photo bwg
Apple
scab, caused by the fungal disease, Venturia inaequalis is one of the most
serious and disfiguring diseases of ornamental and fruit apples. Although the
name implies it is limited to apples, this disease can be found on many members
of the rose family. The hosts can include apples, hawthorns, fire thorns,
cotoneasters, and pears.
The symptoms of this
disease are typically olive brown lesions on the leaves and sometimes on new
succulent shoots. The scabs are indistinct at the margins and if the infection
is severe they may coalesce into larger patches. The patchy lesions can be
found on flowers, fruit, leaves and twigs. The tree responds to this infection
by prematurely dropping infected leaves. Repeated severe infections can cause die back
after several years.
The disease overwinters in
dropped leaves and affected twigs remaining on the tree. Wet spring conditions spread
spores and allow them to germinate on the leaves, initiating new infections.
The warmer the temperature, the shorter the amount of time it takes to initiate
and infection. The peak period of infection occurs when the trees are starting
to flower and continues until full bloom. Secondary infections will occur
throughout the spring and summer when conditions are wet for long periods of
time.
Some cultural control,
including disposal of diseased leaves and twigs away from infected trees as
well as the removal of all leaves in the fall will help to reduce the intensity
of future infections. Planting disease resistant varieties and avoiding mass plantings
also reduce the amount of disease inoculum and the potential for sever
outbreaks.
There are a number of
broad spectrum fungicides that are labeled for control of apple scab. Typically
they require several applications starting at bud break and continuing every
two weeks to get suitable control.
