Journal of the Oklahoma Native Plant Society, Volume 6, Number 1, December 2006


80 Oklahoma Native Plant Record
Volume 6, Number 1, December 2006

Critic’s Choice Essay

Cedar-apple Rust
Clark L. Ovrebo

Department of Biology, University of Central Oklahoma, Edmond, OK 73034

The photograph on the cover
illustrates a phenomenon of nature that can
be seen in the Oklahoma springtime at
about the same time that the redbuds are in
flower and the morels are fruiting. The
orange-colored masses represent a stage in
the life cycle of cedar-apple rust,
Gymnosporangium juniperi-virginianae, and this
stage is occurring on the eastern red cedar
(Juniperus virginiana).

Rust fungi are obligate plant
parasites; that is, they require a living host in
order to obtain nutrition and survive. Rust
fungi have among the most complicated of
all fungal life cycles with most rusts
requiring two hosts to complete their life
cycles. You may have heard of the wheat
rust (Puccinia graminis) that, throughout the
history of cultivated crops, has been a
devastating plant pathogen. Its second host
is the barberry (Berberis spp.).

As the common name suggests, the
cedar-apple rust divides its time between
Juniperus species and apple or flowering crab
trees. Since we have the photo of the stage
on the Juniperus, that is where we will begin
our examination of the life cycle (Fig. 1).

At first, brown gall-like structures
that are rather hard and less than 2 inches in
diameter form (Fig. 2). They can be seen
developing on cedar trees in the wintertime
and have been referred to as “cedar apples.”
Then, in response to spring rains, the galls
expand considerably and send out the telial
horns (Fig. 3). These orange, finger-like
gelatinous structures, which are masses of
teliospores, grow from the galls. The
teliospores are two-celled and later a
basidium grows from each and releases four

basidiospores. The basidiospores are carried
by the wind to distances of up to three
miles, where they land and infect the leaves
of an apple tree. In late spring or early
summer light yellowish orange spots form
on the upper surface of the leaves (Fig. 4).
Small flask-shaped structures called
spermagonia appear on the leaf surface. The
spermagonia are sticky and produce
spermatia (spores) that insects carry to
another spermagonium where fertilization
takes place. The hyphae (fungal filaments)
that result from fertilization grow toward
the lower surface of the leaf where small
pustules called aecia are formed. The aecia
release aeciospores during mid-summer that
are wind-dispersed to Juniperus trees and the
infection process starts over. The entire life
cycle takes about two years to complete,
with the longest developmental stage on the
Juniperus.

The most damage is done to the apple
trees, so the rust is of concern to apple
growers because of their commercial
importance. Trees may lose the infected
leaves and apple production and quality will
be diminished. Fungicidal sprays are
available to treat both tree species. For
more information and additional photos on
the cedar-apple rust, visit
http://www.ento.okstate.edu.

ONPS
Ovrebo, C.L.
https://doi.org/10.22488/okstate.17.100048



Figure 1 Life cycle of Gymnosporangium juniperi-virginianae

Figure 2 Gymnosporangium juniperi- Figure 3 Telial horns. Figure 4 Rust spots on apple.
virginianae “Cedar-apple gall”. Photo by L.B. Stabler Photo courtesy of
Photo by author. Oklahoma State University

Department of
Entomology and Plant
Pathology.

spermagonium

aecium

aeciospores

basidiospores
teliospore

basidium
m

telial horns

developing
“cedar-apple gall”

apple leaf x.s.

Ovrebo, C.L.

Oklahoma Native Plant Record
Volume 6, Number 1, December 2006

81