
ANTHRACNOSE - Many types of fungi cause anthracnose
on various hosts. The types usually involved with tree anthracnose in
this area are Discula and Glomerella. Dogwood anthracnose caused by the fungus (Discula destructiva)
is the most serious disease of dogwoods in the landscape and our forests. Other
species of anthracnose fungi attack the above hosts and can be equally harmful. The fungus involved causes dieback or
even death of infected trees. The early symptoms begin in mid to late May as
leaf spots with tan or purple borders. In wet weather, these spots can
rapidly enlarge and kill entire leaves. The infected leaves may remain hanging
on the branches, especially in wet weather. The disease will then spread from
the infected leaves into the twigs and branches and cause dieback of the limbs.
If the dieback reaches the main trunk, the entire tree could be killed.
In order to distinguish anthracnose from other leaf spots, examine the
underside of the leaves for numerous small tan to brown dots, about the size of
a pin head (sometimes they're so small a hand lens or microscope is required to
see them), scattered within the infected tissue. These dots are the source of
spores that will be spread by rain, dew or insects to healthy leaves and neighboring
trees. Under severe disease conditions, the flower bracts of dogwoods can
become spotted. Prune out all dead or dying twigs and
limbs during dry weather (refer to drought page). In the fall,
leaves should be raked up and removed to help prevent resurgence next
spring. Also, if possible, remove any dead leaves still attached to the
branches. Fertilizing affected trees is not recommended.
Fertilizing will cause new succulent growth that is highly susceptible to
attracting the fungus. In areas with severe disease, consider
replanting using resistant dogwood species such as kousa
dogwood, (Cornus kousa),
or hybrids between kousa and flowering dogwood such
as 'Celestial', 'Milky Way' , 'Stardust' , 'Steeple' , 'Stellar Pink' and
others. When disease pressure is severe, fungicides can be used on trees
in landscapes in the spring at bud break, followed by additional sprays every
10-14 days until leaves are fully expanded. Trees should also be sprayed once
in the fall after the leaves have changed color, but before leaf drop.