Insect and Disease Fact Sheet Compliments of
New Century 
1-877-79TREES
Cedar Rust Diseases of Ornamental Plants
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There
are a number of "cedar rust" diseases in which the fungus completes
its life cycle on two plant hosts; one in the cypress family and one in the
rose family (the rosaceous host). Discussed here are three common cedar rust
diseases in the northeast U.S. 1.
Cedar apple rust – The fungus alternates between Eastern red cedar and
mostly apple and crabapple. 2.
Cedar hawthorn rust – The fungus alternates between junipers and
hawthorn, crabapple, and apple in addition to several other rosaceous hosts. 3.
Cedar quince rust – The fungus alternates between junipers and a
wide range of rosaceous hosts. The
most noticeable in the landscape is hawthorn. In
some cases these diseases are minor problems, but cedar quince rust and cedar
hawthorn rust can be a major problem on hawthorns and cedar apple rust is a
major economic consideration in commercial apple production. Diagnostic Symptoms Cedar
apple rust: On junipers, tan to brownish round to kidney-shaped fungal galls
are present in winter and early spring (Figure 2). With moist weather, gaudy
bright orange |
Figure 1. Close-up of cedar apple rust on crabapple
leaf. |
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masses of gelatinous spores develop from these galls, and
galls swell to several times their original size (Figure 3). Spore masses are
several inches in diameter, with a central core and radiating hornlike
tendrils, and are highly visible during moist weather in mid-spring. On
apple and crabapple, bright orange-yellow leaf spots develop on upper
surfaces of leaves in late spring (Figure 1), followed by light colored, and
fringed cup-shaped structures: on lower leaf surfaces several weeks later.
Damage on junipers is generally minor and involves presence of the galls and
twig dieback. On apples and crabapples, fruit infections and leaf drop also
can occur. |
Figure 2. Cedar apple rust gall on juniper. |
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Cedar
hawthorn rust: On junipers, galls are somewhat smaller than with cedar apple
rust disease. Galls continue to produce spores on junipers for more than one
year, compared to only one season of spore production with cedar apple rust.
On hawthorn, leaf spots are similar to above and occasionally green twigs are
deformed by the fungus (Figures 4 and 5).Cedar quince rust: Infected areas on
jumper are much less spectacular than with cedar apple rust, with a cushion-like
mat of orangish fungal growth developing on spherical
galls in spring (Figure 6). Cedar quince rust causes the greatest damage of
the three rusts to ornamental rosaceous hosts, especially to hawthorns,
because of extensive, unsightly fruit infestations, stunting and death of
fruits and swelling and distortion of twigs. |
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Infected
leaves brown and die. Fruits become covered with orangish-pink
spore horns (Figure 7). Unsightly spherical cankers developing on stems can
last more than one year. Disease Cycle and Condition
Favoring Disease Rust
fungi have complicated disease cycles with a number of different spore types
that will not be detailed here. A crucial
factor relative to control on these cedar rusts, however, is that there is no
repeating spore cycle on the rosaceous hosts. In other words, spores produced
on hawthorn will not re-infect hawthorns or other rosaceous plants; they will
only re-infect junipers later in the season. Spores produced on juniper will not
re-infect junipers; they will only infect the rosaceous host. The alternating
host plant is necessary for survival of the fungus. |
Figure 3. Cedar apple rust gall with gelatinous spore
horns on juniper. |
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Spores
produced on the juniper host are blown during moist weather to the rosaceous hosts
in mid-spring at a time when new growth has emerged. The funguses then causes leaf spots on upper
leaf surfaces and while growing in the leaf two strains of the fungus mate
and emerge as a new spore form on the lower leaf surface. These spores are
then blown back to junipers in mid-summer to fall, develop galled areas on
the junipers over a one and a half year period and the cycle begins again. Windborne
spread of spores between the hosts of several hundred yards is not unusual
and spread can be a matter of miles. |
Figure 4. Cedar hawthorn rust on upper surface of
hawthorn leaf. |
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Control Hints 1.
Application of fungicides.
Protective fungicides can be applied several times starting with
pre-bloom on hawthorn and bud break on crabapples if the disease is
chronically a problem at a given site.
These applications are to protect the plant from spores being disseminated
from the juniper host in mid-spring.
Since there is no repeating cycle of this disease on the rosaceous
host, further applications after this springtime spread from juniper are
unnecessary. 2.
When you diagnose cedar rust disease from infected hawthorn or
crabapple fruits and leaves it is far too late to spray for that year. Sprays
are rarely recommended to protect the juniper host from spores being
disseminated from the rosaceous host in late summer and fall. |
Figure 5. Cedar hawthorn rust on lower surface of
hawthorn leaf. |
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3.
Eradication of the other host plant. One approach sometimes suggested is to
eliminate junipers from around plantings of rosaceous hosts, and vice
versa. Concerted efforts to eradicate
junipers were historically tried in concentrated apple growing regions. This
practice is limited to some extent by practicality in terms of the widespread
occurrence of junipers, long distance spread of the fungi involved, the
rights of juniper lovers, and the fact that in most situations cedar diseases
are not so serious that such extreme measures are needed. Nevertheless, it is prudent to separate
highly susceptible junipers and rosaceous hosts to the extent possible in
nursery and landscape situations. One simple
practice where only a few plants are involved is to remove galls from
junipers. This is easier to do with
cedar apple rust and cedar hawthorn rust, since galled areas are more
inconspicuous with cedar quince rust. 4.
Use plants with genetic resistance.
A number of juniper species and cultivars and a number of rosaceous
plant species and cultivars have varying levels of resistance and
susceptibility to these three diseases. |
Figure 6. Cedar quince rust on juniper.
Figure 7. Cedar quince rust on hawthorn fruit. |
Information obtained through the Ohio State Extension Factsheet HYG-3055- 96
Insect and Disease Fact Sheet Compliments of
New Century
1-877-79TREES