Agriculture Reference
In-Depth Information
Management.
Good orchard management practices to maintain healthy
trees and open canopies help reduce the incidence of the disease. Avoid us-
ing overhead watering. Because this disease is caused by a bacterium, sulfur
is not particularly effective in controlling it. Fixed coppers and Bordeaux mix
can help control the disease, but they are phytotoxic to many stone fruits. In
particular, do not use sulfur products on apricots. One strategy is to begin
with a copper spray just before bud break followed by either weekly antibi-
otic treatments or alternating treatments of copper and antibiotic beginning
at bloom. Note that copper damage on stone fruits can mimic symptoms of
bacterial blight.
By far the most important strategy is to select varieties that are resistant
to the disease. Also avoid planting near orchards containing susceptible vari-
eties. High levels of inoculum can overwhelm even resistant varieties.
Black Knot
Caused by the fungus
Apiosporina morbosa
, black knot is widespread in
North America and was once a serious orchard disease. Diligently following
the management practices we have discussed should reduce the disease to a
minor annoyance. Black knot is most often found on wild stone fruits, such
as chokecherry, and in abandoned or poorly managed orchards. In orchards,
black knot is most often a problem on plums and prunes, but it can infect
apricot, peach, sweet cherry, and tart cherry.
Symptoms.
The disease attacks only the wood and appears most commonly
on twigs and small branches, but it can infect larger branches and trunks.
The fungus forms very distinctive greenish to black corky swellings on the
infected limbs, resembling a hot dog that has been skewered lengthwise. The
fungus extends at least several inches underneath the bark from the ends of
the visible swelling.
Management.
Good cultural control practices are critically important for
managing this disease. Follow the general cultural programs we have dis-
