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Introduction
Several
different fungi in the genus Rhytisma infect the leaves of maples
and cause raised, black spots to form on upper leaf surfaces. The diseases
are called "tar spots" because their appearance so closely resemble
droplets of tar on leaf surfaces. Tar spot is rarely serious enough to
threaten the health of trees but sometimes there can be so many spots that
the tree becomes unsightly. Heavy infections can also cause early leaf
drop-- a circumstance that causes the greatest consternation to homeowners
because lawns are littered and must be raked before autumn officially
arrives. In several upstate New York communities tar spot on Norway maple
is particularly troublesome because of early leaf drop.
Symptoms
The first symptoms of infection by a tar spot fungus
usually show up in mid-June as small (less than 1/8 inch diameter), pale
yellow spots. The spots enlarge and their yellow color intensifies as the
season progresses. On red maple and silver maple, a single black spot
usually develops in each yellow spot by early August. The black spot grows
in diameter and thickness until, by late September, it truly does look
like a spot of tar. (See Figure 1 on Diagnostic Lab Fact Sheet). The
surface of the spot may have a pattern of wavy indentations or ripples.
Another form of tar spot affects striped and Norway maples. ON these trees
20 to 50 small spots, each no larger than a pin-head, appear in late July
or early August (see figure 2 on the Diagnostic Lab Worksheet). On striped
maple, the spots do not enlarge much after they first appear. On Norway
maple, however, the spots grow and eventually coalesce to yield a larger
black mass up to 1 1/2 inches in diameter.
Figure 1:
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Figure
2: | |
Disease cycle
The fungi that cause tar spots overwinter on
infected leaves that fall to the ground. The following spring, just as new
leaves are unfolding, the fungal tissue in the leaves on the ground
ripens. The surfaces of the spots split and minute, needle-like spores
escape. The spores are carried about by wind and if they land on new
leaves of a susceptible host they may germinate, penetrate the leaf
tissue, and start a new disease cycle.
Management
Strategies
Where tar spot occurs at levels
so low that the appearance of trees is not affected, keep it that way by
raking and destroying infected leaves in the fall. This will reduce the
number of overwintering spots which can produce spores the following
spring. Unacceptably high levels of infection can be brought under control
with two or three applications of an appropriate fungicide as new leaves
expand in the spring. In these cases, fungicides may have to be applied
every year.
Created, KLS, 8/99
This publication contains pesticide recommendations. Changes in
pesticide regulations occur constantly, some materials mentioned may no
longer be available, and some uses may no longer be legal. All pesticides
distributed, sold, and/or applied in New York State must be registered
with the New York State Department of Environmental Conservation (DEC).
Questions concerning the legality and/or registration status for pesticide
use in New York State should be directed to the appropriate Cornell
Cooperative Extension Specialist or your regional DEC office.
READ THE LABEL BEFORE APPLYING ANY PESTICIDE.
__________________________________________________________________________________
The Plant Disease Diagnostic Clinic at Cornell
University is located at 334 Plant Science Building, Ithaca, NY,
14853. Phone: 607-255-7850, Fax: 607-255-4471, Email: kls13@cornell.edu |
