Introduction: Cedar-apple rust can infect leaves and fruit of most cultivars in the
mid-Atlantic region. A notable exception is 'Delicious', which is nearly immune (Table of apple cultivar susceptibility)
(Table of selected species and varieties of
Crabapple, Juniper and Hawthorn with resistance to rust diseases).
II. Symptoms: The most conspicuous symptoms on apple are
bright orange, glistening lesions on the leaves (photo 2-10). Lesions which are not
inhibited chemically may form small tufts of spore-producing structures (aecia) on the
lower surface of the leaf by July or August. Cedar-apple rust appears on fruit first as
bright orange, slightly raised lesions (photo 2-11), but may take on a more brown and
cracked appearance as the fruit enlarges (photo 2-12). Usually some of the orange color
remains at harvest as evidence of the early season infection. Sporulation of fruit lesions
is less common than that of leaf lesions. Stem infection causes a slight swelling of the
stem and may result in abscission of the young fruit. On the cedar tree, cedar-apple rust
produces brown, globular galls ranging in size from 1/4 inch (6-7 mm) to nearly two inches
(50 mm) in diameter. These are dimpled like a golf ball in the dormant season, but produce
gelatinous, orange spore horns during spring rains (photo 2-13).
III. Disease Cycle: The fungus overwinters on galls on the cedar
tree. Wetting of galls in the spring initiates expansion of the spore horns and production
of basidiospores which are carried to the apple tree to infect leaves and fruit during
extended wetting periods. Basidiospores may be produced within four hours at 52 to 77 F
(11.1 - 25 C), and an additional four to six hours of wetting will permit severe infection
under heavy inoculum conditions (Table 2-2). Basidiospores do not form at temperatures
below 46 F (7.8 C) and the length of wetting required for infection is extended
considerably at temperatures below 50 F (10 C). Lesions begin to appear 10 to 14 days
after infection. In late summer, spores produced on the lower surface of infected apple
leaves reinfect foliage of nearby cedar trees. These infections develop into galls which
produce spores in the spring following the next full growing season. A cedar-apple rust
gall produces spores only one season. All of the lesions seen on the apple tree result
from spores produced on the cedar; there is no secondary infection within the apple tree.
Table 2-2. Approximate wetting period required for
cedar-apple rust infection at different air temperatures. a
|Average temperature (F)
||Average temperature (C)
|79 - 86
||26.1 - 30.0
a Adapted from APS Compendium of Apple and Pear Diseases.
b NB = no basidiospores formed at this temperature.
c NSI = no severe infection observed at this temperature.
d NI = no infections observed at these temperatures.
Monitoring: Note the presence of red cedar trees within 1/2mile (0.8 km) of the
orchard and Quince Rust and survey for cedar rust galls (photo 2-13) and quince rust
cankers (photo 2-16). For both fresh and processing apples, determine cedar-apple rust
infection periods by observing duration of leaf wetness and average temperatures during
the wet period (Table 2-2, chapter 2). Monitor rust gall maturity on red cedar trees
(photo 2-13). Lesions begin to appear 10 to 14 days after infection. Awareness of wind
direction from a large inoculum source may aid in selection of the most effective
fungicide before or after potential infection periods.
At midseason, collect rust galls from red cedar trees and determine if they
are still capable of sporulating by placing them in water in a white cup. If the horns
expand on wetting (photo 2-13) and the water is colored orange within a few hours, the
galls are still capable of producing spores during future wetting periods. Because there
is no secondary infection within the apple tree, monitoring of the apple tree serves only
to pinpoint fungicide selection or timing weaknesses in the control program. Rust lesions
appear on foliage and fruit within about 14 days and two to four weeks, respectively,
V. Management: Fungicides that are
effective against the rust diseases should be applied periodically from the pink stage of
bud development through third cover to protect the emerging leaves and developing fruit.
Removing cedars located within a 2-mile radius of the orchard interrupts the life
cycle of the fungus and makes control with fungicides easier. Removing all cedars
within 4 to 5 miles of the orchard will provide complete control.
Chemical control -
Chemical control - home
Option 1 -
Virginia Home Orchard
Management Guide web site
Option 2 - Virginia Home Orchard
Management Guide pdf file (Acrobat
Text prepared by K. S. Yoder and A. R.
Download this file in pdf format (Acrobat Reader required).