Agriculture Reference
In-Depth Information
Apple Scab
Apple scab is one of the most serious apple diseases in North America and
has severely limited commercial production in eastern North America. Crop
losses of 70 percent or greater are common following cool, wet springs if the
disease is not controlled. The disease is caused by a fungus (
Venturia in-
aequalis
) that infects leaves, blossoms, fruit, and sometimes new shoots and
bud scales. Severe infections can defoliate trees and, if repeated, weaken the
trees and decrease yields. Although infected fruit is edible, the scabby sur-
faces and deformed shapes can render it unmarketable.
Symptoms.
Symptoms often occur first on the undersides of young leaves as
velvety brown to olive-green spots with indistinct edges. As the leaves un-
fold, the spots appear on the upper leaf surfaces and leaf stems (petioles).
The spots gradually grow and often merge, forming rough, brown scabs that
can cover entire leaves. Heavily infected leaves, or those whose petioles have
been infected, often fall from the trees. Symptoms on the fruit are similar
and develop into rough, corky scabs.
When the flower and fruit stems (pedicels) are infected, the flowers and
fruit can fall prematurely. The fruit often becomes deformed as growth stops
in infected areas but continues in uninfected parts of the fruit. Large cracks
can develop on the fruits, making them susceptible to infection by opportun-
istic pathogens and saprophytes (microorganisms that live on dead tissues).
Early infections often appear first on the blossom end of the fruit. Late-sea-
son infections may not appear until the fruits are in storage.
Causes and timing.
The apple scab pathogen can overwinter on the bud
scales, but more commonly it survives on fallen leaves. Primary infections
usually come from ascospores that develop in fruiting bodies on fallen leaves
and fruit. Ascospores are dispersed from the fruiting bodies about the time of
bud break and peak during bloom. Young tissues become infected more eas-
ily and quickly than older leaves and fruits. Once lesions form on the leaves,
