different rust diseases occur on apple and crabapple. All are caused
by different species of the fungus Gymnosporangium and have
various junipers and red cedars (Juniperus species) as an
alternate host. Apples are generally most susceptible to infection
by the rust fungi during the period from early bloom until about
30 days after bloom.
Rust on Apple: Pale yellow spots appear on the upper surface
of leaves during May or June. Spots are up to l/4 inch in diameter,
turn orange with time, and often have a reddish border. Small black
fungal bodies (pycnia) form within the spots and may exude an orange
fluid. In time, yellow spots develop on the underside of the leaf.
These spots thicken, and during late spring and early summer, a
number of small, orange-yellow tubular projections (aecia) appear.
These develop into open, cylindrical tubes that split toward the
base into narrow strips and curl backward. Infected leaves may turn
yellow and drop. Defoliation of rusted leaves is most common in
dry summers. On fruit, similar yellow-orange spots appear, usually
at or near the calyx end. These spots usually occur on immature
fruit and are much larger than the spots on leaves (up to 3/4 inch
in diameter). The light green color of the young fruit becomes a
darker green around the infected area. The tube-like aecia may form
on the slightly raised fruit lesions. Infected fruits are often
stunted and misshapen, and may drop early.
rust on apple leaves.
Photo from the Minnesota Department of Natural Resources Archive,
Minnesota Department of Natural Resources
Quince Rust on Apple: Cedar-quince rust only affects the fruit
of apples. Infected fruit become puckered at the blossom end and
later develop a sunken, dark green area. The flesh under the sunken,
dark green area becomes brown and spongy. The formation of pycnia
and aecia on infected fruit is rare. Apples are susceptible to cedar-quince
rust during the period from early bloom through third cover.
Cedar-Hawthorn Rust on Apple Leaf: Spots similar to those
caused by cedar-apple rust develop on apple and crabapple. Larger,
gray to brown spots form on leaves of hawthorn. Few aecia form on
apple and crabapple. Fruit infection on apple is rare. Defoliation
and deformation of fruits and twigs may occur on hawthorns.
Cedar-apple rust is caused by the fungus Gymnosporangium juniperi-virginianae,
cedar-quince rust by Gymnosporangium clavipes, and cedar-hawthorn
rust by Gymnosporangium globosum. The disease cycle for all
three rusts is essentially the same. The disease cycle for cedar-apple
rust will be outlined as an example. The fungus overwinters as mycelium
in galls in juniper. Large yellow to orange gelatinous sporehorns
are formed on the galls in the spring and spores (teliospores) are
produced. Each teliospore germinates and produces four to eight
sporidia or basidiosproes. As sporehorns begin to dry, the sporidia
are forcibly discharged into the air and carried by wind to nearby
apple leaves, fruits, and twigs. About 30 days after apples have
bloomed, the sporehorns have discharged all their spores and most
apple leaves are no longer susceptible. Within five or six hours
after landing on the leaf, sporidia become attached to the surface,
germinate, and penetrate the cuticle and upper leaf surface. After
10 to 14 days, the yellow spots develop on the upper leaf surface.
The orange to black pycnia develop in the spots and several weeks
later, the aecia form on the underleaf surface. The aecia produce
another type of spore (aeciospores) that are carried by wind to
junipers. When the aeciospores contact a juniper twig, they become
firmly attached and germinate in warm moist weather of late summer
or early fall and penetrate the twig. A young, pea-size, greenish-brown
gall develops. The gall enlarges the following year, but does not
produce sporehorns with teliospores until the second spring. The
complete disease cycle requires almost two years.
Grow resistant or immune apples,
crabapples, and junipers. Before buying trees, check with the
URI Gardening Hotline for lists of
rust resistant cultivars from The Sustainable Trees and Shrubs
of New England manual.
Destroy nearby, worthless or wild junipers infected
with rust galls.
Where rusts are a severe problem, follow a recommended fungicide