Crown gall
Figure 1
Crown gall on apple roots
Cause
The bacterium, Agrobacteria tumefaciens.
Primary hosts
in Missouri
Members of the genera Populus, Salix, and Prunus. The MU Extension Plant Diagnostic
Clinic commonly diagnoses crown gall on the roots of apples, peaches, blackberries
and walnuts and on the stems of euonymus and poplar.
Figure 2
Crown gall on euonymus stems.
Symptoms
and diagnosis
Swellings or galls may form above ground on stems or branches or below ground
on roots (Figures 1 and 2). The galls are usually rounded with a rough surface
and a spongy texture. The galls may darken and crack with age. The galls can be
confused with galls made by insect or mites and also by physiological responses
to wounding or grafting. However, the interior of the gall caused by an insect
will have chambers or cavities where the insect developed. The interior of a gall
due to crown gall will have a mass of disorganized vascular tissue. It is often
necessary to isolate the bacteria through diagnostic methods in order to make
a positive identification.The effect that crown gall may have on a plant may range
from having little to no impact on the growth and production to plant, to declining
plant vigor, to plant death. The disease has a greater impact on young plants
and is of great concern to the nursery industry.
Life cycle
Agrobacterium tumefaciens is a bacterium that lives in the soil and infects
plants through wounds. Common methods of entry are through wounds made by cultivation,
pruning or insect feeding or through natural causes, such as wind and hail. Once
the bacterium enters the plant it inserts a portion of its DNA (called T-DNA)
from the tumor-inducing portion of the bacteria's chromosome, into the chromosome
of a healthy plant cell. The expression of the T-DNA causes the overproduction
of plant hormones that lead to undifferentiated cell growth in that region of
the plant; thus forming a gall. Small galls usually develop within 2 to 4 weeks
after infection when temperatures are 20 degrees C or above. The expanding tissues
can sometimes disrupt the flow of nutrients and water. Bacteria are released from
the gall into the soil as the gall decays. The bacteria are spread to new plants
and areas through splashing rain, irrigation water, insects, tools and plant parts
used for propagation. The bacteria can survive up to two years in the soil with
out a plant host.
Integrated
management strategies
The most effective method to manage crown gall is through prevention and sanitation.
Plant disease-free, healthy nursery stock in uninfested soil. Avoid planting into
areas that have previously had plants infected with crown gall for 2 to three
years. Destroy infected plants. Use care during propagation to avoid transmitting
bacteria to healthy plants. Sterilize pruning tools frequently using a disinfectant
such as 70 percent alcohol or 20 percent solution of household bleach. Avoid mechanical
wounding to young plants.
The biological control agent, Agrobacterium radiobacter, strain K84 has been
very successful in preventing crown gall in apples, peaches, blackberries and
some ornamental crops. It does not work for grapes. This strain is not pathogenic
and is antagonist to pathogenic bacteria. It is very effective when used as a
preventative, but will not cure infections after they have occurred.
Updated 6/8/09