Verticillium wilt is a common disease of many different kinds
of plants including ornamental (landscape) plants, fruit trees,
and vegetables. In western Washington, the disease is especially
common on maples, particularly the smaller ornamental maples.
The disease is often serious, causing reduced plant beauty, reduced
growth, or death.
The leaves on diseased maples may suddenly wilt and die, or
may suddenly drop at any time during the growing season. These
symptoms may occur over the entire tree, or only on one side,
or only on one or several branches. The leaves may or may not
be stunted and yellowed. Diseased maples may die quickly, or
may live for years.
The disease can be more positively identified by cutting lengthwise
into the trunk or an affected branch. In living wood, olive-green
streaks are usually present (Fig. 1). In dead wood, less distinct
brownish streaks are usually present. This streaking may be located
anywhere in the wood, and may be found more commonly in the roots
or lower trunk, but is usually present in the branches also.
The olive-green streaks should not be confused with those occurring
around wounds in healthy trees.
This disease is caused by the fungus Verticillium albo-atrum.
The fungus generally invades the plant through the roots, but
can also infect through wounds on the aboveground parts of the
plant, causing the symptoms mentioned above. The fungus can survive
in the soil for many years, even after the diseased plant has
been removed.
Wilting, dropping, yellowing, or stunting of leaves, especially
over the entire tree, may occur simply because of poor care of
the tree, and Verticillium wilt may not be involved. Always be
sure to give your maple good cultural care.
CONTROL
Verticillium wilt is not effectively controlled by chemicals,
but there are some cultural control measures which can help.
Prune out and destroy affected branches. Be sure to prune
back to wood which shows no streaks, and be careful to make the
final cut with sterilized tools to avoid fungus spread. Sterilize
shears or other pruning tools in rubbing alcohol after pruning
infected portions of trees. Washing the pruning tools in soap
and water before sterilizing, if practical, is a good idea.
The fungus can also be spread if the infected branches are
not properly destroyed. Destroy the removed branches by burning,
or place them in the garbage or take them to the dump.
Apply only limited amounts of nitrogen fertilizer, just enough
to insure normal growth. However, there are conflicting reports
as to the benefit of applications of high nitrogen fertilizer.
Be sure to water the tree as necessary.
Completely remove dead trees or shrubs, including the root
system; destroy as mentioned previously. This will dispose of
a great deal of the fungus and lessen its spread, but some of
the fungus will still be in the soil. Therefore, avoid moving
any soil from the contaminated area to uncontaminated areas.
Remember that the fungus can survive in the soil for many years.
Any tools (such as shovels, shears, saws, etc.) which were used
in the removal of the infected plant should be cleaned and/or
sterilized to avoid spreading the fungus.
Although the fungus can infect many kinds of plants, some
plants are considered resistant or immune to Verticillium wilt.
Such plants don't often become diseased. If you want to plant
an ornamental or landscape plant in an area known to be contaminated
by Verticillium, then choose a resistant or immune plant.
Some plants regarded as resistant or immune to Verticillium
wilt: many Cornus spp. (dogwoods); Juglans spp.
(walnuts); most Malus spp. (apples and crabapples); Pyrus
sp. (pears); Quercus spp. (oaks); Crataegus
spp. (hawthorns); Liquidamber styraciflua (sweet gums);
Salix spp. (willows); Platanus spp. (sycamores
and planes); Betula spp. (birches); conifers (such as
yews, firs, pines, etc.); and monocots (such as grasses, irises,
lilies, daffodils, tulips, gladioluses, etc.).
