Insect and Disease Fact Sheet Compliments of
New Century 
1-877-79TREES
Verticillium Wilt of Landscape Trees and Shrubs
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Verticillium wilt, caused by the fungi albo-atrum
and V. dahliae, is a serious vascular disease of
hundreds of woody and herbaceous plant hosts.
Food crop hosts include everything from raspberries and strawberries
to tomatoes and potatoes. Some
of the many common woody ornamental host plants include ash, barberry, catalpa,
elm, magnolia, maple, Russian olive, redbud, smoketree,
tuliptree, and viburnum.
One group of plant not susceptible to Verticillium
wilt is all the gymnosperms, including conifers such as pine and spruce. While
many landscape plants are affected, Verticillium
wilt is not a major problem in natural forested areas. Diagnostic Symptoms Wilting
of leaves and dieback of branches, often one at a time or on one side of the
tree, are the most severe
symptoms. This can occur over a number of years, with remission of symptoms
in some years, or can rapidly progress top plant death in-a year or two
(Figure 1). Other symptoms of Verticillium wilt may
include marginal browning and scorch of leaves, abnormally large seed crops,
small leaves, stunting, poor annual growth, and sparse foliage. |
Figure 1. Above ground symptoms of Verticillium
wilt on maple. |
Sometimes
large areas of cambial tissue die from infections by the fungus and
opportunistic fungi such as Nectria develop in elongated
cankers. Late season infections may not be noticeable until plants come out of
dormancy, with branch dieback evident.
All of
the above symptoms can also be caused by other stress factors. A good field
symptom that can set Verticillium wilt apart
diagnostically is the discoloration of xylem and cambial tissue, visible as streaks
if you cut into the wood. This discoloration is variable for different plants;
generally greenish to blackish on maple, yellowish green on smoketree,
and brown on ash. This streaking is not totally diagnostic on two, counts: 1)
other fungi and other factors can cause discoloration; and 2) on some hosts and
on youngest twigs, infection is not always accompanied by discoloration.
However, vascular discoloration is a good field symptom that can then be followed
up for confirmation by a diagnostic lab such as The Ohio State University Plant
& Pest Diagnostic Clinic.
Disease Cycle and Conditions
Favoring Disease
The Verticillium fungus can survive for many years in soil
making effective crop rotation difficult. The fungus infects plant roots through
wounds and in some cases direct penetration of root tissue. Verticillium
also is transmitted from plant to plant by grafting and budding.
From
root infections, the fungus spreads upward in the plant-through the vascular
stream. The results of infection are tissue damage and plugging of xylem,
robbing stems and leaves of needed water and minerals.
The fungus
is returned to the soil as plant parts fall or die, and tiny resistant fungal microsclerotia are spread by wind, in soil and on
equipment. Many weed hosts are also susceptible; therefore the cycle of contaminated
soil is hard to break. Development of Verticillium
wilt is favored by factors that stress roots, including wounding and droughty
conditions.
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Control 1. Disease resistance. - If Verticillium wilt is diagnosed at a particular landscape pr
nursery site it is prudent to replant into that area with a plant that
exhibits resistance to this disease. A few common examples of plants typically
free of this disease include: crabapple, mountain ash, beech, birch,
boxwood, dogwood, sweet gum, hawthorn, holly, katsura
tree, honeylocust, oak, pear, London planetree, and sycamore, rhododendron, willow, and zelkova. The red maple cultivars Armstrong, Autumn Flame,
Bowhall, October Glory, Red Sunset, Scarlet and Schlessinger have also been reported as resistant. 2. Keep
plants as healthy-as possible. - Proper transplanting practices, proper
water management to avoid droughts, a good fertility program and pruning out
dead branches are all good plant health care management practices. These can
help limit infections and help limit the effects of these infections to some
extent. Pruning out infected branches is useful as a general horticultural
practice for overall plant vigor and aesthetics, but does not eliminate Verticillium from the plant since infections spread from
the roots. Fungicides are not effective for control of this disease. |
Figure 2. Above ground symptoms of Verticillium
wilt on maple. |
Sections of maple trunk showing Verticillium
wilt discoloration on cambial tissue.
Information obtained through the Ohio State Extension
Fact Sheet HYG-3053-96
Insect and Disease Fact Sheet Compliments of
New Century
1-877-79TREES