| Cedar-apple rust (Gymnosporangium juniperivirginianae),
cedar-hawthorn rust (G. globosum),
and cedar-quince rust (G. clavipes) are closely
related rust diseases that require two hosts to
complete their life cycle. All three rusts can infect
most varieties of eastern red cedar (Juniperus
virginiana) as well as many other junipers and an
alternate host. Cedar-hawthorn rust, in addition to
affecting apples, crabapples, and many hawthorns sometimes
infects pears, quince, and serviceberry.
Symptoms and Diagnosis
Symptoms on juniper: The fungus produces reddishbrown
galls on twigs and small branches of
susceptible junipers. After reaching a diameter of
about 1/8 to 1/2 inch, circular dimple-like
depressions will appear resembling the surface of a
golf ball. In the spring, small chestnut brown
structures protrude from each of the dimples. During
wet spring weather, these projections greatly expand
into an orange mass of spore-bearing, jelly-like
tendrils known as telial horns. Blown by the wind, the
microscopic spores infect neighboring hawthorn trees.
The galls produce spores at least two years in
succession after which they dry out and harden. They
can remain on the juniper host for several years.
Symptoms on hawthorn: Cedar-hawthorn rust causes
discoloring and yellow to nearly black spots on the
leaves, fruit, petioles or new twigs. These spots contain
black pimple-like fruiting bodies that produce spores.
Life Cycle
The disease overwinters as galls on infected
junipers. During wet spring weather, "horns" extrude
from the galls and produce spores. These spores are
then windblown or carried by insects to the leaves
of nearby susceptible hawthorn plants. The spores
then turn brown, infect the leaf tissue, and form
yellow spots on the leaves.
In the late summer, spores are produced on
hawthorn leaves. They are windblown back to
needle bases or cracks in juniper twigs. After
infection of the juniper, small galls form, thus
completing the cycle. It takes two years to produce a
spore-bearing gall.
Integrated Pest Management Strategies
1. For both hawthorns and junipers, prune out
diseased branches. The spread of cedar-hawthorn
rust can be limited by reducing infested plant parts.
2. Live with the disease. The disease rarely kills
trees. It can, however, disfigure plants when twigs
are infected.
3. Use fungicidal sprays. Both junipers and
hawthorns can be protected from infection with
fungicide applications during periods of spore
production. Spray hawthorns prior to spring
bloom with a fungicide. Pesticides registered for
use include chlorothalonil (Daconil) and
mancozeb. Thorough uniform coverage of plant
surfaces is essential for good disease control.
Spray foliage to the point of runoff. Repeat on a
7–10 day schedule while cloudy and/or wet
conditions favorable for disease development are
common. Spray junipers three times, at two-week
intervals, beginning in mid-July, with a fungicide
such as Acme Bordeaux Mixture.
4. Avoid planting susceptible hawthorns within a
two-mile radius of junipers. (This may be
impractical in an urban environment.)
5. Replace infected trees with genetically resistant
hawthorn varieties. |
