Apple Scab & Black Knot
Members of the apple and cherry families provide many of our common
small flowering trees but each family is susceptible to a disfiguring
fungal disease. During spring these two diseases may infect disease
Cool, wet weather in spring creates ideal conditions for the fungal
disease apple scab. Apple scab affects susceptible apples and ornamental
crabapples. Leaves on severely infected trees develop gray blotches,
turn yellow and drop in midsummer. Apple scab will also affect flowers,
fruits, and in rare cases twigs.
Apple scab does not permanently damage a tree. However, severe
defoliation two or three successive years can weaken a tree to the
point that it becomes prone to winter or drought injury.
Reduce the chance of apple scab infection with the following practices.
Rake and destroy leaves in the fall to reduce the disease potential
the following spring. Maintain trees by watering during summer droughts
and fertilizing every five years. Remove trees that defoliate yearly
and replace with resistant varieties. In new plantings always use
scab resistant crabapples. Spotting may occur on resistant varieties
but the leaves should not yellow and drop. A number of crabapple
varieties have shown excellent resistance in trials including Adirondack,
Louisa, Prairifire, Purple Prince,
Harvest Gold, Red Jewel, and Sugar
Correctly applied fungicides may help reduce apple scab damage.
Fungicides act as protectants and must be present before the disease
begins. Make the first application when new green growth first appears.
Repeat every seven to ten days until frequent and prolonged rainy
periods are finished in early July. Thorough coverage of buds, young
leaves, and fruit is essential. Spray on calm days or nights when
temperature is between 40° and 80° F. Benomyl and Captan
are two of the fungicides registered for controlling apple scab.
Carefully read and follow label directions.
Ugly growths on plum and cherry trees are a disease called black
knot. Black knot is a common disease of plums, cherries, and other
members of the Prunus genus. The disease can be controlled
with pruning and fungicide sprays. Black knot causes elongated,
rough, black swellings on twigs, branches and trunks. The growths
begin velvety olive green then the following year turn hard, brittle,
and coal black. Stems girdled by black knot will die outward from
the growth. In spring, fungal spores are released from existing
knots and blown in the wind to other trees. Trees will gradually
weaken and die if effective control measures are not taken.
Do not purchase trees with visible knots or abnormal swellings
on the branches. Resistant plants are available. Most infections
occur between budbreak and two weeks after bloom if the weather
is wet with 55-77°F temperatures. A copper fungicide should
be applied every two weeks from the time the leaf buds open until
three weeks after flower petals fall. Fungicides prevent new infections
but do not cure disease already present. There are many formulations
of copper fungicides. The fungicide you use must be labeled for
black knot fungus on your type of plant.
Besides spraying you must prune out all knots to remove old infections.
Prune all infected wood in late winter or as soon as new knots appear.
Cut four to eight inches behind any black knot growths. Knots on
the trunk or large limbs may be carefully cut out with a knife or
chisel, removing about an inch of healthy bark beyond the swelling.
Destroy infected material by burning, burying, or sending to municipal
Alertness and care will reduce the incidence of apple scab and
black knot in our landscape
February - March 2001: Hybrids
& Heirlooms | Put the Right Plant In the
Right Place | Windbreaks Can Help Save
On Energy Costs | Apple Scab & Black Knot