Notes on Apple Diseases
Black rot and frog-eye leaf spot
Identification
Black rot causes many different symptoms in apple orchards. Leaf symptoms
are usually called frog-eye leaf spot. First, small purple spots form
on the leaves. These spots gradually enlarge and eventually develop into
lesions with a light tan interior, surrounded by a dark purple perimeter.
Heavy infections of frog-eye leaf spot can cause leaves to turn yellow
and drop.
Black rot symptoms on tree trunks and limbs appear as cankers, which
are reddish brown in colour and are slightly sunken. As the wood ages
it becomes shrunken and layers of bark begin to peel back.
Black rot also causes small purple black spots on the fruit. These spots
eventually enlarge to form concentric brown rings.
Period of activity
The heaviest infection occurs during the four to six week period following
petal fall. However, secondary infections from black rot can occur throughout
the summer.
Thresholds
There are no thresholds for black rot in Ontario apple orchards. Remove
and destroy wood infected with black rot to prevent spread of the disease
within the orchard.
Management notes
In orchards where black rot is a problem, avoid the use of DMI fungicides
such as Nova or Nustar for apple scab, because these products do not provide
control of black rot. Use a full rate protectant program of Captan or
Maestro for several years, as these products also have good activity against
black rot.
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