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University
of Rhode Island GreenShare Factsheets
Cedar
Rust Diseases of Ornamental Plants Gymnosporangium
spp.
Gymnosporangium
rusts infect trees and shrubs in the cypress family alternately
with those in the rose family. On cypress hosts they cause galls,
stem swellings, and twig dieback. Symptoms on Rosaceous hosts include
colorful spots on leaves and fruits. Of the 36 species of Gymnosporangium
in North America, there are only 3 of concern in the Northeast.
Alternate hosts are eastern red cedar and juniper, and apple and
crabapple trees. Junipers become infected in late summer or autumn.
Galls, formed from a combination of plant and fungus tissue, are
first evident as green swellings on leaves. When mature, the kidney-shaped
brown galls are up to 2 inches in diameter. During spring rains,
the galls swell and extrude bright orange gelatinous horn-like projections
called telia which are comprised of fungus spores infectious to
apple and crabapple leaves and fruit. Galls on junipers die after
producing spores for one year, but remain attached to the twig for
several years. Some twig dieback may result, although damage to
junipers is generally minor. Small,
yellowish-orange spots appear on infected apple and crabapple leaves
in the spring. These spots continue to enlarge and turn more orange,
often with a red border. The fungus grows through the leaf, and
fringe-like structures arranged in a ring appear on the underside
of the leaf in midsummer. These structures produce spores which
are infectious to junipers. Leaf drop may result from a heavy infection.
Fruit may also be susceptible to infection, evidenced by a general
russetting and the same symptoms as leaves. Hawthorn
rust is very similar to cedar apple rust, except that it attacks
a wider host range in the Rosaceae family. Hawthorn rust alternates
between eastern red cedar, common and prostrate juniper and apple,
crabapple, hawthorn, and sometimes pear, quince and serviceberry.
On juniper, the reddish-brown galls are smaller, usually less than
12 mm (1/2 inch), and more spherical than those caused by cedar
apple rust. The telia are chestnut brown. Galls produced by hawthorn
rust may produce spores for 3-5 years. Significant twig dieback
can result from this disease.
On hawthorn, leaf spots are similar to those caused by cedar apple
rust. If infection is heavy, hawthorn leaves may yellow and drop
prematurely. Fruit and green wood may also be affected.
Quince rust is the most damaging of the three rust diseases, with
the widest host range in the Rosaceae family-- 480 sp. in 11 genera
(apple, mountain ash, chokeberry, cotoneaster, hawthorn, medlar,
pear, photinia, quince, flowering quince, and serviceberry), although
it is most often seen on hawthorns in the Northeast. Quince rust
generally affects stems and fruit rather than leaves, unlike cedar
apple rust and hawthorn rust. It is also a perennial disease, as
the fungus overwinters in cankers on twigs.
On junipers, quince rust infects leaves and young wood, causing
swellings that girdle twigs and branches. From these swellings,
spores are produced which are infectious to Rosaceae hosts. Diseased
twigs and branches often die. Some twigs survive and a perennial
infection ensues in which spores are produced annually. Heavy infection
results in dieback and loss of vigor. On
Rosaceous hosts, infection is usually characterized by a spindle-shaped
swelling on twigs. Fungal mycelium grows from the site of infection
through the stem to the buds, causing them to open prematurely.
Leaves emerge stunted and misshapen. Hawthorn leaves yellow and
drop when infected. Quince rust girdles most infected stems in the
second year, causing death of the twig. The fungus overwinters at
the edge of the canker. Infected fruits of hawthorn and serviceberry
shrivel and die. White tube-like structures up to 6 mm (1/2 inch)
in length emerge from cankers on twigs and branches, as well as
from infected fruit, releasing masses of bright orange spores which
are infectious to junipers. Since rust
has an alternate host cycle requiring both susceptible junipers
and Rosaceous hosts to be present, the most obvious method of control
is to eliminate one of the host groups. This may not be a practical
option, since junipers are widespread and spores are wind-dispersed
over long distances. Junipers can be examined and galls of cedar
apple rust or cedar hawthorn rust removed to prevent infection of
Rosaceous hosts. The most effective means of control is to plant
cultivars with proven resistance to rust disease. Resistant juniper
(Juniperus) cultivars and varieties: J. chinensis
'Foemina'
J. chinensis var. sargentii
J.
communis 'Aureospica'
J. communis var. depressa
J. communis var. saxatilis
J. communis 'Suecica'
J. sabina
J. squamata var. fargesii
J. virginiana 'Tripartita' Resistant
hawthorn (Crataegus) cultivars:
C. crus-galli
C. intricata
C. laevigata 'Autumn Glory'
C. phaenopyrum
C. pruninosa
C. viridis 'Winter King' Resistant crabapple
(Malus) cultivars: Adams
Centurion
Christmas Holly
David
Dolgo
Harvest Gold
Henningi
Indian Summer
Jewelberry
Liset
Mary Potter
Molten Lava
Ormiston Roy
Pink Spires
Prairifire
Ralph Shay
Robinson
sargentii
Sentinel
Silver Moon
Strawberry Parfait
White Cascade
Adapted
from Ohio State University Cooperative Extension, 1999.
Wayne Sinclair, Howard T. Lyon, and Warren T. Johnson, 1987,
Disease of Trees and Shrubs, Cornell University Press: NY. , 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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