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University
of Rhode Island GreenShare Factsheets
Apple
Rusts Pyrrhaltla
virburni
Coleoptera: Chrysomelidae
Several
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. Cedar-Apple
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. Cedar-
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 spray
program.
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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