susceptible, new apple
leaves and f flowers. If these apple tissues remain wet over
Fig 3.8 Apple black spot symptoms in the field.
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