| Crown gall is a plant disease caused by the soil-inhabiting
bacterium, Agrobacterium tumefaciens.
The bacterium causes abnormal growths or galls on
roots, twigs, and branches of euonymus and other
shrubs primarily in the rose family. The bacterium
stimulates the rapid growth of plant cells that results
in the galls. In addition to being unsightly, the galls
weaken and stunt the growth of the plant. Although
galls can disrupt the flow of water and nutrients up
the roots and branches, they usually do not cause
total plant death. The disease can spread to other
susceptible plants through contaminated soil and
tools. Most chemical treatments are not effective.
The galls on forsythia, viburnum, highbush blueberry,
American elm, hickory, maple, oak, and privet are
believed to be caused by a fungus, Phomopsis sp.
Since its cultural controls are the same as for
bacterial crown gall, both are treated together herein.
Chemical treatments, however, will differ. Chemical
treatments for both are of very limited value to the
home gardener. Only those relating to bacterial
crown gall have been addressed.
Symptoms and Diagnosis
Galls are most commonly found near ground level
on the roots and lower branches of the plants. As the
galls enlarge, they become woody and hard. The
outer layer turns brown and corky. The plant may be
weakened and stunted with some branch or tip
dieback. Symptoms may not develop immediately
after infection. Galls grow most rapidly during the
warm months of the year.
Life Cycle
Crown gall forming bacteria inhabit the soil and
survive for many years. The bacterium can initially
be brought in on the roots of infected plants. It then
spreads by soil and water movement or
contaminated pruning tools. Bacteria enter the plant
through wounds possibly caused by chewing
insects, cultivation damage, or from grafting and
pruning tools. The crown gall bacterium has been
known to survive more than two years in the soil in
the absence of susceptible plants. It can live for
several years in decomposing galls buried in the
soil. Crown gall is likely to be more serious in limed
soil than in acid soils so soil pH could be important
in limiting the disease.
Integrated Pest Management Strategies
1. Prune out infected material. Crown gall cannot
be eliminated from a shrub even though the
infected plant may live for many years. To
improve the appearance of the plant, prune out
and destroy infected stems below the galled area.
Sterilize the pruning shears after each cut with a
mixture of one part bleach to nine parts water.
2. Destroy infected plants. Destroy the infected
plant. The bacterium will remain in the soil so it
is important to plant a resistant plant species. If
the same species needs to be planted in the area,
remove and replace the soil or consider
soil sterilization.
3. Sterilize the soil. Soils known to be infected with
crown gall bacteria can be sterilized using
chemicals, heat, or antibiotics. This is not
practical for most home gardeners. A biological
control has been introduced using a bacterium,
Agrobacterium radiobactor strain 84. This
bacterium was discovered to be antagonistic to
crown gall bacterium. It is available for use as a
preplant treatment by dipping nursery stock in a
suspension of the live bacteria in water.
4. Exclude the problem. When purchasing forsythia
and euonymus plants, inspect them carefully for
signs of galls. Do not purchase plants that show
gall-forming symptoms.
5. Replant with a more resistant plant species. The
following plants recommended by the University
of Illinois show greater resistance to bacterial
crown gall: barberry, hornbeam, true cedars,
ginkgo, golden raintree, tulip tree, mahonia,
spruce, linden, boxwood, catalpa, beech, holly,
larch, magnolia, black gum, pine, Douglas fir,
bald cypress, hemlock, birch, firethorn, redbud,
smoke tree, sweet gum, deutzia, serviceberry,
yellowwood, yew, and zelkova. |
